Contents

Chapter 1: The Invisible Hand

Chapter 2: What Is Metabolism Anyway?

Chapter 3: What Is This Going to Cost Me?

Chapter 4: How Humans Evolved to Be the Nicest, Fittest, and Fattest Apes

Chapter 5: The Metabolic Magician: Energy Compensation and Constraint

Chapter 6: The Real Hunger Games: Diet, Metabolism, and Human Evolution

Chapter 7: Run for Your Life!

Chapter 8: Energetics at the Extreme: The Limits of Human Endurance

Chapter 9: The Past, Present, and Uncertain Future of Homo energeticus

Chapter 1: The Invisible Hand

A Small Matter of Life and Death

Nine-year-olds burn 2,000 kcal; for adults, it’s closer to 3,000 kcal, depending on how much you weigh and how much fat you carry. You breathe most of it out as carbon dioxide and turn a small fraction of it into water (but not necessarily sweat).

Planet of the Apes

The consensus view that energy expenditure was the same across mammals was based on measurements of BMR, and that seemed to be a glaring issue. BMR is measured with the subject at rest (nearly asleep), so it doesn’t represent all the calories the organism burns each day, just a fraction. Also, BMR can be tricky to measure. If the subject is agitated, cold, sick, or young and growing, the measurement can be elevated—and unsurprisingly, much of the primate data came from very young, tractable monkeys and apes.

A Sloth in the Family Tree

Orangutans were burning just one-third of the energy expected for a placental mammal their size. Their daily energy expenditure fell within the lowest 1 percent of placental mammals. The only species with lower expenditures for their body size are three-toed sloths and pandas.

In the wild, males don’t reach maturity, and females don’t have their first baby until about fifteen years old. Females reproduce incredibly slowly, with seven to nine years between pregnancies, the longest birth spacing of any mammal. Orangutans depend on fruit, but there can be months with so little fruit available that they’re reduced to tearing the bark from trees and scraping the soft inner layer off with their teeth for sustenance. These food shortages seem to affect their social behavior, as they’re the only ape that lives alone; there’s not always enough food to feed a group.

Their metabolic engines had evolved to run slowly, conserving fuel to fend off depletion and death. But the consequences were stark: growth and reproduction require energy, and a lower metabolic rate inevitably meant a slower life history. This, in turn, meant orangutan populations were slow to rebound from natural or man-made disasters. Their low metabolic rate made orangutans more vulnerable to extinction in the face of habitat destruction.

Primate Power

Primates burn only half as many calories as other placental mammals. To put that in human terms, consider that the average daily energy expenditure for human adults is between 2,500 and 3,000 kilocalories per day. They showed that a typical placental mammal our size burns well over 5,000 kilocalories per day. But it’s not like those other mammals are incredibly active; they walk a couple of miles per day at most and spend much of their time eating and resting. Their bodies simply burn energy faster, much faster, than our diminished primate metabolism can sustain.

Primate BMRs were similar to those of other mammals, even though daily energy expenditures differed drastically. The discrepancy between BMR and total daily expenditure reflects the large primate brains.

This Is Us

Humans have larger brains but smaller livers and guts (the stomach and intestines) than other apes. Brains, guts, and livers are all energetically expensive organs—each ounce of tissue burns a ton of calories because the cells in those organs are incredibly active.

In humans, the energy saved by having a smaller gut and liver perfectly offsets the energy cost of our larger brain. Based on that critical observation and the observation that human and ape BMRs were broadly similar to other mammals, Aiello and Wheeler argued that the critical metabolic changes in human evolution were changes in allocation, increasing the calories channeled toward the brain while decreasing energy to the guts. In this scenario, daily expenditure stays the same. Humans didn’t spend more energy than apes; they just spent their energy differently.

Katherine Milton showed that leaf-eating primates with large guts to digest their fibrous diet had smaller brains than fruit-eating species in the same forests.

Humans have the highest daily energy expenditure, burning about 20 percent more than chimpanzees and bonobos, about 40 percent more than gorillas, and about 60 percent more than orangutans after accounting for differences in body size. BMR differed, too, by the same proportions. Humans in their sample carried twice as much fat (about 23 to 41 percent body fat) as the other apes (about 9 to 23 percent). 

Darwin and the Dietician

Our metabolic engines were not crafted by millions of years of evolution to guarantee a beach-ready bikini body, keep us fit, or even necessarily keep us healthy. Instead, our metabolism has been shaped by the Darwinian directive to survive and reproduce. Rather than keeping us trim, our faster metabolism has led to an evolved tendency to pack on more fat than any other ape.

Chapter 2: What Is Metabolism Anyway?

Demystifying Metabolism

One calorie is defined as the energy needed to raise the temperature of one milliliter of water (one-fifth of a teaspoon) by one degree Celsius (1.8 degrees Fahrenheit). It’s a tiny amount of energy—too small to be a valuable unit of measure when we talk about food. Instead, when we talk about “calories” in food, we’re talking about kilocalories or 1,000 calories.

Along the way, life had to confront some significant challenges in basic chemistry. Oils had to mix with water. Oxygen, a chemical that burns and kills, had to be harnessed for life. Fats and sugars, holding more energy per gram than nitroglycerin, had to be burned carefully for fuel. All the work our bodies do is powered by mitochondria, living within your cells. Mitochondria have their own DNA and their own two-billion-year evolutionary history, including saving all life on Earth from certain doom. And much of the work done to digest your food into usable bits is done by the microbiome. 

Follow the Pizza

Carbohydrates are starches, sugars, and fiber. They come mainly from the plant-based portions of your food. Fats (which include oils) come from both plant and animal sources. Proteins come primarily from animal tissue and the leaves, stems, and seeds of plants (including beans, nuts, and grains).

Carbohydrates

Carbohydrates come in three primary forms: sugars, starches, and fiber. Sugars and starches are digested and either used to build glycogen stores or burned for energy. They can also be converted into fat. Fiber regulates the digestion and absorption of sugars and starches and feeds the intestinal microbiome.

Sugars are just small carbohydrates—little chains of carbon, hydrogen, and oxygen atoms. The monosaccharides are glucose, fructose, and galactose. The other sugars—sucrose, lactose, and maltose—consist of two monosaccharides stuck together and are called disaccharides. Sucrose (table sugar) is just glucose and fructose bound together. Lactose (milk sugar) is glucose and galactose. Maltose is two glucoses.

Starches are simply a bunch of sugar molecules strung together in a long chain (polysaccharides/complex carbohydrates). The most common sugar molecule in plant starch is glucose, and plant starch molecules can be hundreds of glucose molecules long. Starch is how plants store energy, which is why it’s in massive supply in energy storage organs of plants like potatoes and yams. Nearly all plant starch is a mix of just two polysaccharides, called amylose and amylopectin. No matter what foods they come from, starches and sugars all get digested into one of the three monosaccharides.

In the mouth, amylase in your saliva begins the digestive process. Once in the stomach, the hydrochloric acid kills off bacteria and is then pushed on. In the small intestine, the starches and sugars are hit with enzymes produced by the intestine and pancreas to break them down further. The pancreas sits just beneath the stomach and is attached to the small intestine with a short duct. It’s most famous for its insulin production. The pancreas also produces most of the several dozen enzymes used in digestion (along with bicarbonate, which neutralizes the stomach acid as it enters the intestine).

• If your genes no longer produce lactase, the lactose cannot get broken down into glucose and galactose, and the bacteria in the large intestine eat it, producing gas and the side effects of lactose intolerance.

Since much of the carbohydrate in your diet comes from starch, and starch is made entirely from glucose, about 80 percent of the starches and sugars that you eat end up as glucose. The rest is broken down to fructose (about 15 percent) or galactose (about 5 percent). Of course, if you eat a diet high in processed foods full of sugar (i.e., sucrose, which is glucose plus fructose) or high-fructose corn syrup (which is about 50 percent fructose and 50 percent glucose mixed with water), the percentage of fructose might be a bit higher for you, and the percentage of glucose a bit lower.

These sugars are absorbed through the intestinal wall and into the bloodstream. The walls of our intestines are full of blood vessels, and blood flow to our guts more than doubles after a meal to carry away nutrients. The result is a rise in blood sugar after a meal, particularly one high in carbs. If the food you eat is processed, low in fiber, and easily digested, the carbs are digested quickly, and the sugars rush into the bloodstream, creating a massive spike in blood sugar. Those foods are said to have a high glycemic index, which is the rise in blood glucose measured two hours after ingesting a particular food relative to the rise you’d experience from eating pure glucose. Foods that are harder to digest (more complex carbohydrates, fewer sugars, more fiber) take longer to digest and absorb, resulting in a long, low rise in blood sugar—and a low glycemic index.

Fiber is a class of carbohydrates (there are many varieties of fiber) that our bodies can’t digest. These rigid, stringy molecules give plant parts their strength and structure. Fiber from our food covers the intestinal walls like a wet knit blanket, forming a lattice-like filter that slows the absorption of sugars and other nutrients into the bloodstream. That’s why the glycemic index—the rush of sugar into the blood—is about 25 percent higher for orange juice, which doesn’t have much fiber compared to a piece of orange.

Most of the microbiome lives in the large intestine, or colon, where it plays a critical role in dealing with fiber and all the other stuff we can’t digest in the small intestine. These bacteria digest much of the fiber we eat, using enzymes our cells can’t make and producing short-chain fatty acids that our cells absorb and use for energy. Our microbiome also digests other stuff that escapes the small intestine, aids in immune system activity, helps produce vitamins and other essential nutrients, and keeps the digestive tract running properly. The effects on our health, from obesity to autoimmune diseases, are wide-ranging, and new discoveries are happening every day.

Blood sugar that isn’t burned immediately is packed away into glycogen stores in your muscles and liver. Glycogen is a complex carbohydrate similar to plant starch. It’s easy to tap into when energy is needed but relatively heavy because it holds an equal proportion of carbon and water (hence the term “carbohydrate”).

When your body’s energy needs are met, and your glycogen stores are full, the excess sugar in your blood is converted to fat. Fat stores are a bit more difficult to use for fuel—there are more intermediate steps to convert them to a burnable form. But fat is a much more efficient way to store energy than glycogen because it’s energy-dense and doesn’t hold water. 

Fats

They are digested down into fatty acids and glycerides and then built back up into fat in your body, which is eventually burned for energy.

Fats (including oils) are all hydrophobic molecules, which means they won’t dissolve in water. But like all life on Earth, our body’s systems are water-based.

Bile is a green juice produced by your liver and stored in your gall bladder, which is a small, thumb-sized pouch that sits between the liver and small intestine, connected to both with short ducts. When fats enter the small intestine from the stomach, the gall bladder squirts bile onto the mush of food. Bile acids (also called bile salts) act like detergents, breaking up the globs of fat and oil into tiny emulsion droplets. Once the fat is emulsified, enzymes called “lipases,” produced by the pancreas, are added to the mix and break these emulsion droplets down to an even smaller size, to microscopic droplets called micelles, just a hundredth the diameter of human hair. These micelles form, break apart and form again like the bubbles in a fizzy drink. Each time they break apart, they release the individual fatty acids and glycerides (which are fatty acids attached to a glycerol molecule) they were holding, the basic building blocks of fats and oils.

Fatty acids and glycerides are absorbed into the intestinal wall and re-formed into triglycerides (three fatty acids attached like streamers to a glycerol molecule), the standard form of fats in the body.

The evolved solution to preventing lumpy blood (due to fat not mixing in water-based solutions) is to pack triglycerides into spherical containers called chylomicrons. This keeps the fats from clumping together but results in a package too big to be absorbed through capillary walls and into the bloodstream, where they need to go for distribution throughout the body. The fat molecules, packed in chylomicrons, are dumped into the lymphatic vessels. Part surveillance system, part garbage collection, the lymphatic vessels have their own network throughout your body, picking up debris, bacteria, and other detritus and bringing it to the lymph nodes, spleen, and other immune system organs to be dealt with. It’s well suited to pick up big particles like chylomicrons stuffed with fat. The lymphatic vessels also collect all the plasma that leaks out of your blood vessels and returns it to your circulatory system, so it offers a port of entry into the bloodstream. Specialized lymph vessels called lacteals, embedded in the intestinal wall, pull chylomicrons into the lymph system and then dump them directly into the circulatory system, just upstream of your heart.

White, fat-filled chylomicrons are so big after a fatty meal that they can give the blood a creamy hue. Eventually, they are ripped apart, and their contents pulled into waiting cells for storage or use. Lipoprotein lipase enzymes in the blood vessel walls first break the triglycerides into fatty acids and glycerol, which are pulled into waiting cells by aptly named fatty acid transporter molecules before being reassembled into triglycerides. Most fat is stored in fat cells (adipocytes) and muscles, forming a reserve fuel tank. These stored triglycerides are the fat that we feel in our belly and thighs or see marbled into a nice cut of steak.

Problems arise when our bodies start storing substantial amounts of fat in our liver and other organs, which can lead to liver failure and a range of other health issues. The causes of fatty liver aren’t always clear, but obesity is a significant risk factor. A small proportion of the fats we eat are used to build structures like cell membranes, the myelin sheaths that coat our nerves, and parts of our brain. Some of the fatty acids needed to build these tissues can’t be made by reformulating others and so are considered essential fatty acids—you need to get them from the food you eat. 

Like carbohydrates, the reason your body goes to all the trouble to digest and store it—is to burn it as fuel. All animals are evolved to store energy as fat because it holds an incredible amount of energy in a small package, 255 kilocalories per ounce. That’s on par with jet fuel, more than five times the energy density of nitroglycerin, and nearly a hundred times better than a typical alkaline battery. Some fats are burned immediately after digestion, fresh from your gut. But most of the time, between meals, your body draws on stored fats for fuel. The triglycerides that make up your stored fat are broken down into fatty acids and glycerol and used to make energy.

Proteins

Unlike fats and carbohydrates, proteins aren’t a primary energy source (unless you’re a carnivore). The primary role of protein is to build and rebuild your muscles and other tissues as they break down each day. Your body does burn protein for energy, but it’s a minor contributor to your daily energy budget.

The cells within your stomach wall make an enzyme precursor called pepsinogen, which the stomach acid converts into the enzyme pepsin, which breaks the protein up. This process continues in the small intestine as food leaves the stomach, with enzymes secreted by the pancreas.

All proteins get digested down into amino acids (a nitrogen-containing amine group connected to a carboxyl acid). Amino acids are distinguished by their tails, which are always some configurations of carbon, hydrogen, and oxygen atoms. There are hundreds of amino acids on Earth, but only twenty-one are used to build proteins in living plants and animals. Nine of these are considered essential for humans, meaning our bodies can’t make them on their own; we need to get them from our diet. The others your body can make by itself if required, usually by breaking down and reformulating other amino acids.

They are absorbed through the small intestine walls and into the bloodstream. From the blood, the amino acids are pulled into cells to construct proteins, which are chains of amino acids strung together.

The construction of proteins from amino acids is one of the primary jobs of DNA. A gene is just a stretch of DNA that lines up a particular sequence of amino acids to make a protein (some genes are regulatory, meaning they don’t assemble proteins themselves but instead activate or suppress protein-assembling genes). Variants in DNA sequence can result in different amino acid lineups and thus slightly different proteins, contributing to biological differences among individuals. Amino acids are also used to make a variety of other molecules like epinephrine and serotonin.

They are eventually converted back into amino acids and travel through the bloodstream to the liver. The amine group in the amino acid has a very similar structure, NH2, to ammonia, NH3. Accumulating ammonia from breaking down amino acids would be fatal, so we have an evolved mechanism to convert that ammonia to urea, which then travels via the bloodstream to the kidneys to be excreted in the urine.

We pee out the equivalent of fifty grams of protein each day. Exercise adds to that total by increasing muscle breakdown. We have to eat enough protein to replace what we lose each day, lest we find ourselves in protein deficit. If we eat more protein than we need, the extra amino acids are converted to urea and cleared out by the urine. 

After the nitrogen-containing head is chopped off, converted to urea, and sent on its way, the tails are used to make glucose (gluconeogenesis) or ketones, both of which can be used for energy. Proteins are typically a minor part of the daily energy budget, providing around 15 percent of our calories each day. But they are a vitally important emergency energy supply if we’re starving.

Burn, Baby, Burn

ATP molecules are like microscopic rechargeable batteries, which are “charged” by adding a phosphate molecule onto a molecule of adenosine diphosphate, ADP. A gram of ATP holds about fifteen calories of energy (that’s calories, not kilocalories), and the human body only holds about fifty grams of ATP at any given time. That means each molecule cycles from ADP to ATP and back over three thousand times per day to power our body.

When we use food to make energy, we’re making ATP. Let’s start with one glucose molecule, the predominant form of sugar that our bodies use for energy (the story is essentially the same for fructose and galactose).

• First, glucose (C6H12O6) is converted to a molecule called pyruvate (C3H4O3) in a ten-step process that is powered by two ATP molecules but produces four ATP molecules, resulting in a net gain of two ATP. It’s a relatively fast process, and it’s what we use to power short bursts of activity like a 100-meter sprint or to power lift at the gym.
• This first stage of metabolism is called anaerobic because it doesn’t require oxygen. If there’s not enough oxygen present, either because we’re not breathing effectively or (more likely) because our muscles are working too hard, too fast for oxygen supply to keep pace with all of the pyruvate being produced, the pyruvate gets converted to lactate. Lactate can be reconverted to pyruvate to be used for fuel, but if it builds up, it can also become the dreaded lactic acid, which makes our muscles burn when we’re working hard and pushing our limits.
• The second stage, aerobic metabolism, is where we need oxygen. If there’s sufficient oxygen in the cell, the pyruvate produced at the end of the first stage is pulled into the mitochondria. Pyruvate is converted to acetyl coenzyme A or acetyl CoA. Along comes oxaloacetate, which is hitched to acetyl CoA and pulls it along a circular track called the Krebs cycle. The train will make eight stops, and at each, some carbon, hydrogen, and oxygen passengers get on or off. The coming and going of those atoms generates two ATP. By the final stop, only the oxaloacetate is left. It’s hitched up to another acetyl, CoA, and the cycle repeats.
• Importantly, some passengers are robbed as they get on and off the Krebs cycle train; their electrons are stolen away by the molecules NADH and FADH. These NADH and FADH molecules scurry away to the back alleys of the mitochondria and unload their purloined electrons into a special receptor complex in the intermembrane space. When the stolen electrons are deposited into the inner membrane complex, positively charged hydrogen ions chase the negatively charged electrons and are trapped in the intermembrane space. The hydrogen ions are like fish caught by a weir: they flow through the inner membrane, pulled by the electrons, only to find themselves trapped and crowded in the intermembrane space.
• With all the positively charged hydrogen ions packed together, an electrochemical force pushes them out to balance the charge on either side of the inner membrane. But there’s only one way for the hydrogen ions to escape the inner membrane space: a special portal in the inner membrane that’s built like a turnstile. The hydrogen ions stream through the turnstile, driven by the electrical charge. As the turnstile spins, it forces together ADP and phosphate molecules, producing thirty-two ATP. The complex choreography of electrons and hydrogen ions dancing along the inner membrane, called oxidative phosphorylation, is the primary energy generator that powers your body.
• The carbon and oxygen atoms, which make up 93 percent of the mass of a glucose molecule, are converted to carbon dioxide in the conversion of glucose to pyruvate and in the Krebs cycle. The hydrogens bind with oxygen at the end of oxidative phosphorylation, forming water, H2O. We eat carbohydrates only to breathe them out.

Burning Fat, Getting Fat, and Going Keto

We use the same steps of aerobic respiration to burn fat. Instead of starting with a glucose molecule, we begin with a triglyceride molecule. Triglycerides are broken into fatty acids and glycerol and converted to acetyl CoA (glycerol is transformed to pyruvate first). And just like glucose, the atoms of carbon, oxygen, and hydrogen that make up those fatty acids and glycerols are exhaled as CO2 or formed into water.

If we’re burning a lot of fat, whether we’re on an extremely low-carb diet or starving, some of the acetyl CoA generated will be converted to molecules called ketones. Most ketone production occurs in the liver. Ketones are sort of a traveling version of acetyl CoA and can travel in the bloodstream to other cells, be reconverted to acetyl CoA, and used to generate ATP. Like many metabolic conversions, most ketone production is done in the liver, but they are used throughout the body.

If you consume no carbohydrates, the only way to generate acetyl CoA is by burning fat. Sure, you can also burn proteins by converting amino acids into ketones or glucose (some amino acids even form molecules that can jump into the middle of the Krebs cycle). But protein is typically a minor player in terms of daily calories. Fat is the primary fuel on a low-carb diet, and if you eat fewer calories than you burn, the deficit will be met by burning stored fat for energy. Some of this fat will be processed into ketones before burning. The brain generally uses only glucose for metabolism, but if there’s no glucose available, it will switch to burning ketones.

The dark side of converting fats to energy is that the tracks run both ways. A sugar molecule (glucose or fructose) can be converted to acetyl CoA and then jump on the fatty acid track instead of entering the Krebs cycle, and the sugar gets converted into fat. It’s the same process used to convert fat into acetyl CoA; just run in reverse.

Got more sugars than you need? Send the extra glucose and fructose to glycogen. Are glycogen stores full? Send the excess sugar to acetyl CoA. If the Krebs cycle train is overcrowded because energy demands are low, start sending acetyl CoA to fat. And there’s always plenty of space available in fat. Glycogen stores fill up, and you can’t store excess protein, but there’s no limit to how much fat you can layer on.

Poisoned by Plants

Oxygen steals electrons and binds to other molecules, altering them entirely and often tearing them apart.

At first, the new oxygen produced by plants was absorbed by the iron in dirt and rocks, creating massive, oxidized “red beds” in the Earth’s crust. Then the oceans absorbed as much oxygen as they could hold. After that, the atmosphere began to fill, climbing from 0 percent to over 20 percent oxygen as photosynthetic plants across the globe belched out the noxious stuff unabated and uncaring. As oxygen levels soared, it began to snuff out life, an event known as the Great Oxygen Catastrophe.

Aliens Within: Mitochondria and the O2 Joy

Oxygen-using bacteria swept across the planet, diversifying into new species and families. In the vicious cell-eat-cell world of early, simple life, the proliferating aerobic bacteria would have been a delicious new menu item. When a cell eats another cell (whether it’s an amoeba in a backyard stream gobbling up a paramecium or an immune cell in your bloodstream killing an invading bacterium), it engulfs its prey, bringing the victim inside its membrane to dismantle and burn for fuel. But as uncountable numbers of aerobic bacteria were eaten over hundreds of millions of years, a small handful escaped destruction. Instead, against the odds, they survived intact, living on within their new host. Symbiosis.

With a dedicated energy-producing bacterium on board, these hybrid cells outcompeted others in the battle for turning energy into offspring.

Mitochondria within our cells retain their own strange loop of DNA, a telltale vestige of their bacterial past. And we dutifully feed and tend to them like treasured pets, our heart, and lungs dedicated to the task of supplying our mitochondria with oxygen and carting away their CO2 waste. Without them and oxidative phosphorylation, we couldn’t sustain the energetic extravagance we take for granted. 

Oxygen is the final electron acceptor in what is known as the electron transport chain. This bucket brigade passes electrons along the inner membrane of the mitochondria, pulling hydrogen ions into the intermembrane space. Without oxygen, the electron transport chain stops, the Krebs cycle backs up, and the mitochondria shut down. When electrons jump onto oxygen at the end of the electron transport chain, they attract hydrogen ions, forming water, H2O. Your mitochondria form more than a cup of water each day from the oxygen you breathe in.

Chapter 3: What Is This Going to Cost Me?

On the Shoulders of Giants

Since oxygen and CO2 are indirect measures of energy expenditure, some crucial details need to be considered when using them to measure metabolism. First, it takes a few minutes of activity before the body reaches a steady-state of oxygen consumption and CO2 production. Short bursts of activity, like sprinting or powerlifting, don’t last long enough to reach a steady state. They rely on anaerobic metabolism that doesn’t consume oxygen, making them difficult to measure. Also, the amount of energy burned for a given amount of oxygen consumed or CO2 produced changes a bit depending on whether you’re burning more carbs, proteins, or fats. Conveniently, the mix of fuels can be calculated from the ratio of oxygen consumption to CO2 production (called the respiratory exchange ratio or respiratory quotient).

Often, energy expenditures are reported in metabolic equivalents or METs. One MET is defined as 1 kilocalorie per kilogram of body mass per hour, roughly the energy cost of resting.

• ACTIVITY: METS: NOTES
• Resting: 1.0: Sleeping is a bit lower, 0.95 METs
• Sitting: 1.3: Same for reading, watching TV, computer work
• Standing: 1.8: On two legs
• Yoga: 2.5: Hatha style
• Walking: 3.0: 2.5 mph (4 km/hr) on a firm, level surface
• Sports: 6.0–8.0: Soccer, basketball, tennis, other aerobic sports
• Housework: 2.3–4.0: Cleaning, laundry, mopping, etc.
• High Intensity: 10–13: Navy SEAL training, boxing, vigorous rowing, etc.

Getting Around: The Costs of Walking, Running, Swimming, and Cycling

Walking Cost (kcal per mile) = 0.36 × Weight (pounds)

• Using the equation for walking cost, you’d find that an average 150-pound person burns 54 kilocalories to walk a mile (0.36 × 150 = 54). A smaller person, at 100 pounds, would burn 36 kcal. If we want to factor in the effort to carry a backpack or a baby, we just add the weight of those items to the “Weight” term before multiplying by 0.36. So, a 180-pound person carrying a 20-pound backpack has a total weight of 200 pounds and will burn 72 kcal to walk a mile.

Running Cost (kcal per mile) = 0.69 × Weight (pounds)

Swimming Cost (kcal per mile) = 1.98 × Weight (pounds)

Bicycling Cost (kcal per mile) = 0.11 × Weight (pounds) (at 15mph)

• Cycling costs increase exponentially with speed and are also affected by wind, road surface, tire design, and pressure (which affect rolling resistance).

At about 3,000 pounds, a Toyota Prius burns a gallon of gasoline (28,800 kcal) to travel 55 miles, meaning its cost per pound (0.175 kcal per mile) is about 60 percent greater than traveling by bike.

Climbing (kilocalories per foot in elevation) = 0.0025 × Weight (pounds)

Effects of Speed, Training, and Technique

The primary way that speed affects cost is straightforward: the faster we move, the faster our muscles have to do the work of moving our bodies, the quicker we burn calories. If running a mile costs 100 kcal, we’ll burn 600 kcal per hour running 6 mph (10-minute mile pace) or 1,000 kcal per hour running ten mph (6-minute mile pace). In other words, the rate at which we burn energy (kcal per minute, or kcal per hour) will increase directly with speed.

There’s a surprising implication: regardless of how fast you run, you’ll burn the same number of calories per mile. That means you burn the same number of calories to run three miles at your fastest pace as you do to jog it casually—you just burn the calories faster (and finish sooner) when you run fast.

The same isn’t true for swimming, walking, and cycling. For those activities, speed also affects our gas mileage, the energy burned per mile.

Walking at our most economical pace, about 2.5 mph, burns roughly 50 kcal per mile for a 150-pound person. We can think of that as an energetically optimal speed since it requires the least energy per mile. Walking faster, at four mph, will burn roughly 40 percent more energy, about 70 kcal per mile. At around five mph, the cost of walking exceeds the cost of running; it’s cheaper to run at that speed than it is to walk.

Walking costs (kcal/mile) increase with speed because of the inherent mechanics of a walking gait. We rise and fall with each step, our center of gravity following a roller-coaster trajectory as we walk. That up-and-down movement gets harder to do as we move faster. When we switch to a run, our legs transform from rigid struts to springy pogo sticks, and we bounce from step to step. We still rise and fall with each step, but the springlike mechanics of running result in a flat cost versus speed relationship.

When you swim or bike, you move your body through fluid (water or air), and you lose energy fighting drag. The faster you move, the more drag fights to slow you down. The effect is powerful in swimming: increasing your speed from 2 mph to 3 mph will increase the energy burned per mile by about 40 percent. For cycling, the costs of fighting drag aren’t too noticeable below about ten mph (which is one reason that air drag isn’t a factor in running). But above ten mph, the effect of drag really grows. A 150-pound cyclist will spend 15 kcal more per mile to increase her speed from 10 to 20 mph; rising from 20 to 30 mph will cost 25 kcal more per mile. And all of this assumes there’s no wind, which will affect drag by increasing or decreasing the flow of air relative to the rider. Cycling at 20 mph into a 10-mph headwind will result in the same drag as going 30 mph in still air.

Technique and equipment have similarly unimpressive effects. In Capelli and colleagues’ study of swimming energetics, they report the exact costs per mile for athletes swimming freestyle, backstroke, or butterfly (breaststroke was noticeably more costly). You can swim in nearly any style, and it has little effect on the cost per lap. The same goes for running.

Miles per Donut

Consider a typical 150-pound adult. Even if they get their recommended daily allowance of 10,000 steps per day (about five miles), that’s only about 250 kcal—just about the same number of calories as a 20-ounce bottle of soda (240 kcal) or half of a Big Mac (270 kcal).

A Body at Rest

Background energy expenditure goes by several names: basal metabolic rate, basal energy expenditure, resting energy expenditure, resting metabolic rate, and standard metabolic rate, among others.

Basal metabolic rate, or BMR, is the most well-defined measurement: it’s the rate of energy expenditure measured in the early morning with the subject lying down, awake but calm, with an empty stomach (no food for the previous six hours), in a comfortable temperature. If one or more of these criteria isn’t met, the measurement is usually called resting energy expenditure or some variant, with an explanation of the conditions in which the measurement was taken. Basal metabolic rate is the energy your body burns when it isn’t doing any physical work, digesting any food, or working to stay warm.

The bigger you are, the bigger your organs are, and the more work they do each day. It’s not surprising, then, that BMR (in kcal per day) increases with body weight (in pounds) as:

• Infants (0 to 3 years): BMR = 27 × Weight − 30
• Children (3 years until puberty): BMR = 10 × Weight + 511
• Women: BMR = 5 × Weight + 607
• Men: BMR = 7 × Weight + 551

Energy expended per pound is much steeper for small organisms (including small humans) than for large ones. That’s why the slope for infants in the equation above is four or five times steeper than it is for men and women. Second, our metabolism changes as we mature, and our bodies shift their physiology from growth to reproduction. Body composition also changes at puberty, with women putting on more body fat than men. Fat doesn’t expend as much energy as other tissues, and so, on average, the calories burned per pound are lower for women than men.

If most of your weight is fat, your BMR will probably fall below the predicted value. If most of your weight is lean tissue, you’ll likely land above. That’s one big reason people notice their metabolisms “slowing down” as they grow old: we tend to trade muscle for fat as we hit middle age and beyond.

Muscle, Skin, Fat, and Bone

For a typical U.S. adult, muscle accounts for 42 percent of body weight but only 16 percent of BMR, about 280 kcal per day (about 6 kcal/day per pound). Your skin weighs 11 pounds but burns only 30 kcal per day; your skeleton weighs slightly more but burns even less. Fat cells are more active than you might think. They make hormones and traffic in glucose and lipids to maintain the energy supply to the body. Still, each pound of fat burns only about 2 kcal per day, for a total of about 85 kcal per day for a typical 150-pound adult with 30 percent body fat.

Heart and Lungs

During exercise, your heart’s output can easily triple. Amazingly, all this work is done for a low cost of about two calories per beat. Not kilocalories, just 2 calories (0.002 kcal). With a resting heart rate of 60 beats per minute, your heart burns about 8 kcal per hour, the energy equivalent of two M&M’s. The heart accounts for about 12 percent of BMR. Lungs, for comparison, are more than twice as large but burn only about 80 kcal per day, or about 5 percent of BMR.

Kidneys

In addition to maintaining precisely the right amount of water in your body, the kidneys handle the enormous task of clearing out waste and toxins, filtering 180 liters of blood a day. Millions of microscopic sieves (the nephrons) clean every drop of blood thirty times per day, pumping salts and other molecules in and out to eliminate the bad stuff and keep the good. The kidneys also perform an essential metabolic task called gluconeogenesis, converting lactate, glycerol (from fat), and amino acids (from proteins) into glucose. All of this metabolic work takes a lot of energy. Together, your kidneys weigh only half a pound, but they burn about 140 kcal per day, accounting for 9 percent of BMR.

Liver

It is the primary storage depot for glycogen and does most of the work converting glucose to glycogen and glycogen back into glucose. It metabolizes fructose into fat for storage or a burnable glucose form. The liver breaks apart unused chylomicrons and stores the fat or repackages it into other lipoprotein containers (including the low-density lipoproteins, or LDLs, and the high-density lipoproteins, or HDLs). The liver is the main site of gluconeogenesis, converting fats and amino acids to glucose when needed and turning the nitrogen-bearing head of amino acids into urea to excrete in the urine. The liver is also the primary site of ketogenesis, and it breaks down a wide range of toxins, from alcohol to arsenic. This ceaseless metabolic work burns about 300 kcal per day, 20 percent of BMR.

Gastrointestinal Tract

The human gastrointestinal tract weighs about 2.5 pounds and burns about 12 kcal per hour, and that’s just at rest on an empty stomach. Digestion costs much more, about 10 percent of the daily calories consumed, or 250 to 300 kcal per day for the typical adult. It’s unclear how much of the energy burned by the gut is attributable to our microbiome. A recent study in mice by Sarah Bahr, John Kirby, and colleagues suggests the calories burned by the microbiome might account for as much as 16 percent of BMR in humans, which would mean that the resting energy expenditure of the gastrointestinal tract (about 12 kcal per hour) is attributable almost entirely to gut bacteria.

Brain

Your brain weighs a little less than 3 pounds but burns about 300 kcal per day, accounting for 20% of BMR. The high cost of brain tissue is the main reason large brains are so rare among animals. Only under rare circumstances does evolution favor channeling tons of energy into a large brain rather than directly into survival and reproduction.

It runs almost entirely on glucose. Neurons, the gray-matter cells that do the work of cognition and control, sending and receiving signals, do little of their own housekeeping. Instead, the glial cells (white matter), which outnumber the neurons nearly 10 to 1, do much of the support work, providing nutrients and cleaning up waste.

Most of the brain’s work lies entirely outside of our conscious experience. The brain is ceaselessly busy sending and receiving signals to regulate and coordinate every aspect of life, from body temperature to reproduction. Thinking accounts for a tiny fraction of this work, and consequently, the costs of cognition are small.

But while thinking is incredibly cheap, learning is quite energetically expensive. Learning is a physical process within the brain. Neurons, with their sinuous dendrites and axons stretching out like the branches of trees, form new connections (called synapses) with other neurons to make new neural circuits. Other synapses and circuits are broken or “pruned.” Our brains develop, strengthen, and prune synapses throughout our lives, but by far, the most active period is in childhood when we’re soaking up the world around us. Work by Christopher Kuzawa and colleagues has shown that in children three to seven years old, the brain accounts for over 60 percent of BMR, three times more than in adults.

Beyond BMR

Thermoregulation:

• Mammals and birds have evolved to run hot, and this ramped-up metabolic rate enables us to grow and reproduce more quickly. But there’s a catch: the complex metabolic stew of chemical reactions that keep us alive needs to be kept within a narrow range of temperatures. If our body temperature rises or falls just a few degrees from normal (98.6°F, or 37°C), we can die. All birds and mammals have a thermoneutral zone, a range of environmental temperatures where body temperature is maintained without any effort. For humans, the thermoneutral zone is roughly between 75°F and 93°F.
• Humans are masters of creating thermoneutral microenvironments next to our skin using clothing and our built environment. Our natural insulation, fat, can shift our thermoneutral zone as well. Adults who are obese have a thermoneutral range that’s a couple of degrees colder than adults who aren’t.
• We can burn brown adipose tissue or brown fat, which makes up a tiny proportion of your body fat. Brown fat creates heat by modifying the electron transport system in its mitochondria: the protons sequestered in the intermembrane space are allowed to leak back through the membrane without producing any ATP. The energy that would be captured in the ATP is lost as heat. 
• The second way we can generate heat is through shivering, which is simply involuntary muscle contraction. Mild cold exposures, like hanging out in shorts and a T-shirt in a 65°F room, can raise your metabolic rate 25 percent above your BMR (that’s an additional 16 kcal per hour for most of us). In extreme cold, shivering can cause our resting metabolic rate to climb above three times our BMR, a much larger effect than burning brown fat.

Immune Function:

• We are under constant assault from bacteria, viruses, and parasites.
• To respond to infection, immune system cells proliferate and manufacture a broad range of molecules. All of that metabolic work burns calories. A study of twenty-five U.S. college men who reported to a student health clinic found their BMRs were 8 percent higher, on average than normal. Notably, the study excluded men who were running a fever. Ramping up body temperature to kill an infection with fever—an ancient mammalian defense—would increase BMR even more.
• Sam Urlacher found that Shuar kids five to twelve years old have BMRs about 200 kcal per day higher than kids in the United States and Europe, a 20 percent difference. The energy demand of fighting infection steals calories away from growth. When our immune system responds to an infection, it makes a number of molecules (immunoglobulins, antibodies, and other proteins) that circulate in the blood. Sam found that Shuar kids with more of these markers in their blood grew slower than those who had fewer. The cost of the immune response, in terms of both calories and growth, is probably one big reason that indigenous populations like the Shuar, Tsimane, and Hadza tend to be short-statured.

Growth and Reproduction:

• Our bodies are built from a conglomerate of protein, fat, and carbohydrates. The energy content of those building blocks is the same as it is in food: 4 kcal per gram of carbohydrate (like glycogen) or protein (like muscle), 9 kcal per gram of fat. Living tissue also holds a good deal of water (about 65 percent of its weight), with no calories. As children grow, the energy content of new tissue, which is a mix of about 75 percent lean tissue and 25 percent fat, works out to about 1,500 kcal per pound. To that, we have to add the energy burned to do the work of disassembling the nutrients from our diet and reassembling them into tissue, which comes to about 700 kcal per pound. The cost of growth, then, is about 2,200 kcal per pound.
• Adding a higher proportion of fat costs more, whereas adding a higher proportion of lean tissue costs less because the energy content of fat is more than twice that of protein. One way to see that difference is to look at the energy burned when losing weight. The energy we expend to lose weight has to be equal to the energy content of the tissue that’s been lost. Since the tissue we lose during weight loss is primarily fat, the general rule is that it takes about 3,500 kcal to burn off one pound.
• For mothers, the cost of growth is only a fraction of the cost of pregnancy and nursing. A newborn baby weighs, on average, between seven and eight pounds. The cost to grow that much baby is only about 17,000 kcal. But the mother also adds tissue of her own (typical weight gain during pregnancy is about twenty-five to thirty pounds) and has to pay the daily metabolic cost of keeping all that new tissue—the fetus plus herself—alive. The total cost of a healthy nine-month pregnancy is about 80,000 kcal. That’s 27 percent more energy than the typical Hadza woman spends walking over a year.
• Milk production for mothers whose children are exclusively breastfeeding costs about 500 kcal per day, which comes out to about 180,000 kcal per year— more than hiking the Appalachian Trail. Some of that energy comes from fat stores built up during pregnancy (about 3,500 kcal per pound lost). And, just as in pregnancy, most of that energy fuels the baby’s BMR and other expenditure. Only a tiny portion goes to the growth of new tissue.

The Game of Life

More energy for growth and reproduction can also mean larger offspring, which have a better chance of surviving to reproduce themselves. Any other expenditures—immune defense, brains, digestion—are worthwhile only to the extent that they improve the ability, over the long term, to channel energy into reproduction.

Mammals and birds evolved turbocharged metabolisms, burning ten times more calories per day than their reptilian ancestors. In each case, this radical metabolic acceleration was favored by natural selection because it increased energy for growth and reproduction. Mammals grow five times faster than reptiles and channel about four times more energy into reproduction. Birds have similarly high rates of growth and reproductive output.

Reptiles, fish, insects, and other cold-blooded, slow-metabolism groups remain incredibly successful despite the advance of mammals and birds. The earliest members of our group, the primates, evolved a much slower metabolic rate and life history around sixty-five million years ago. Short-term growth and reproduction were reduced, but the slower metabolic rate also stretched out the life span, and lifetime reproductive success improved. 

For each major group of vertebrates—placental mammals (primates and non-primates), marsupials, reptiles, birds, fish, amphibians— metabolic rate increases with body size in a distinct curve. Just as we saw with human BMR, calories per day rise steeply among small animals but grow shallower with larger species. This is Kleiber’s law of metabolism.

• Using measurements of BMR for a range of species, Kleiber argued that metabolic rate increased with body mass to the ¾ power, or mass0.75. Nearly a century later, we now know that the same is true for total daily energy expenditures, not just the portion spent on BMR. Groups differ in the height of the curve (e.g., reptiles have lower curves than mammals), but all have an exponent (the shape of the curve) around 0.75.
• Daily energy expenditure is a function of body size: larger animals burn more calories per day. But an exponent less than one means that small animals burn a lot more energy per pound of tissue than large animals. For reasons that still aren’t well understood, the cells of small animals work harder and burn energy faster than the cells of large animals.

Rates of growth and reproduction follow these same distinctive curves. Within birds, mammals, and reptiles, growth and reproduction rate increase with body mass with exponents in the neighborhood of Kleiber’s 0.75, ranging from 0.45 to 0.82. That means that, for their body size, small animals grow faster and reproduce more than larger animals. A 220-pound caribou female will produce one 14-pound calf each year, equivalent to 6 percent of her body weight. In that same amount of time, a 1-ounce female mouse will produce about five litters of seven pups each, equivalent to 500 percent of her body weight. The difference corresponds fairly well with the mouse’s ten times higher rate of cellular metabolism. Growth rates compare the same way. Mice grow to thirty times their birth weight in just forty-two days; caribou grow to fifteen times their birth weight, which takes nearly two years.

One Billion Heartbeats

The biology of death is an area of intense and active research. Still, researchers have long been aware of an apparent connection to metabolism: the slower a species burns energy, the longer it tends to live.

We know that faster metabolic rates don’t inevitably mean a shorter life. Small birds, for example, tend to have faster metabolic rates than mammals of the same body size but generally live longer.

The free radical theory proposes that aging is the accumulation of damage caused by toxic by-products of oxidative phosphorylation. In the electron transport chain, the end process of making ATP in the mitochondria, oxygen molecules are occasionally transformed into free radicals (also called reactive oxygen species), which are oxygen molecules that have lost an electron. These mutant oxygen species are voracious, and they rip electrons from surrounding molecules, causing damage to DNA, lipids, and proteins. Harman argued that aging is the accumulation of damage (sometimes called oxidative stress or oxidative damage) from these free radicals. Since free radicals are an unavoidable by-product of making ATP, it follows that our cells’ metabolic rates (which are also their rates of ATP production) determine how quickly we age and die.

• The free radical theory of aging has its problems. Studies of antioxidant consumption in humans and other animals don’t always show the expected effects on life span. The difficulties in finding clear, strong links between metabolism and longevity have left some researchers lamenting whether such links exist at all.

The Devilish Arithmetic of Daily Energy Expenditure

The factorial method is crude, but it seems to give reasonable results. And it remains alive and well today: it’s still used by the World Health Organization to figure out daily caloric needs for populations they work with, and it’s the math behind every online calculator that estimates your daily calorie needs from some combination of height, weight, age (all used to estimate your basal metabolic rate) and your level of physical activity (used to assign an average daily PAR value). However, it’s just a good guess.

• It assumes that daily energy expenditure is simply BMR plus the costs of physical activity and digestion. That view has become so accepted and widespread that it’s hard to imagine any other perspective. It’s what every student in nutrition and metabolism is taught, what every aspiring doctor learns in medical school, and it’s the guiding faith of every exercise weight-loss program. The bottom line is that your daily activity level has almost no bearing on the number of calories you burn each day.

Don’t Bother Asking:

• In a recent study of 324 men and women across five countries, adults underreported actual food intake by 29 percent on average. That’s the equivalent of forgetting an entire meal every day. Reported energy intake didn’t track expenditure whatsoever. Diet surveys are just random number generators, and the data they provide on daily calorie consumption is useless. The only way to make them worse than useless is to treat them like accurate data and base a nutritional program on them.

The Ballad of Nathan Lifson:

• The hydrogen and oxygen atoms in our body water pool are in constant flux, entering the body water in our food and drink and leaving the body as urine, feces, sweat, and the water vapor in our breath. In his early work, Lifson figured out that the oxygen atoms in the body water pool have an alternative way to leave the body. When carbon dioxide is formed during the metabolism of some carbon-based molecule, one of the oxygen atoms in the new CO2 molecule is taken from the body water. That oxygen atom is then exhaled in the CO2 that we breathe out. The bottom line is this: hydrogen leaves the body only as water; oxygen leaves as water and CO2.
• Lifson realized that if he could track the rate of hydrogen and oxygen atoms leaving the body, he would be able to calculate the rate of CO2 production. And since you can’t burn energy without making and exhaling CO2, Lifson knew that if he could measure the rate of CO2 production, he could measure energy expenditure. As long as he could track their hydrogen and oxygen flow rates with the occasional urine sample, the subjects could do whatever they liked.
• If 10 percent of the hydrogens in the subject’s body water were deuterium on Monday, but only 5 percent of them were deuterium on Wednesday, he would know that half of the body water from Monday had been flushed out and replenished with normal H2O. He then could use those measurements to calculate the rate at which hydrogen atoms were lost. The same approach would tell him the rate of loss for oxygen atoms. The difference between those two rates had to reflect the rate of CO2 production. 
• The amount of isotope needed for the measurement is proportional to body size. That made studies in mice or other small animals relatively cheap, but posed a challenge for human work. In 1955, the amount of isotope needed for a 150-pound human would cost more than $250,000 in today’s dollars. By the 1980s, deuterium and oxygen-18 were cheap enough to measure an adult human for just 1 percent of what it would have cost in the 1950s or ’60s.

The Doubly Labeled Water Revolution

These days, labs like the author’s can measure a person’s daily energy expenditure using doubly labeled water for about six hundred dollars.

The most significant predictors of daily energy expenditure are the size and composition of your body. Bigger people are made of more cells, and more cells doing more metabolic work burn more calories each day. Some of our organs and tissues burn more calories than others. Most importantly, fat cells burn a lot less energy each day than lean tissue, the cells that make up our muscles and other organs. If fat cells make up a larger proportion of your body weight, you will burn fewer calories each day than a person who weighs the same but is leaner. Since women tend to carry more body fat than men, women tend to burn fewer calories each day than men who weigh the same.

You can plug your body weight into the appropriate equation and calculate estimated daily energy expenditure. Notice, though, the ln function in each equation. That means you need to take the natural logarithm of weight before multiplying the result by 786 (females) or 1,105 (males) and subtracting the appropriate intercept value. A 140-pound woman has an estimated daily energy expenditure of 2,300 kcal per day. For a 160-pound man, expected daily energy expenditure is 3,000 kcal per day.

Daily expenditure rises steeply with body size in children. Their cells burn much more energy each day than larger, older people. If you’ve ever held a baby close and felt her heartbeat thrumming in her tiny chest, you have a sense of how hard her body is working. Each pound of a typical three-year-old kid burns about 35 kcal per day. That number steadily declines through childhood and adolescence, flattening out at around 15 kcal per pound each day in our early twenties. The curved relationship between body weight and daily energy expenditure means that we need to be thoughtful when we compare energy expenditures among individuals.

Often, people simply divide energy expenditure by weight to compare metabolic rates among different-sized people. The assumption underlying that approach is that energy expenditure per pound ought to be the same for everyone. But that’s not how it works. Because the relationship between size and expenditure is curved the way it is, smaller people inherently burn more energy per pound than larger people.

Many populations fall above or below the trend line—their expected daily expenditure—by 300 kcal per day or more. This is the dirty little secret of online BMR and daily expenditure calculators. There is a ton of variation in metabolic rates even after we account for body size and gender. When you plug your info in and get a daily expenditure or BMR back, you need to take that number with a large pinch of salt.

Chapter 4: How Humans Evolved to Be the Nicest, Fittest, and Fattest Apes

Selfish, Lazy Vegetarians

Early primates had dexterous, grasping hands tipped with fingernails instead of claws. One persuasive theory of primate origins is that early primates coevolved with flowering plants, which also got their evolutionary start after the dinosaur extinction. In this scenario, primates adapted to eating fruits of these plants, unintentionally providing them with a means of dispersing their seeds throughout the forest in their poo. Plants with more attractive fruits were distributed more effectively and had better reproductive success. An evolutionary partnership was formed, with plants selected to produce fleshy, sugary fruits and primates adapted to seek them out and eat them.

Rather than concentrating all their reproductive effort over a few short years, primates had longer reproductive careers that lowered the consequences of encountering a poor season or two. Slower growth also meant more time for learning during development, with more opportunities for innovation and creativity.

Over millions of years, they expanded into a diverse group with two main branches: the lemurs and lorises on one side and monkeys on the other. Around twenty-one million years ago, a new shoot sprouted from the monkey branch: the apes. For fifteen million years, they proliferated and expanded across Africa, Europe, and Asia. There were dozens of species. Then, for reasons that remain obscure, the bushy ape bough was pruned to just a few branches. By six million years ago we lose nearly all traces of apes in the fossil record. Only a handful of hominoid species persist today: chimpanzees, bonobos, and gorillas in equatorial Africa; orangutans, and several species of gibbons (“lesser apes” in the casual condescension of primate taxonomy) in the rain forests of Southeast Asia. The only other ape lineage to survive was ours, the hominins.

Around seven million years ago in Africa, a population of apes gradually split in two. One of the resulting populations would become the founding stock of the chimpanzee and bonobo lineage. The other population was the founders of the hominins. From the fossil record, we know that the earliest hominins walked on two legs and had stubby, less lethal canine teeth. Otherwise, they were very apelike: chimpanzee-sized bodies and brains; long arms, long fingers, and grasping feet for scrambling high up in the trees. This first chapter of hominin evolution lasted from seven to four million years ago.

The second chapter of the hominin lineage, from about four to two million years ago, is known from a much more complete fossil record. This is the era of the genus Australopithecus, including the famous Lucy and her kin, Australopithecus afarensis. Several species come and go in the fossil record throughout this period, each with their own anatomical distinctions. Still, there are common trends. The grasping foot of earlier hominins like Ardi is gone, morphed into a foot much more like ours with the big toe in line with the others. This, along with changes in the pelvis, suggests these species were more proficient on the ground, burning fewer calories to walk and perhaps venturing a bit farther each day than either living apes or the earliest hominins. Teeth get larger, the enamel much thicker.

• Brain size ticks up a bit in Australopithecus, from just under half a quart to just above (but still only about a third the size of ours).
• In 2015 researchers reported large, rudimentary stone tools from a 3.3-million-year-old site in northern Kenya. We don’t know what these tools were used for, or whether they represent a widespread phenomenon or just a short-lived, early experiment. Regardless, they suggest that at least some Australopithecus species were a bit more clever and resourceful than living apes, who use rudimentary tools to fish for termites or crack nuts but aren’t known to manufacture stone tools.
• And yet, for all the anatomical diversity and hints of creativity, these hominins were most likely apelike in terms of their metabolism. We can be confident in that assessment because, like the living apes today, species from the first two chapters of hominin evolution were essentially vegetarians. They may have hunted small game occasionally or looted termite mounds, as chimpanzees and bonobos do. But a look at their teeth and their tree-climbing adaptations tells us Ardi, Lucy, and the others were getting the vast majority of their calories from plant foods. An apelike, plant-based diet, in turn, tells us these species didn’t need to walk very much to find food. It’s a general rule of ecology that plant-eaters don’t travel very far each day, because plants are plentiful and don’t run away. Living apes rarely cover more than a mile or two in a given day.

But around 2.5 million years ago, hominins started behaving in strange, un-apelike ways. Rather than hunting the occasional monkey or small antelope, they began targeting zebras and other big animals. Stone tools begin to show up all across East Africa in large numbers, and animal fossils from sites in Kenya and Ethiopia show signs of butchery. Meat was no longer a rare delicacy; it was a regular part of the menu. This was the dawn of hunting and gathering, the start of the third and latest chapter of hominin evolution. It marks the early emergence of our genus, Homo. The big dietary innovation that would change our metabolism and our evolutionary destinies wasn’t the food these hominins ate; it was the food they gave away.

Human the Sharer

Men and women both make essential contributions in hunting and gathering societies, but neither is enough on their own. What makes hunting and gathering so successful isn’t the hunting or the gathering, it’s the and. 

In stark contrast, the living apes hardly ever share. Sure, mothers of all ape species will occasionally share some food with their infants or young children. Orangutan mothers in the wild share food with their young kids about one out of every ten meals, usually foods that are difficult to obtain. Sharing among adult apes is even less common.

• Gorillas have never been observed sharing food among adults in the wild. Adult chimpanzees in the Sonso community in the Budongo Forest of Uganda share food about once every two months, and much of what passes for “sharing” is more like tolerated theft. Bonobos share the most, but even they fall well short of the human norm.
• Apes, despite their intricate, lifelong social relationships, live lives of dietary solitude. 

Humans are social foragers. We routinely bring home more than we need, with the intention of giving it away to our community. That means we have one another as a safety net; if someone comes home empty-handed, they won’t go hungry. This allows us to diversify and take risks, to develop complementary foraging strategies—hunting and gathering—that maximize the potential for big gains while limiting the consequences of failure. Some group members hunt, and will occasionally bring home a big game bounty of fat and protein. Others gather, providing a stable, dependable source of food to get through the days when the hunters are unlucky. It’s an incredibly flexible, adaptable, and successful strategy. And the foundation of it all is the inviolable, ironclad, unspoken understanding that we will share.

The Metabolic Revolution

To the extent that the recipient is related to you and shares the same genes, their reproductive success is partly yours. But the discounting is steep: even your child shares only half your genes. The costs of acquiring extra food would need to be low, and the payoff to the receiver really high, for sharing to be worth it. It’s easy to understand why no other apes—in fact, hardly any other species at all—have hit upon sharing as a successful strategy.

The earliest hard evidence for sharing comes from cut-marked bones on large animals like zebra. No hominin could eat a zebra by himself, no matter how hungry. And targeting a zebra, dead or alive, would require teamwork, either to hunt it or to push other hungry carnivores off the corpse. Teamwork pays only if there’s an agreement to share the spoils. Perhaps hominin sharing grew from apelike hunting, with some individuals giving more than the limited, grudging scraps we see with chimpanzees.

Or perhaps hominin sharing grew from the sort of fruit-sharing behavior we see among female bonobos at Wamba. A strong case can be made that wild tubers were an important shared food early on.

Positive Feedback and Virtuous Cycles:

• The caloric investment in brains tells us that cognitive sophistication was so critically important for those hominins that it was worth spending precious calories on more brain power. Physical activity must have increased substantially as well. Relying on meat for a good portion of the diet requires a lot of work each day to get food.
• Regardless of how they hunted, hominins had embarked on a high-energy strategy, hunting and gathering, spending lots of calories on intellect and effort with the expectation of even greater, shared returns. Populations grew and ranges expanded. Homo erectus, the first hominin species to go global, appears in East Africa nearly two million years ago and quickly spreads out across the Old World. Within 100,000 years, its range extended from southern Africa, through central Eurasia, and all the way to East Asia, with stone tools recovered in China and fossils as far away as Indonesia.
• Greater brainpower improved our ancestors’ capacity to locate and procure the best fruits, tubers, and game, while simultaneously improving their abilities to plan and scheme together. Greater endurance allowed them to cast a wider net, hunting down prey and exploiting the bounties of a much larger home range. And sharing, tied it all together. With the newfound capacity to acquire more food than they needed, and the social contract to share the surplus, hominins found themselves awash in energy.
• In each generation, the smartest, fittest, and friendliest individuals were the ones who tended to survive the best and reproduce the most. An arms race developed within the hominin lineage, the early, incipient changes snowballing into an ever more grotesque species, with big bulbous heads, delicate faces, and hairless, sweaty bodies.
• In less than two million years, brain size triples in the genus Homo. Stone tool sophistication increases in parallel. By one and a half million years, hominins were making symmetrical, tear-shaped “hand axes.”
• By 500,000 years ago, hominins are controlling fire (debated). Language abilities must have been developing throughout this period as well, though it is fiendishly difficult to track its evolution. By the time Homo sapiens emerges in Africa around 300,000 years ago, trade networks for highly prized raw materials stretch for miles, and natural red pigments are being used for decoration and perhaps symbolic art. By 130,000 years ago, if not earlier, humans along the coast of southern Africa were harvesting shellfish on an annual schedule, paying attention to the seasons and the tides to get the best catch. Our species expands out of Africa and into Eurasia around 120,000 years ago, echoing the earlier waves of Homo erectus, bringing art and innovation wherever we go. By 40,000 years ago, we’re painting lurid murals on cave walls from Bordeaux to Borneo.
• Our VO2 max, a common measure of peak aerobic power, is at least four times that of chimpanzees’. We carry more muscle in our legs (though less in our arms) than other apes, and we have a much greater proportion of fatigue-resistant “slow twitch” muscles. Our blood holds more hemoglobin to ferry oxygen to working muscle. And our naked, sweaty skin keeps us cool, protecting us from overheating even when exercising in hot conditions. All of this allows us to go farther and faster than any of the other apes.
• Division of labor like that needs a strong commitment to sharing to make it work. We spend the first decade or two of our lives soaking up the shared resources of generous community members, learning to be a functioning, productive adult. Our brains burn so much energy on learning, building, and pruning neural connections as information floods in that our body’s growth slows down during the early elementary school years. In hunting and gathering societies like the Hadza, people don’t become self-sufficient until late in their teenage years.
• Adult hunter-gatherers, both men and women, can easily bring home thousands of extra food kilocalories per day, far more than they need for themselves. This is the extra energy that fuels our faster metabolic engines and greater daily energy expenditures. The extra energy is shared with children, as well as with their moms and other caregivers. In fact, because the energetic burden of reproduction is shared, with moms getting lots of help, mothers in hunter-gatherer societies typically have a kid about every three years, a much faster pace than that of ape mothers who do all the work themselves. It’s the human life history paradox: each kid takes longer to grow up, but we still manage to reproduce faster than our ape relatives. And it’s our commitment to sharing and unique metabolic strategy that make it work.
• Brain size creeps up into the low end of the modern human range by about 700,000 years ago, in a species called Homo heidelbergensis that is found throughout Africa and Eurasia. Their big brains and technological sophistication suggest that long childhoods and super-productive adult foraging was established well before our particular species, Homo sapiens, evolved in Africa. Likewise, their big, expensive brains and hunter-gatherer lifestyles tell us they likely had the same accelerated metabolic rates that we see in humans today, burning more energy than their Australopithecus forebears.
• As our Homo sapiens ancestors expanded throughout Africa and across the globe, they found they weren’t alone. The world was already full of humanlike species: Neanderthals in Europe, Denisovans in central Asia, relict populations of Homo erectus in Asia, an erectus-like species in southern Africa called Homo naledi, and a miniaturized species called Homo floresiensis, nicknamed the Hobbit by paleoanthropologists, in the islands of Indonesia.
• It’s often been argued that we were simply smarter or more creative, but it’s not at all clear that was the case. Neanderthals had brains a bit larger than ours and were making cave art, playing music, and burying their dead long before we showed up. Perhaps we brought new diseases into Eurasia with us as we expanded across the globe, wiping out the Neanderthal and Denisovan populations in the same way that European diseases devastated Native American populations after contact.
• One compelling explanation is that humans persisted because we were friendlier. Richard Wrangham at Harvard University, as well as Brian Hare and Vanessa Woods, his colleagues at Duke, have argued that Homo sapiens became hyper-social through a long process of self-domestication. In this scenario, individuals (particularly men) who tried to get their way through violence and intimidation were ostracized/executed by members of their group. Over time, friendliness and the gene variants that promoted it were favored; mean people didn’t have as many kids. Humans took the sharing behavior of earlier Homo species to the next level. Our communities began to function as hyper-cooperative superorganisms, like beehives or ant colonies. In this scenario, our greater social cohesion was our key advantage over Neanderthals and Denisovans as we spread into Eurasia. When we found ourselves on the same landscapes as Neanderthals and other hominins, our hyper-cooperative strategy won out.

The Downside

An integral part of being hyper-social, sharing apes is our insatiable need to belong to a group. From childhood we are keenly aware of who our tribe is. We pick up the language, the appearance, the signifiers of our group, and we adopt them. We want to belong. This makes a good deal of sense when we consider the evolutionary importance of sharing. Without our group, we’re dead. And we need to know who to be nice to. The social contract demands that we are generous with those in our community.

Just as important is understanding who is not in our group. Sharing with outsiders is an enormous risk. If they aren’t part of our tribe, they might not reciprocate. Even worse, they might be hostile. 

We divide our world into an in-group and an out-group. Penn State and Pitt, Steelers and Patriots, Republicans and Democrats, citizens and immigrants, my race and yours, Tutsi and Hutu, Muslims and Christians… It matters very little whether the groups are defined by something meaningful or completely arbitrary. Members of our group are family for life. Outsiders might even not rate as human.

The other downside of our evolved metabolic strategy is our evolved propensity for metabolic disease. Obesity, type 2 diabetes, and heart disease don’t evoke the same moral horror as genocide, but they kill more people globally each year than violence. These diseases aren’t inevitable.

The faster metabolism and greater daily energy expenditures of the hominin metabolic revolution put our hunter-gatherer ancestors at an increased risk of starvation. Greater daily energy needs mean sharper consequences when food is in short supply. Of course, sharing helps mitigate most of this risk. But there are many potential threats to our energy supply, from prolonged illness wiping out our appetite to unpredictable weather wiping out local plants or game. With a faster metabolism demanding a continuous supply of calories, selection to buffer us against energy shortages led to a second, complementary solution: more fat.

Raise an ape in a zoo, with lots of food and limited exercise, and they get big but they don’t get fat. Their bodies use the extra calories to build more lean tissue, bigger muscles, and other organs. As a result, zoo apes weigh considerably more than they do in the wild, but they stay lean. In contrast, hominins like us evolved to store a lot of those extra calories away as fat, a rainy day fund to survive future food shortages, prolonged illnesses, or other disruptions in our energy supply. Too many of us end up with far more fat than our bodies need, and the negative health consequences that come with it.

Our hominin bodies are also evolved to support, and in fact depend on, the high levels of daily physical activity that were the norm throughout the past two million years of hunting and gathering. We have evolved to require daily exercise. Without it we get sick.

Chapter 5: The Metabolic Magician: Energy Compensation and Constraint

Hard Living

Our ancestors were hunting and gathering only a few hundred generations ago. In the industrialized human zoos that we’ve built for ourselves in the United States, Europe, and other developed societies, we have become much more sedentary. Modernization has brought with it a number of important innovations that improve and extend our lives, from indoor plumbing to vaccines and antibiotics. But by several measures we’ve also become much less healthy. Obesity, type 2 diabetes, heart disease, and the other major killers in the developed world are virtually unheard of among hunter-gatherers and subsistence farmers. Many in public health believe that these diseases of civilization are due in part to a reduction in daily energy expenditure resulting from our sedentary modern lifestyles.

Things Get Weird

Hadza men and women were burning the same amount of energy each day as men and women in the United States, England, the Netherlands, Japan, Russia. Somehow the Hadza, who get more physical activity in a day than the typical American gets in a week, were nonetheless burning the same number of calories as everyone else.

Constrained Daily Energy Expenditure

Daily energy expenditure wasn’t simply responding to differences in daily activity. Instead, the body seemed to be maintaining daily energy expenditure within some narrow window, regardless of lifestyle. I call this view of metabolism “constrained daily energy expenditure.”

Like the Hadza, the Shuar live an incredibly active lifestyle, hunting, fishing, and gathering plant foods from the wild. They also farm a bit, using hand tools and a lot of hard work to grow and harvest starchy staples like manioc and plantain. Sam measured daily energy expenditures among five- to twelve-year old Shuar kids and compared them to kids from the United States and U.K. The Shuar kids were more physically active, and they also had elevated BMRs due to higher levels of parasites and other infection. Nonetheless, their daily energy expenditures were identical to those of U.S. and British children.

Further south, in Bolivia, Mike Gurven and his team measured daily energy expenditure in men and women among the Tsimane, who, like the Shuar, make a living hunting, fishing, and farming in the Amazonian rain forest. We analyzed the doubly labeled water samples in my lab. The Tsimane rack up as much physical activity each day as the Hadza, roughly ten times that of Americans. Tsimane men and women showed slightly elevated daily expenditures, but not due to physical activity. Like Shuar kids, Tsimane adults have elevated BMR because of their high rates of parasitic and bacterial infection—their immune systems are working overtime. Once you account for their incredible immune activity, there’s no evidence of higher daily energy expenditures as a consequence of their strenuous lifestyles.

Even among industrialized countries, there’s no correspondence between measured physical activity and daily energy expenditure, activity energy expenditure, or PAL ratios. People who work harder don’t necessarily burn more calories.

They pooled everyone together and adjusted their daily energy expenditure to account for the effects of body weight, fat percentage, age, and other characteristics, and plotted their adjusted expenditures against daily physical activity. There was a ton of variability among people even after accounting for body size and body fat. The effect of activity was weak and it petered out at higher activity levels. People who were moderately active burned about 200 kilocalories more, on average, than total couch potatoes, but there was no difference between moderately active adults and those with the highest levels of physical activity.

Constrained daily energy expenditure seems to be the rule among warm-blooded animals. Several laboratory studies in rodents and birds have measured daily energy expenditures while increasing daily physical activity—not so different from Westerterp’s half-marathon study. Again and again, we see the same result: daily energy expenditure doesn’t change even as the animals work harder and harder.

Trying to Outrun Obesity

The modern explosion in obesity and all its downstream effects can’t be blamed on decreasing energy expenditures in industrialized countries. Doubly labeled water studies in the industrialized world, which stretch back to the 1980s, seem to confirm this: daily energy expenditures and the PAL ratio have stayed the same in the United States and Europe for the past four decades, even as obesity and metabolic disease have skyrocketed.

Second, constrained daily energy expenditure means that increasing daily activity through exercise or other programs will ultimately have little effect on the calories burned per day. Weight change is fundamentally about energy balance: if we eat more calories than we burn, we gain weight; if we burn more than we eat, we lose weight. The widespread evidence that daily energy expenditure is constrained tells us that durable, meaningful changes in daily energy expenditure are extremely difficult to achieve through exercise. If the energy burned is really difficult to budge no matter how much we exercise, we’d be better off battling obesity by focusing on the amount of energy we eat. Exercise will keep you healthy and alive. It just won’t do much for your weight.

Our metabolic engines are exquisitely tuned to match the energy we burn each day with the energy we eat, and vice versa. (In fact, that’s probably why animals evolved constrained daily energy expenditure in the first place: to match expenditure to the amount of food available.) Even transient increases in daily energy expenditure are met with increases in energy intake. When we burn more, we eat more.

All studies that try to achieve weight loss through exercise show the same pattern: the longer the study lasts, the less that weight loss meets expectations. For the first couple of months in a new exercise program, results are all over the place. People generally lose weight, but there’s a huge amount of variability in how they respond in the short term (some people even gain weight). But after a year, even with someone watching them exercise so they don’t skip or cheat, the average amount of weight lost is less than half of what’s expected. By two years, the average amount of weight lost is less than five pounds, and many, as we see with the Midwest studies, will lose nothing.

The other important change is that exercising drives us to eat more. Our brains are exceptionally good at adjusting our hunger levels so that we make up for any increase in expenditure by increasing intake.

Obese people burn just as much energy each day as thin people, after accounting for differences in body size and composition (in fact, if you don’t correct for body size, obese people tend to burn more calories each day simply because they’re larger). And daily energy expenditure, high or low, doesn’t predict anything about your likelihood of gaining weight.

We Are All the Biggest Loser: Metabolic Responses to Overeating and Undereating

By week 30, Biggest Loser contestants’ BMRs had dropped nearly 700 kcal per day, or about 25 percent. The reduction in BMR wasn’t just a function of weighing less; it was far greater than expected from weight loss alone. Their cells had reduced their metabolic rate, working and burning energy more slowly. And the changes weren’t temporary. When Hall and colleagues checked in with the contestants again six years after the show, their BMRs were still lower than expected. 

There are ancient, evolved responses to negative energy balance in humans and other animals. When our bodies sense that we’re not eating enough to meet our daily energy requirements, we start throttling things down. The body struggles to balance its energy budget so that expenditure doesn’t exceed intake. Our thyroid gland reduces the amount of thyroid hormone produced, which is like taking your foot off the gas pedal. Our cells slow down, which lowers BMR and daily energy expenditure. At the same time, the hormones and brain circuitry that control hunger increase our drive for food. We become ravenous, fixated on food as our body directs our mental energies toward finding something to eat. 

Ancel Keys and colleagues at the University of Minnesota took thirty-two young men and put them on a semi-starvation diet for twenty-four weeks. The men ate just 1,570 kcal per day, less than half of their estimated daily energy expenditure at the start of the study. They lost 25 percent of their body weight. Unsurprisingly, their irritability and moodiness increased, while their interest in sex and other activity fell. They were constantly hungry, obsessed with food. BMR dropped 20 percent below that expected for their body weight.

• All of these changes went away when the men were allowed to eat again. As their weight came back, their bodies called off the alarm. Unlike the Biggest Loser contestants, their BMRs returned to normal, as did their mood and interest in sex and other hobbies. They were no longer in starvation mode. Notably, the men overshot their initial weight when they regained after the study, putting on a couple more pounds of fat than they had had at the start of the study.

BMR and daily energy expenditure don’t dictate weight change, they respond to weight change. The Biggest Loser contestants were in starvation mode during and after the competition. Their lower BMRs and daily expenditures were a desperate, evolved strategy to keep expenditures in line with their severely reduced intake. In the years following the show, contestants who ate the most and regained the most weight gave their bodies the strongest signals that the danger of starvation had passed. Their BMR and daily expenditures rebounded along with their body weight.

The Brains Behind the Operation

In concert with your brain stem, the hypothalamus senses energy coming in by monitoring the blood for factors like glucose and leptin, and neural signals from the taste buds, stomach, and small intestine that relay information on the size and macronutrient content of a meal. The hypothalamus can also sense when we’re in negative energy balance, monitoring levels of ghrelin, leptin (which decreases when fat cells are depleted), and other cues. In response, the hypothalamus can crank our metabolism up or down by controlling the activity of the thyroid gland and the production of thyroid hormone. It can also change our hunger levels, adjusting the amount of food we need to eat to feel full.

The variables (leptin, ghrelin, blood glucose, stomach fullness, food flavors) are the same for everyone, but our immediate environment, genetics, and past experience shape the way the system weighs each variable and responds. For example, lower leptin levels generally push the hypothalamus to activate the hunger response, but the precise threshold at which leptin will trigger your hunger response will have a lot to do with your genes, eating habits, and the typical levels of leptin circulating in your blood.

When we’re in starvation mode, the hypothalamus acts quickly. The objective is to survive the lean period to reproduce sometime in the future when conditions improve. Within days, thyroid hormone plummets. BMR goes way down. If food restriction is severe and lasts for a long time, our organs will actually shrink. But not all organ systems are hit equally hard. We know from careful studies of the bodies of victims starved to death in war and famine that the brain is spared. The spleen, on the other hand, shrinks dramatically.

The hypothalamus controls nearly every system in the body, from stress response to reproduction, and can manipulate specific functions. For example, humans are quick to put reproduction on the back burner when times are tough. Subjects in starvation experiments lose their interest in sex. Women often see a decline in estrogen levels and, if the food restriction is sufficiently severe, will stop ovulating.

In the immediate term, hunger is increased in order to match intake to expenditure (exercise). If high levels of daily activity persist for weeks or months, though, other changes are made. Other systems, including reproduction, immune function, and stress response, are suppressed, making room in the budget for greater activity costs. 

Behavior may change as well, inducing us to rest more and fidget less. We should expect these responses to follow an evolutionary logic, cutting nonessential tasks first and prioritizing our long-term reproductive success. In three to five months, we’ll be acclimated to our new exercise regimen. Daily energy expenditure will be nearly the same as before we started.

Today, in the grips of an obesity epidemic, the average American adult gains about half a pound per year, an error of around 1,750 kcal. That’s only about 5 kcal per day, or less than 0.2 percent of daily energy expenditure. In other words, without thinking much about it, we match our daily energy intake to within 99.8 percent of our daily expenditure (and vice versa).

A Smarter Way of Thinking About Metabolism and Obesity

Our Paleolithic brain is overwhelmed by our modern environment. Rather than perfectly matching intake to expenditure, we have a tendency to overeat—not by much, usually, but the error is consistent and it adds up over time as fat.

When we blame our metabolism for our struggles with obesity, or we rely on exercise to increase daily expenditure and lose weight, or we fall for the latest metabolism-boosting scam, we are making a fundamental mistake about the way metabolism works. The global obesity epidemic cannot be a problem of energy expenditure. For one thing, as we see with the Hadza, daily energy expenditures are the same today in the industrialized world as they were in our hunter-gatherer past. Our bodies are incredibly adept at responding to changing activity levels to keep daily energy expenditures within a narrow window. But more crucially, blaming obesity on slow metabolism gets the cause and effect of weight change completely backward. Our metabolism doesn’t dictate energy balance, it responds to energy balance.

It’s still the case that obesity is fundamentally a problem of taking in more fuel than our engines burn. But rather than pretending we’re in the driver’s seat, we should be asking why the evolved mechanisms that normally match intake precisely to expenditure are failing in our industrialized world.

Chapter 6: The Real Hunger Games: Diet, Metabolism, and Human Evolution

More Data, Less Shouting

In 1999, David Dunning and Justin Kruger, psychologists at Cornell University, had a brilliant insight that seemed to explain why incompetent people are so annoying: their very incompetence blinds them to how incompetent they are. To test this hypothesis, they had dozens of Cornell undergrads take tests in logic, grammar, and the ability to identify humor. Then they asked the students to rate themselves on how well they thought they did. To no one’s surprise the worst performers—those least knowledgeable—routinely rated themselves as experts at what they were doing.

Paleo diet evangelists have distinguished themselves by projecting a hardnosed, steely-eyed view of human nature and evolution. Humans, they assure us, have evolved to eat meat. They push high-fat, low-carb diets that send the body into ketogenesis, arguing that our ancestral diet was all bison and no berries. Paleo proponents, particularly the self-styled carnivores, reject the notion that vegetarian or vegan diets are healthy or natural, dismissing plant-based recommendations or cautions about fat as politically correct pandering or corporate propaganda. In their view, no self-respecting hunter-gatherer would eat a starchy, carb-rich diet, and they sure as hell wouldn’t eat any sugar. Vegans can be just as bad. 

There are three lines of solid evidence that tell us something about the diets our ancestors ate: the archaeological and fossil record, ethnographies of living hunter-gatherers, and functional analyses of the human genome. The details differ and it’s easy to get lost in the weeds, but the overarching message from each is clear: we evolved as opportunistic omnivores. Humans eat whatever’s available, which is almost always a mix of plants and animals (and honey).

Archaeology and the Fossil Record:

• For the first four to five million years of hominin evolution, the different species we see in the fossil record (including the famous Lucy skeleton and her Australopithecus kin) had molars with rounded cusps for eating plant foods. They had long arms and slightly curved fingers as well, which tells us they were climbing into trees often, presumably for fruit and other plant foods. Sure, they probably hunted monkeys or other small game occasionally like chimpanzees and bonobos do today. Insects might have been a regular part of the menu, too, much the same way that chimpanzees target honey and eat ants and termites. But all the evidence from the long early period of hominin evolution points toward a heavily plant-based diet.
• One innovation during this period might have been the exploitation of tubers. Australopithecus species, which are found in the fossil record roughly four to two million years ago have really large molars with thick enamel. Their teeth also preserve scratches that suggest sediment in their food, and the isotopic signature of the enamel is similar to that of wild tubers.
• At around 2.5 million years ago, we see a momentous dietary shift with the origins of hunting and gathering.
  • As the genus Homo began hunting and scavenging more, meat became an ever-larger part of the diet. We see cut marks from stone tools on animal bones starting about 2.5 million years ago, and that continues right up to the present day.
  • At 1.8 million years ago, the Homo erectus population we were excavating at Dmanisi was eating antelope and other animals.
  • By 400,000 years ago, Homo heidelbergensis was regularly taking down wild horses and other big game.
  • By 100,000 years ago, Neanderthals were regularly eating reindeer and mammoth. The cave floors of Neanderthal sites are often thick with the butchered remnants of their meals, and their position as meat-eaters in the food web is evident from the telltale isotopic signatures of their bones (animals that eat other animals have elevated levels of the isotope nitrogen 15, which gets concentrated as you move up the food chain).
  • Our own species was equally adept at hunting, with charred bones from a staggering number of species found in ancient hearths.
• Eating animals means more energy—particularly fat—in each bite of food, which meant less food was needed to meet daily energy demands. The need for big molars and other digestive machinery was reduced. Natural selection favored smaller teeth and guts, freeing up energy for other tasks. Today, our digestive tracts are 40 percent smaller, and our livers 10 percent smaller, than they would be if our digestive systems were proportioned like our vegetarian great ape brethren. These reductions free up about 240 kcal per day, which we spend on bigger brains and other energetically expensive adaptations.
• Some of the newest and most exciting research on hominin diets comes from analyses of food particles trapped in the plaque stuck to the teeth of fossil hominins.
• Neanderthals were the quintessential big game hunters, but they balanced all that meat with carb-rich grains, starchy tubers, sweet fruits, and nuts. Amanda Henry has found similar evidence in the fossilized teeth of members of our own species from this period.
• Archaeological excavations in Jordan have recently uncovered an ancient oven and charred bread remnants dated to over 14,000 years ago, thousands of years before the emergence of agriculture. The bread flour was made from wild cereals.

Ethnography:

• When we plot the proportion of calories from plants and from meat against latitude, two things are immediately obvious. First, there is a lot of variation. Within 50° latitude of the equator (that is, south of Winnipeg, Canada, and north of the Falkland Islands), you can find meat-heavy diets, plant-heavy diets, and everything in between. People eat whatever is available. And this brings us to the second point. In really cold climates, more than 50° from the equator, populations eat a lot of meat. (It’s worth noting, though, that Arctic populations worked to get plant foods wherever they could, even pillaging rodent burrows to steal their stores of wild tubers.) Why do Arctic groups eat a lot of meat? Because plants don’t grow there, at least not very well. We eat what’s around.
• Honey (including the stuff the Hadza get) is just sugar and water, with nearly the same proportions of fructose and glucose as high-fructose corn syrup. In fact, our blood sugar and fat metabolism respond identically to honey, high-fructose corn syrup, and table sugar (sucrose, which is formed from fructose and glucose). If carbs—especially sugar—were particularly bad for you, these high-carb cultures should all have diabetes and heart disease. Instead, they have exceptionally healthy hearts and virtually no cardiometabolic disease. Diets among populations like the Hadza, Tsimane, and Shuar are also low in fat, which makes up less than 20 percent of their calories each day (the typical American diet is 40 percent fat). In fact, outside of the far north, there aren’t any well documented hunter-gatherer groups (like the Hadza) or horticulturalists (like the Tsimane and Shuar) with diets that are high in fat.
• Humans can be healthy eating a broad range of diets, and have done so in the past. There is no single Paleo diet.

Genetics:

• Pastoralists like the Maasai offer a great example of a local adaptation to diet. Milk is a large part of the diet in pastoralist cultures, and much of the energy in milk is provided by lactose, a disaccharide sugar made of glucose and galactose. Like all mammals, we need the enzyme lactase to break lactose into its glucose and galactose during digestion.
  • Infants make lactase in abundance to digest their mother’s milk, but in most people, and in all human populations prior to ten thousand years ago, the gene that makes lactase usually shuts off after childhood. That’s a problem for lactose intolerant adults who eat dairy, causing all sorts of digestive distress as lactose sugars pass unscathed into the large intestine, where they’re digested by gas-producing bacteria. In pastoralist populations, a mutation in the lactase gene arose about seven thousand years ago that causes it to stay active into adulthood.
  • In a pastoralist society, this mutation conferred a big advantage to those who carried it. Those unbloated, dairy eaters had more calories at their disposal. They survived better and had more kids.
  • Remarkably, this happened twice, independently, among early pastoralist groups in East Africa and northern Europe. Today, the descendants of these early pastoralists carry the lactase persistent version of the gene that doesn’t switch off.
• All humans have more copies of the gene that makes salivary amylase (an enzyme in your spit that digests starch) than other apes, resulting in twice the amount of amylase in our spit and reflecting the importance of starchy foods in the hominin diet.
  • But while all humans living today carry plenty of salivary amylase genes to digest starch, populations vary in the number of gene copies. Cultures with deep traditions of eating more carbohydrate tend to carry even more copies of the salivary amylase gene, increasing their levels of salivary amylase even further and improving their ability to digest starch.
• A variant of the NAT2 gene, which produces an enzyme involved in several metabolic pathways, is thought to have become more common in farming cultures in response to decreasing levels of dietary folate. Farming in African and Eurasian cultures, and the resulting shift in the types of fatty acids in the diet, appears to have driven changes in the fatty acid desaturase genes (FADS1 and 2), which are important in lipid metabolism.
• Indigenous groups living in the Atacama Desert of Chile have adapted to the naturally high levels of arsenic in their groundwater, with natural selection favoring a variant of the gene that speeds up clearance of arsenic from the body. 
• Arctic populations have also adapted to eating a lot of meat, but not in a way that most Paleo diet proponents would predict. Work with Inuit populations in Greenland and Canada has shown that the FADS genes have changed in these groups as well, presumably in response to the high fat content (particularly omega-3 fats) in their diet, which has traditionally included a lot of seal and whale blubber.
  • But remarkably, most people in these groups can’t go into ketosis. Instead, they carry a mutant variant of the gene CPT1A that essentially prevents the production of ketones (the “normal” variant of the gene regulates the ketone production in the mitochondria). The non-ketogenic variant was so advantageous among the Inuit and other Arctic cultures that it is ubiquitous among these populations today.

Magical Ingredients: Sugar, Fat, and Testicles

When it comes to your metabolism, there are very few foods shown to have any measurable impact beyond the normal costs of digestion. “Energy-boosting” drinks and supplements, like Dr. Oz’s detox water, are universally bullshit. “Negative calorie” foods that supposedly take more energy to digest than they contain, like celery and leafy greens, are also a myth, though filling up on low-calorie, high-fiber veggies is a good way to lower your daily calorie intake.

Drinking ice water won’t change the amount of energy you burn each day. Even for foods proven to ramp up metabolic rates, the effects are usually modest. The 100 milligrams of caffeine in a cup of coffee will increase your daily energy expenditure by around 20 kilocalories, the equivalent of five M&M’s.

Fat Versus Sugar

At the core of the anti-sugar argument is a plausible mechanism that really could promote obesity, diabetes, and other metabolic disease. Called the carbohydrate-insulin model, it works as follows: eating carbohydrate-rich foods, particularly those high in easily digested sugars, raises your blood glucose levels (blood sugar). In response, the pancreas produces the hormone insulin. Insulin has wide-ranging effects throughout the body, but one important role is to move glucose out of the blood and into cells to store as glycogen or to make ATP. But there’s a limit on how much glycogen our body can hold, and insulin stimulates the conversion of excess glucose into fat and inhibits the pathways that mobilize and burn fatty acids.

Low-carb proponents often complain that mainstream science has ignored the carbohydrate-insulin model, but in fact a number of scientists over the last decade or so have sought to test its predictions.

• The low-carb ketogenic diet was no different than the high-carb baseline diet in promoting fat loss. Daily energy expenditure was slightly higher (57 kcal/d) on the ketogenic diet, but by much less than the carbohydrate-insulin model predicts.
• Large-scale real-world studies examining the effects of low-fat and low-carb diets on weight loss generally find that they are equally good (or bad). The DIETFITS study, which was funded in part by Taubes and the Nutrition Science Initiative, randomly assigned 609 men and women to either a low-carb or low-fat diet. After twelve months, both groups had lost thirteen pounds and 2 percent body fat, on average. Resting energy expenditure dropped in both groups, as we’d expect for people losing weight, but there were no differences between diets (if anything, average resting energy expenditure trended slightly lower in the low-carb group).
• Low-carb diets were tested in a large real-world sample, and they fared no better (and no worse) than the traditional low-fat approach.
• Cancer deaths in the U.S. peaked around 1990, a decade before the decline in sugar consumption. The amount of sugar consumed (including high-fructose corn syrup) peaked around 2000, but the prevalence of overweight, obesity, and diabetes have continued to climb even as people eat less sugar.
• Sugar certainly isn’t healthy (it holds zero vitamins, fiber, and other nutrients, for a start), and sugary foods are easy to overconsume. But there’s little evidence that calories from sugar (including high-fructose corn syrup) are any worse or better for your weight or metabolic health than the calories from fat.

Why Low-Carb Keto Diets (and Others) Succeed

The reason that low-carb diets work is simple: they reduce energy intake and impart negative energy balance. You burn more calories each day than you eat. Low-carb diets may be particularly effective in the short term because they force the body to burn through your glycogen. On a very low-carb diet (usually 20 grams or less of carbohydrate per day), the carbohydrate metabolic pathway shuts down. As that happens, glycogen stores are depleted—the last passengers to take the carbohydrate line into the mitochondria. Unlike fat, glycogen holds water. Because the body stores glycogen in its hydrated form, with three or four parts water per glycogen, burning it also leads to water loss and a rapid reduction in body weight.

Once glycogen stores are depleted, the body relies on the fat metabolic pathway to provide energy. You’ll start burning your stored fat, but only if your daily energy expenditure exceeds your intake.

It’s possible that low-carb diets are helpful for people with type 2 diabetes, since a large dose of carbohydrates can send blood sugar levels soaring to unhealthy levels in people who lack the usual response to insulin.

And it doesn’t seem to matter much whether you restrict the calories at each meal or skip some meals altogether. Intermittent fasting, in which you abstain from eating for large portions of the day, has been widely touted for weight loss. In randomized control trials similar to the Dansinger study, people assigned to intermittent fasting diets are no more successful at losing weight and keeping it off than those assigned to traditional calorie restriction diets.

Hungry Hungry Hypothalamus

Sensory information from your taste buds and guts, along with nutrient contents and hormones circulating in the bloodstream, provides your hypothalamus with a detailed account of the calories coming in and going out. The hypothalamus reacts accordingly, manipulating your hunger and metabolic rate to keep you in energy balance. Normally, this system does an incredibly good job matching intake and expenditure. When we eat enough to meet our needs, we feel full and stop. When we burn our stores of glycogen and fat, we get hungry and eat. If we happen to overeat or starve, our metabolic rate responds appropriately to correct the imbalance.

But the strange and wonderful universe of foods we’ve developed in the industrialized world have exposed a weakness in the system. For far too many of us, the foods we eat overwhelm the usual checks and balances that moderate intake. In short, our modern diets are too delicious. We like food for the same reason we like everything: it triggers the reward system in our brains. Like all animals, from the simplest worms to the most complex primates, we have brains that are evolved to reward behaviors that improve our chances of survival and reproduction. Sex, sugar, social connection… all the essential, universal cravings are built into us from the beginning. We are prewired with neurons waiting to sense “good” things and release reward molecules like dopamine and endocannabinoids in response, to keep us going back for more. The evolutionary logic is simple: organisms with reward systems that are well tuned to their social and physical environments seek out more food and more sex, and tend to have more offspring that inherit their neural reward systems.

Counteracting our desire to eat palatable foods is a set of signals that reduces the reward they bring and makes us feel full. As food is digested and absorbed into the bloodstream, our pancreas releases insulin and our fat cells release the hormone leptin, both of which act in our brain to muffle the reward response to food. Stretch receptors in the stomach and hormonal and neural signals from the digestive tract communicate to our brain that we’re filling up. Protein intake is monitored as well, making us feel fuller the more we eat (in fact, there’s compelling evidence that we monitor the amount of protein we’re eating and don’t feel satisfied until we’ve had enough). All of these satiety signals essentially turn the volume down on the reward signals that food provides and make us feel full, leading us to stop eating, even if the food is delicious.

Modern diets overwhelm our hypothalamus and its ability to balance intake and expenditure in two ways. First, we’re bombarded with far more variety than our hunter-gatherer ancestors ever encountered. This variety sabotages our ability to judge intake by jumping from one set of reward neurons to another. Our brain shuts down the reward response for flavors it’s experiencing but leaves others exposed, a phenomenon called sensory specific satiety.

The other major problem with modern foods is that they are literally designed to be overeaten. Much of the food we buy at the supermarket, the canned and packaged foods, has been engineered beyond anything our ancestors would have recognized. Fiber, protein, and anything else that will make you feel full is removed. Sugar, fat, salt, and other things to tickle your reward system are added. As a result, added sugars and oils are the two leading sources of calories in the American diet today, accounting for fully one-third of the energy we consume. Our evolved reward systems are unprepared for the intensity and breadth of reward signals that these processed foods provide. Our hypothalamus is too slow to shut down our appetite, and we overconsume.

How Does Anyone Avoid the Obesity Trap?

The revolution in genetic research over the past two decades has uncovered over nine hundred gene variants associated with obesity. Just as we’d suspect, nearly all of these genes are active primarily in the brain, clearly pointing to the brain as the epicenter of dysregulation in obesity. The food reward system is complex and expansive, as are the systems that regulate hunger, satiety, and metabolic rate. The myriad pieces of those systems are built by our genes, and those genes vary a bit from person to person. Some genetic variants make our reward and satiety systems more prone to overeating, others make them more resistant.

One obvious strategy to manage our weight and maintain good metabolic health is to build our diet around foods that are filling and nutrient rich without packing in a lot of calories.

Diets that work, including both low-carb and low-fat varieties, are effective because they cut out low-satiety foods and help us feel full on fewer calories. Vegetables, fruits, meat, and fish can all be part of a healthy diet, as long as we avoid foods that prod us to overconsume. Low-carb enthusiasts rightly point out that sugary foods are too easy to overeat: they jangle our reward systems without making us feel full. Sugar-sweetened beverages (sodas and sports drinks), fruit juices, and processed carb-rich foods are dangerous because they carry lots of reward response without any of the fiber that make whole fruits and vegetables so satiating. But fatty foods, particularly processed foods devoid of protein, can cause the same problem.

Getting calorie-rich processed foods out of your house and off your desk at work, and replacing them with protein- or fiber-rich alternatives (like plain nuts, fruit, or fresh veggies), can help reduce the number of calories you consume each day while still feeling full. Cooking for yourself more often can also help, as most restaurants are in the business of making delicious food that’s easy to overeat. We can also try to lower the stress in our lives. Emotional and psychological stress, as well as physical stress like sleep deprivation, can cause dysregulation in our neural reward systems that can lead to overeating. Our brains can also learn to substitute food reward for the emotional and psychological rewards we crave when we’re feeling isolated, scared, or sad.

Chapter 7: Run for Your Life!

The fact that our closest evolutionary cousins don’t need to be active to stay healthy tells us that exercise isn’t like water or oxygen, some required element that all animals need to survive. Our need to exercise is peculiar. As our hominin ancestors evolved into hunter-gatherers, the body adapted to the incredible physical demands it entails. Muscles, heart, brain, guts—everything was affected. This transformation fundamentally changed the pace at which our cells work, accelerating our metabolic rates to meet the energetic demands of our high-octane strategy.

Exercise Gets Everywhere

Men who can do more than ten pushups in one go reduce their risk of a heart attack by more than 60 percent compared to men who can’t. Aerobic fitness is associated with better cardiometabolic health—and longer, healthier lives as well. The benefits of staying strong are particularly important as we age. One standard measure of fitness for older folks is a 6-minute walk test, wherein a person walks as far as they can in six minutes. Older adults who can cover at least 1,200 feet in that time have half the risk of dying in the next decade compared to those who can’t make 950.

Vigorous exercise (6 METS) gets the blood rushing through your arteries, triggering the release of nitric oxide, which keeps them open and elastic. Pliable vessels keep blood pressure low and are less likely to clog or burst, the catastrophes that cause heart attacks and strokes.

Moderate activity (3 to 6 METS, things like a brisk walk, an easy bike ride, or gardening) is great, too. It helps with the trafficking of glucose out of the blood and into cells, and it is known to improve mood, stress, and can even help treat depression.

A Different Way of Thinking About Exercise Energetics

As you increase the amount of energy burned on physical activity, the energy available for other tasks is diminished. Constrained daily energy expenditure changes the way we think about the role of exercise in our daily energy budget. With a fixed energy budget, everything is a trade-off. Instead of adding to the calories you burn each day, exercise will tend to reduce the energy spent on other activities.

In a long-lived species like us, the evolved metabolic strategy is different. Sam Urlacher’s work with Shuar kids has shown that children fighting an infection increase the energy spent on immune defense while reducing their growth. Apparently, when times get tough, humans play the long game, allocating energy to maintenance and survival. When exercise starts to take up a large chunk of the constrained daily energy budget, we see the same sort of prioritization at work. Activities that aren’t essential are shut down first. Essential activities are protected until the bitter end. As a result, exercise has wide-ranging effects on how our metabolism is managed and where our calories are spent, which has enormous effects on our health.

Inflammation:

• When your body is under attack from bacteria, viruses, or parasites, the body’s first line of defense is inflammation. Immune system cells are sent to the site of infection, a ton of signaling molecules called cytokines are released into the bloodstream, and the tissue swells. The inflammation response is energetically costly but essential.
• Big problems arise when inflammation targets the wrong things, attacking our own cells or some harmless grain of pollen rather than a true threat. Depending on the tissues involved, inflammation can lead to everything from allergies to arthritis to arterial disease and more. Inflammation can also affect the hypothalamus, promoting overeating and other dysregulation.
• We’ve known for decades that regular exercise is an effective way to lower chronic inflammation, and that lower inflammation means less risk of heart disease, diabetes, and other metabolic disease. A constrained daily energy budget helps explain why exercise is so effective at reducing inflammation. When a large portion of the daily energy budget is spent on exercise, the body is forced to be more frugal with the remaining calories at its disposal. Suppressing the inflammation response, limiting it to target real threats rather than sounding the alarm constantly, reduces the energy spent on unnecessary immune system activity.

Stress Reactivity:

• You need a healthy stress response to deal with the real emergencies that life inevitably throws at you. For our hunter-gatherer ancestors, a surge of adrenaline and cortisol—the hormonal cocktail at the heart of the fight-or-flight response—was essential to escape the occasional leopard. These days, it might be the fuel you need to outrun a mugger or dodge a taxi. But as is true with inflammation, when stress response is triggered incorrectly or never shuts off, the result is chronic stress, which is devastating to our health.
• In a Swiss study that used public speaking to induce a stress response in two groups of men: endurance athletes and sedentary non-exercisers, they found their reactions to stress were remarkably different. Both groups showed elevated heart rate and cortisol levels, but the athletes’ response was smaller and dissipated more quickly. Their bodies invested less energy in the stress response, just as the constrained daily energy model would predict.
• Another great example of exercise’s healthy, suppressive effects on stress response comes from a study of college-age women with moderate depression. When they were exercising regularly, their bodies produced 30 percent less adrenaline and cortisol each day. Their depression improved, too

Reproduction:

• With more energy spent on physical activity, less is available for reproduction.
• Comparing testosterone levels in endurance runners to age-matched sedentary men, Hackney found about a 10 percent drop in testosterone, on average, among men who had been training for one year, a 15 percent or so drop for those training for two years, and about a 30 percent drop for those training five years or more, suggesting that it can take years for the body to fully adjust to different levels of exercise.
• Suppressing the reproductive system might sound like a bad thing, but in general it’s quite the opposite. Exercise is one of the most effective ways to decrease the risk of cancers of the reproductive system (like breast and prostate cancer), in part because it keeps reproductive hormone levels in check. In fact, reproductive hormone levels in the sedentary industrialized world are likely much higher than they were in our hunter-gatherer past, judging from the levels seen in the Hadza and other physically active, traditional populations.
• The lower activity levels and easier access to high-calorie foods experienced by women in the U.S. means their bodies can put more energy into reproduction and can recover from the last pregnancy sooner than Hadza moms can.
• Taken to the extreme, exercise can begin to cut into normal reproductive system function. At unhealthy workloads, ovulatory cycles can stop completely, libido can evaporate, and sperm count can plummet.

The Dark Side

At the workloads that most of us are likely to experience, the suppressive effects are good for us. They help keep inflammation, stress response, and reproductive hormones at healthy levels. But at extreme workloads, exercise cuts deeper. Tour de France cyclists like Landis burn over 6,000 kcal each day on cycling, and the race lasts nearly a month. They are pushing their bodies to the brink. The consequence is stark: their bodies shut down other functions, cutting into the essential tasks that keep us healthy.

• They get sick more often and take longer to recover because their immune system is weakened. Injuries take longer to heal. The cortisol bump that helps them wake up in the mornings is muted and they feel fatigued all the time. The reproductive system goes into hibernation. Libido drops. Women have irregular periods or stop cycling altogether. Men’s sperm counts decline. Testosterone, the hormone that helps maintain muscle and keep their competitive edge, crashes—unless, of course, they can artificially elevate it with a few discreet injections.

A 2014 study by Karolina Lagowska and colleagues provided food supplements to thirty-one women endurance athletes (rowers, swimmers, and triathletes) who had irregular ovarian cycles and other symptoms of overtraining. After three months of being plied with extra calories, the women saw their daily energy expenditure increase by a modest amount: they were eating and burning about 10 percent more calories each day, the metabolic effect we’d expect given the body’s usual response to overeating. The women’s weight and body fat didn’t change—they weren’t storing the extra energy; they were using it. Some of those extra calories went to the reproductive system, increasing luteinizing hormone. But it wasn’t enough to make a meaningful impact on ovarian function.

With daily energy expenditure fixed, the only way to increase energy availability is to decrease training workload. Rather than some mysterious aberration or a lack of food, overtraining syndrome is just the logical extension of the same energetic trade-offs that make moderate exercise so good for us.

Of Apes and Athletes

Pro cyclists train about five hours each day, mostly at “vigorous” (6+ METS) levels of exertion. Olympic swimmers regularly log five to six hours of swimming each day during training. That’s about three times more exercise than our bodies are evolved to handle, judging by Hadza standards.

People who got an hour or more of moderate and vigorous activity each day were 80 percent less likely to die than the most sedentary participants. A similar study of 150,000 Australian adults found that an hour of vigorous exercise each day helped counteract the negative health effects of sitting at a desk job all day. In Denmark, men and women in the famed Copenhagen City Heart Study cut their risk of dying in half if they averaged at least thirty minutes of exercise a day.

But Weight, There’s Less

Physical activity changes the way the brain regulates hunger and metabolism. Regular exercise seems to help the brain match appetite to caloric needs. Inflammation might play a role here as well. Overconsumption of energy-rich fatty foods can cause inflammation in the hypothalamus, leading to poor regulation of hunger and satiety signals and weight gain, at least in rat studies. It’s speculative, but perhaps chronic inflammation brought on by inactivity has similar ill effects in the brain.

The second caveat in the relationship between activity and weight is that exercise can also be useful for managing weight once you’re able to lose it. Exercise is a poor tool for achieving weight loss, but it does seem to help people maintain weight loss.

If our hypothalamic hunger and satiety systems continue to target our pre-weight-loss intake, we’ll be pushed to eat more calories than we burn. As a result, we’ll slowly gain the weight back, until our body weight and daily energy expenditure are right back where they were before we lost weight.

Pushing the Limits

By telling the public the truth about exercise not contributing to weight loss, it’s believed they just won’t do it. Longevity isn’t as strong of a motivator as vanity. The danger, though, in selling exercise as a way to lose weight is that it doesn’t work. Eventually, people notice the results don’t match the sales pitch. Some will keep with it anyway, hooked by the many other benefits of exercise—improved mood, clearer minds, stronger body—and willing to overlook the bait and switch. But there would be more happy customers if those of us in public health were honest about what we’re selling. Exercise won’t keep you thin, but it will keep you alive.

Chapter 8: Energetics at the Extreme: The Limits of Human Endurance

A Matter of Time

A person who feels completely exhausted still has plenty of fuel on board. Even when we feel like we’ve reached our absolute limits, there’s still plenty of ATP in our tired muscles and glucose and fatty acids circulating in our blood. And second, neural control of fatigue helped explain the strange effects of mood and perception on performance.

With running, speed has a huge effect on how much energy you burn before you reach your limit. The faster you run, the less energy you burn in total before hitting the wall. Race for a mile, and you’ll collapse after burning 100 kcal. Race a marathon, and you’ll collapse, just as exhausted, having burned 2,600. Our bodies don’t just stop when we run out of fuel. Intensity matters.

One reason that speed affects fatigue is the change in the type of fuel your body burns during exercise. When we’re resting and during low-intensity activity, our bodies burn fat as their primary fuel. As exercise intensity picks up, more glucose is added to the mix of fuel. Some of this additional glucose is supplied from circulating blood sugar; some of it is pulled from glycogen stores in the muscle. Compared to fat, glucose is easier and faster to burn (even if it’s being converted from glycogen). This improved speed of availability helps keep the muscles supplied with ATP as exercise intensity and energy demand climb.

At some point, as running speed and energy expenditure continue to increase, the mitochondria can’t make ATP fast enough to meet demand, even with a steady supply of glucose. If we’re measuring oxygen consumption in the lab when you hit this point, we’ll see oxygen consumption plateau and remain steady even as your speed and energy demand continue to climb. This break point is your VO2 max, the limit of your aerobic capacity. The supply chain that carries oxygen and glucose to your cells, then converts them to ATP via the mitochondria, has reached its limit. It simply can’t supply energy any faster. With your aerobic (oxygen-based) production of ATP maxed out, the muscles are forced to rely on anaerobic metabolism. As anaerobic metabolism grows, CO2 production continues to climb even as oxygen consumption remains stuck. Your blood pH becomes more acidic. Glucose in your cells is broken down into pyruvate, the molecule that will jump into the mitochondria, transform into acetyl CoA, and feed the Krebs cycle, ultimately paying off in tons of ATP. But there’s a traffic jam getting into the mitochondria, and the excess pyruvate is diverted and converted to lactate and then to lactic acid. Your muscles start to burn. You simply can’t push any longer. You slow down or collapse in a gasping heap.

Marathons are exciting because the entire race is run along the edge of a cliff, right up against the VO2 max threshold, each racer monitoring their own body and trying to read their opponents’, looking for the right time to give them a shove. The VO2 max threshold makes shorter races into a sort of blood sport, each competitor trying to find the right mix of oxygen and pain to fuel faster and faster races without blowing up before the finish.

Endurance over Days, Weeks, and Months

How is it that activity expenditure decreases (rather than BMR) when exercise increases? One possibility is that people reduce their non-exercise behavior—what the researcher James Levine called non-exercise activity thermogenesis, or NEAT—to reduce AEE when their exercise workloads increase. The idea here is that the body might unconsciously reduce small, overlooked behaviors that burn calories, like fidgeting or standing, in response to increased exercise demands.

The other possible explanation is that AEE is capturing more than just physical activity. Our bodies have a strong circadian rhythm: resting metabolic rate follows a daily roller-coaster trajectory, up and down, rising to its peak in the late afternoon and hitting its valley in the early morning. We measure BMR during the valley, in the early morning. When we calculate AEE by subtracting BMR and digestion costs from daily energy expenditure, we implicitly ignore the daily rise in resting energy expenditure and instead lump all those nonactivity calories into AEE. The energy compensation we often see coming out of AEE probably reflects decreasing amplitude of the circadian fluctuation in resting energy expenditure. Increasing exercise workload doesn’t necessarily make the valleys of resting expenditure lower, but it squashes the peaks. The resulting decrease in AEE looks like the energy compensation is coming from changes in activity, but in fact it’s due to decreased energy expenditure on everything else—for example, the healthy suppression of immune activity, reproductive hormones, and stress reactivity.

It’s Alimentary, My Dear Watson

He tracked down credible estimates of daily expenditure of world records for ultra-long distances, from the maximum distance run in 24 hours to the 46-day record for the 2,200-mile Appalachian Trail. He searched for endurance events that lasted longer than the Race Across the USA but was coming up empty-handed. The longest lasting, highest energy expenditure activity he could find was pregnancy: nine months long, with daily expenditures of 3,000 kcal per day or more in the third trimester. When we look at these records of human endurance: daily expenditures were higher for shorter events like the triathlon, and lower for longer events like the Tour de France. Still, it was hard to compare among all the studies, in large part because the subjects of each one differed so much in body size, which we know affects metabolic rates. To account for size, he divided daily energy expenditure by BMR. That ratio, called metabolic scope, removes the effect of body size because size affects both daily expenditure and BMR in a similar manner. You can think of metabolic scope as a size-corrected daily energy expenditure.

When he plotted metabolic scope against duration, the result was an elegant crisp line, a graceful arc sweeping down from the high expenditures of the shortest events and out to the lower expenditures of the longest. Those points, that line, marked the boundaries of human endurance.

He then added all the other high endurance studies he could find, from military endeavors to athletes in training. Every one of them fell within the bounds of human capability. Not one of them jumped the border. Even pregnancy fell right along the boundary, marking the far end of our metabolic capacity. Expecting mothers were pushing the same metabolic limits as Tour de France cyclists. Pregnancy is the ultimate ultramarathon.

Endurance Takes Guts

Daily weight loss increased in proportion to daily expenditure. The athletes he looked at weren’t trying to lose weight, they were stuffing their faces with all the high-calorie performance foods they could manage to eat. But they weren’t able to get calories in fast enough to meet demands, and as expenditures climbed higher, the energy deficit grew. Then another piece of the puzzle clicked into place. When they put the daily energy expenditure measurements together with the data on weight loss, they found that every athlete (and pregnant mother) in their dataset was taking in the same amount of energy per day. Across the board, from the Antarctic trekkers to elite distance runners, their bodies were absorbing about two and a half times their BMR. All of the energy expenditure above the two and a half times BMR intake limit was coming out of their fat stores, which is why athletes above that level of expenditure were losing weight.

To put this in terms of calories: no matter the event or circumstances, the maximum amount of energy the body can absorb is around 4,000 to 5,000 kcal per day. Beyond that, and you’ll be in negative energy balance, burning more fat and glycogen than you can replenish each day and slowly melting away.

Your body simply can’t digest and absorb calories any faster. For events that last days, weeks, or longer, it isn’t our muscles that hold us back, it’s our guts.

Tour de France athletes would circumvent these limitations but injecting intravenous lipids and glucose at night between race stages.

Athletes Everywhere

For mothers, the limits on energy absorption may prevent pregnancies from lasting too long. Throughout gestation, for the fetus to grow, mom has to take in more energy than she burns. This is the fundamental rule of pregnancy: mom has to gain weight. But as her weight increases, so does her daily energy expenditure. At nine months, the familiar duration of a typical pregnancy, mom is being pushed to the brink. If the baby gets much larger, mom won’t be able to bring in enough calories to sustain them both. We think the signals of metabolic stress released as moms approach their metabolic limits help trigger the birth process.

If the neonate is even a tiny bit too big, serious and often life-threatening complications arise. And how do babies get too big? They get too much energy from mom, either by annexing a greater share of the nutrients in mom’s bloodstream or by overstaying their welcome. In populations like the Hadza, where pregnant mothers stay physically active through the third trimester and foods are unprocessed and slower to digest, less energy will be available for the fetus to commandeer. Their babies are unlikely to get too big before mom’s metabolic limits trigger birth. In the United States and other sedentary, industrialized populations, mothers are awash in easy calories, and the fetus doesn’t need to compete with the energy demands of physical activity. Perhaps this leads to babies being born a bit later and a bit larger, not by much, but enough to cause trouble. It’s notable that the rates of delivery by cesarean section have skyrocketed over the past half century, along with modern changes in diet and physical activity.

Our digestive limits also put a cap on daily energy expenditure during normal daily life. Over the long haul, as months stretch into years and years into lifetimes, we simply can’t burn more energy than we can bring in. We have to live within our metabolic means. No one can maintain a daily energy expenditure too far above two times their BMR. And guess what? No one does. Around the globe, when we look at daily energy expenditures during normal daily life for the hundreds of populations who have been measured, from Holland to the Hadza, everyone lives their lives well under the 2.5 times BMR limit. In physically active populations like the Hadza, the body adjusts to keep daily expenditures at a sustainable level.

What’s So Special About Michael Phelps?

First off, his self-proclaimed 12,000 kcal is probably about 7,000 kcal. He is also 6’4 and 194 pounds. Meaning his BMR should be about 1900 kcal, give or take 200. Not to mention his above average lean mass raising his BMR to about 2100 kcal.

If Phelps ate 7,000 kcal per day, he’d be absorbing about 6,650 kcal for his body to burn. The remainder would be lost in the bathroom. At 6,650 kcal per day, Phelps would be absorbing a little over three times his BMR. That would put him in the very upper range of energy absorption in our human endurance dataset.

A few athletes in their sample had estimated rates of energy absorption just above three times BMR. Eating 7,000 kcal per day, Phelps would be stretching the limits of the rule of 2.5 times BMR, but not breaking it. 

To become an Olympic champion like Phelps, maybe you also need a digestive tract that’s really good at absorbing calories, so that you can fuel the endless hours in the pool without your body shutting down. Maybe Phelps, Ledecky, and the other ultra-elite athletes in our modern Olympic pantheon are distinguished as much by their remarkable guts as their ferocious strength.

Evolved to Break the Rules

Our metabolic machinery provides a perfect demonstration of our body’s physiological interconnections. The same machinery that limits our athletic endurance also shapes gestation and pregnancy and constrains daily energy expenditure. Tellingly, all of these aspects of our metabolism are enhanced compared to our ape cousins. We have better endurance, larger babies, and higher daily energy expenditures than chimpanzees, bonobos, or any other great ape. Natural selection pushed our metabolic capacity skyward, increasing expenditures across the board.

Chapter 9: The Past, Present, and Uncertain Future of Homo energeticus

The distinction between our internal metabolic engine and the external engines that run our world is largely an invention of language, a verbal sleight of hand we’ve played on ourselves. A calorie is a calorie, whether it’s in the food we eat, the sunlight we trap in a solar panel, or the fossilized plants we burn in our cars. Our two engines, internal and external, are deeply interdependent and intertwined in ways we rarely appreciate. We’ve been burning energy externally, harnessing it for our purposes, ever since our hunter-gatherer ancestors got hold of fire, hundreds of thousands of years ago. As we shaped fire, it shaped us. Just as our metabolism today reflects its evolutionary roots, our modern energy economy, and our dependence on it, is an extension of our hunter-gatherer past.

From Focusing Your Energy to Playing with Fire

Simple tools, from the stone choppers at Olduvai to the knives in your kitchen, are useful because they allow us to concentrate our energy. You have the strength to cut a steak with your bare hands, but only if you can focus your power along the edge of a blade. 

Fire was the great technological leap forward. Stone tools, a bow and arrow, and other simple tools allow you to manipulate the way you store, focus, and release your body’s own energy. With fire, our hominin ancestors had access to a completely new engine. Unlike their internal metabolic engines, our hunter-gatherer ancestors could burn these fires as hot as they liked, for as long as they wanted. Most important, they could harness the power of fire in service of essential evolutionary tasks: growth, maintenance, and reproduction. It was a first in the two-billion-year history of life: external energy expenditure to augment your own metabolism.

It seems fire initially had three uses: cooking food, staying warm, and keeping potential predators away. The use of fire for warmth meant our ancestors didn’t have to shiver through the night. Even mild cold can elevate our metabolic rates by 25 percent, or around 16 kcal per hour. Sleeping cold for eight hours could cost a stone-age hunter-gatherer over 100 kcal. With fire to keep warm, those calories could be spent on other important physiological tasks, like growth, reproduction, and repair. Our ancestors might have also slept more soundly knowing that big cats and other species instinctively shied away from fire.

Cooking completely changed our diets and in turn changed our bodies. Wood fires release about 1,600 kcal per pound of fuel. In a simple campfire, most of that energy is lost to the air. The energy that is captured as heat in the food changes its structure and chemistry. Meat becomes easier to chew. Proteins are denatured, making them easier to digest. Starches that are otherwise indigestible are transformed; their carbohydrates accessible in our guts. The effects are largest with root vegetables, which are full of resistant starches that our guts can’t digest: we get double the calories from a cooked potato as we do if we eat one raw. In short, fire supercharged the hominin diet, increasing the amount of energy per bite and decreasing the energy spent on digestion.

Over time, our hunter-gatherer ancestors evolved to rely on fire to prepare our food. Digestive capabilities were reduced, the energy for a big gut and intensive digestion diverted to other tasks. Some of this extra energy seems to have been allocated to reproduction, just as we’d expect from natural selection. The energy boost from cooking may have also contributed to the evolution of larger, more energetically expensive brains.

Raw Foodists eschew cooking for a variety of philosophical reasons or misguided ideas about the “life force” in food. The largest study of their health and physiology comes from a group of over three hundred men and women following raw food diets in Germany. People eating uncooked diets have a hard time maintaining healthy weight, with many below a BMI of 18.5, the threshold for being considered malnourished. Women on raw food diets often stopped ovulating, and the degree of ovarian disruption was directly correlated with the proportion of uncooked food in the diet. Men’s reproductive function was sometimes compromised as well, with some reporting a loss of libido. Without cooked food, humans’ ability to survive and reproduce—the two nonnegotiable measures of evolutionary fitness—are seriously diminished. Even with access to modern foods and high calorie oils.

Fires could be used to change the landscape, burning swaths of forest or scrub to push game and promote new plant growth. Flame also unlocked a universe of chemistry and new materials.

• Paleolithic hunter-gatherers learned to use fire to harden the tips of wooden spears, a practice Hadza women still use today to prepare their digging sticks.
• Our ancestors discovered that fire-treated rocks often made better stone tools.
• Neanderthals and modern humans learned to use kilns to made bitumen, a strong adhesive derived from birch sap, to glue stone axe heads and other blades onto wooden handles.
• As early as thirty thousand years ago, humans were building fires hot enough to fire pottery.
• Around seven thousand years ago, early farming cultures were figuring out how to smelt ore to make copper and other metals.
• By three thousand years ago, they had figured out how to make iron and glass. One hundred generations later, their descendants would be walking around with smartphones in their pockets and sending rocket-powered robots to distant planets.

The Technology Tsunami

 Over time, we manipulated the metabolisms of our domesticated plants to divert their energy into the starches and sugars that fuel our bodies. Today, the fruits and vegetables we find in the market look like grotesque energy-packed carnival freaks compared to their wild ancestors.

We played the same trick with our domestic animals. By protecting them from natural predators and picking the winners and losers in reproduction, we favored those that allocated more energy into growth and milk production. Under our management, these species evolved into soft, dumb, reliable sources of fat and protein. They provided a metabolic engine for converting grasses and other forage that was inedible to us into milk, blood, and meat for our consumption.

Horses and other large species provided a new kind of engine as well—a source of mechanical work to augment or replace our own physical abilities. As James Watt, inventor of the steam engine, deduced through experiments at the dawn of the Industrial Revolution, a horse can comfortably produce around 640 kcal of work per hour (the definition of horsepower) and sustain that output for ten hours, day after day. That number is even more impressive than it might seem. Muscles are, at best, only about 25 percent efficient at converting metabolic fuel into mechanical work. To produce 6,400 kcal of work in a ten-hour day, a horse burns over 25,000 kcal of energy—and that’s in addition to the energy spent on BMR, digestion, and its other physiological needs.

In early farming cultures around the world, fertility rates accelerated as mothers and babies benefited from the extra calories that domestication provided. In the centuries that followed the adoption of agriculture, family sizes grew by an additional two children per mother. We can see these effects in hunter-gatherer and mixed foraging-farming populations today. A typical Hadza woman will have six children over the course of her lifetime, while a Tsimane woman, with the caloric benefits of some traditional farming, will have nine.

As populations grew, early farmers encountered strange new problems that their hunter-gatherer forebears never had to deal with, like overcrowding and the difficulties of public sanitation. Communicable diseases that would have fizzled out quickly in sparse hunter-gatherer encampments became full-blown plagues, tearing through early farming towns and cities.

Larger populations meant more people living, working, and thinking together. Putting more heads together has a synergistic effect on the development of new ideas, a phenomenon that Joe Henrich, a human evolutionary biologist at Harvard, calls the collective brain. Greater capacity to produce food also meant that people could diversify. Some were free to spend their adult lives on tasks other than food production, a luxury that no hunter-gatherer would recognize. Over three thousand years ago, cultures in the Mediterranean, South Pacific, and elsewhere had figured out how to harness the power of the wind to sail. Watermills appeared over two thousand years ago, as people learned to harness the energy of a flowing river to grind grain, lift water into irrigation systems, and perform a wide range of other work. Windmills joined them a few centuries later. Each invention and refinement expanded our external engines and the energy we could command.

Fossil fuels represent the collective metabolisms of uncountable plants and animals in the distant past, toiling away over millions of years. When we burn them, we’re releasing the energy stored in those ancient organisms.

Oil and natural gas production followed, moving from marginal sources of fuel to mainstays of global energy use following the development of commercial drilling in the mid-1800s. Today, these fossil fuels combine to provide over 35,000 kcal of energy every day for every person on Earth, 80 percent of our species’ external energy expenditure.

Our modern food system requires an immense amount of energy. Food production in the United States consumes roughly 500 trillion kilocalories each year. A third is burned as gasoline or diesel in farm machinery and transportation. Another third is the fossil fuel used to make fertilizers and pesticides. Electricity to run farms, storehouses, and supermarkets takes up most of the rest. Those trillions of kilocalories channeled into food production have profound effects on both the energy cost and energy content of our diet.

Hadza adults acquire roughly 1,000 to 1,500 kcal per hour of foraging.

The Tsimane and other foraging-farming societies, the rate of energy production is around 1,500 to 2,000 kcal per hour.

In the United States in 1900, with industrialization already in stride, an hour of physical labor in a manufacturing job could buy you more than 3,000 kcal of flour, eggs, bacon, and other staples. As the flow of fossil fuel energy increased, so did our purchasing power. Today, an hour’s wage could buy an American laborer roughly 20,000 kcal of those same staples.

• Processing has flipped the usual relationship between cost and energy content on its head. The result is a highly processed, supercharged diet. The energy density of an industrialized diet is 20 percent higher than the Hadza diet, and it requires little or no physical effort to acquire. Our hunter-gatherer ancestors would be stunned.
• Another result of increased energy density is the ability to recover quicker from pregnancy, and yet, birth rates have gone down. Some attribute this to increased life expectancy and others to access to education and family planning services.

Unintended Consequences

It is a basic rule of life that no species can persist if it spends more energy foraging than it acquires in food. Mammals in the wild typically get around 40 calories of food for every calorie they spend foraging. Humans in hunter-gatherer societies like the Hadza or mixed foraging-farming societies like the Tsimane fare a bit worse, with each calorie of work spent on food production yielding around 10 calories of food. Our modern food production system violates the fundamental laws of ecology. When we include the fossil fuel energy consumed in food production, we burn 8 calories for every calorie of food we produce.

The energy consumed in food production is just one part of our energy economy. Each year in the United States, we consume a staggering 25 quadrillion kilocalories. With a population of approximately 330 million, annual U.S. energy expenditure works out to 77 million kilocalories per person. That’s 210,000 kcal per day, equivalent to the daily energy expenditure of a nine-ton mammal. Each American consumes more energy than do seventy hunter-gatherers.

Globally, our species consumes 141 quadrillion kilocalories each year, an average of 47,000 kcal per person per day, nearly sixteen times the energy expenditure of our internal metabolic engine. There are 7.7 billion people on the planet, but we’re burning energy like there’s 120 billion of us.

Human-caused climate change is well under way, with the Earth 0.8°C (1.4°F) warmer than it was in the late 1800s, when fossil fuel use began to take off. The current generation of climate models, which have done an incredibly accurate job predicting our increasingly warm and wild weather, predict an additional 8°C warming globally in the next century or two if we burn the remainder of known fossil fuel reserves.

So far, our species has figured out four ways to generate power at meaningful scale that don’t emit greenhouse gases: hydropower, wind turbines, solar power, and nuclear fission. Hydropower is essentially maxed out. We’ve run out of big rivers to dam, and it causes massive ecological damage in any case. That leaves solar and wind, which currently combine to provide 2 percent of global energy, and nuclear, which generates 5 percent.

Building a Better Zoo

We have to make the cost of food more reflective of its impact on our health. One approach is to increase the cost of unhealthy foods. Taxes on soda and other sugar-sweetened beverages are often unpopular, but they seem to work. Extending those taxes to other highly processed foods might lower their intake as well, and in any case would provide a source of revenue for governments grappling with the ever-expanding health costs of our ever-expanding waistlines. We also need to make healthy unprocessed foods cheaper and easier to find.

We need walkable cities and towns, and real investment in human-powered movement. Cities like Copenhagen are leading the way on this, designing bicycle friendly urban areas that favor people over cars. Bike share systems have enormous potential to help, too, increasing daily physical activity and reducing disease.

With farming, and then again with industrialization, came major revisions to the social contract. Class differences and hierarchies emerged as wealth was consolidated in land and then in capital. This worked well for the upper class, of course, but was a disaster for those stuck at the bottom, who were used as slaves or otherwise exploited for their labor. The rest were caught somewhere in the middle, eager to climb the socioeconomic ladder but also desperate not to be caught in the gears below.

The stresses that result from this socioeconomic arrangement, from fears about money to the aching feeling that we’re being left behind to the daily assaults on our dignity, are new for our species. We don’t seem to handle them well. Living at the unhappy end of the socioeconomic spectrum makes us sick and shortens our lives. People living in poverty suffer from higher rates of obesity, diabetes, heart disease, and other cardiometabolic illness than the wealthy, and the effect is larger than anything we’d expect from diet and exercise differences alone. Likewise, people of color and other marginalized communities have worse health and shorter lives. If we’re serious about changing our environments to improve our metabolic health, then we need to address socioeconomic disparities, not just diet and exercise.

Daily physical activity is lower, for one thing. We are also less socially connected. Families are smaller and more dispersed. Loneliness has become so prevalent that it’s now a recognized medical condition. Modernization also brought us indoors. Time outside can relieve stress and promote physical activity, and it seems to improve cardiometabolic health more than physical activity alone.