Energy Systems

The Metabolism

The continuous process of breaking down organic matter for energy, the forming of new substances within the tissues of the body, and the removal of metabolic waste. It is regulated by hormones, growth factors, vitamins, minerals, and the autonomic nervous system (ANS).

Examples of the metabolism in action include the breakdown of carbohydrates, proteins, and fats into energy (the citric acid cycle), the removal of superfluous ammonia through urine (urea cycle), and the breakdown and transfer of various chemicals. The first pathway discovered was glycolysis, where glucose is broken down into pyruvate supplying ATP and NADH to cells.

The Aerobic Energy System

Otherwise known as cellular respiration. The processes involved are glycolysis, pyruvate oxidation, the citric acid cycle, and the electron transport chain. Using glucose and oxygen to create ATP as an energy source in a mitochondrion. The main byproducts are CO2 and water.

• Aerobic glycolysis is the first phase, which occurs under aerobic conditions. A glucose molecule is broken down into pyruvate, simultaneously producing 2 ATP molecules and 2 NADH molecules. Glycolysis also takes place under anaerobic conditions, but the end result is lactate or lactic acid.
• Citric acid cycle, or Krebs cycle takes place in the mitochondria. The primary metabolic compound of the citric acid cycle is acetic acid (acetyl coenzyme A) produced from fatty acids, carbohydrates and proteins. Hydrogen ions and electrons are transferred to the inner mitochondrial membrane for oxidative phosphorylation (binding energy to ATP molecules through oxidation) and the electron transport chain. The reaction releases NADH and small amounts of ATP and CO2. The citric acid cycle involves 10 steps, each affected by B vitamins and certain minerals such as magnesium, iron, and the liver’s main antioxidant glutathione. The reactions are inhibited by heavy metals such as mercury, arsenic, and aluminum.
• Most of the energy captured is by NADH molecules. For each acetyl coenzyme A molecule, 2 NADH molecules are generated and then used for energy in the reaction that follows (oxidative phosphorylation). The regulation of the citric acid cycle is determined by the availability of various amino acids as well as feedback inhibition (if too much NADH is produced, several enzymes of the citric acid cycle are inhibited, slowing down reactions).
• Oxaloacetate acts as a compound used to fulfill a sudden need to produce energy (e.g., in the brain or muscles). Taking an oxaloacetate supplement may help to boost regeneration of mitochondria in the brain, reduce silent inflammation in the body, and increase nerve cell numbers.
• Oxidative phosphorylation consists of two parts: the electron transport chain and ATP synthase. Oxidative phosphorylation produces the most energy generated in aerobic conditions (ATP). It is a continuation of the citric acid cycle. In the electron transport chain, H+ ions are released into the mitochondrial intermembrane space. Through ATP synthase, the H+ released from the intermembrane space move back into the mitochondrion. Using energy in the process, ATP synthase converts the ADP used for energy into ATP again. Ubiquinone (COQ10) acts as a contributor to the electron transport chain. Statins have been found to be a contributing factor to COQ10 deficiency.
• Fatty acids broken down in the digestive system are used for energy in the mitochondria. During beta-oxidation, the fatty acids are activated by being bound to coenzyme A. The result is acetyl coenzyme A, which is used for energy production in the citric acid cycle. The oxidation of long-chain fatty acids requires carnitine acyl transferases in which the fatty acids are transported from the cytoplasm into the mitochondrion. Such transfer of short and medium-chain fatty acids isn’t necessary as they move there by diffusion.

Anaerobic Energy System

Typically used during high-intensity sports activities. ATP is produced by breaking down glycogen stored in muscles and the liver by utilizing the free ATP molecules immediately available in the muscle cells.

During anaerobic glycolysis, glucose is broken down into pyruvate which is then converted into lactic acid (lactate) during the lactic acid fermentation process.

• The creatine phosphate system is one of the main energy systems for the muscles. 95% of the body’s creatine is located in the skeletal muscles. Creatine phosphate (phosphocreatine) is synthesized in the liver from creatine and phosphate from ATP. Red meat is a source of creatine, but it can also be synthesized from amino acids (arginine and glycine). It significantly increases force generation in skeletal muscles. Creatine is formed and recycled in the creatine phosphate shuttle. The shuttle transports high-energy ATP molecule phosphate groups from mitochondria to myofibrils, forming phosphocreatine through creatine kinase. Used for fast muscle contraction. Unused creatine is transported via the same shuttle into mitochondria where it is synthesized into creatine phosphate. Used phosphocreatine forms creatinine which exits the body in urine via the kidneys. The blood creatinine levels are measured to determine the kidney’s filtering capability. The higher the muscle mass, the higher the volume of creatinine secreted.

The body’s main energy storage systems

Glycogen is a large sized molecule formed of several glucose molecules. It is stored in the liver (10% of the weight), muscle cells (2%), and, to a lesser extent, red blood cells (RBC). In addition to glucose, glycogen binds triple the amount of water. Because of this, a person’s body weight may fluctuate by several kgs with a 24-hour period. The glycogen in the liver acts as an energy reserve for the entire body’s energy production needs, and those of the central nervous system in particular. The amount of glycogen present is determined by physical exercise, the basal metabolic rate and eating habits.

Glycogen stores are useful for regulation of blood sugar between meals and during intensive exercise. Glucose may also be used for energy under anaerobic conditions. Conversely, fatty acids are broken down into energy only in aerobic conditions. A metabolically active glycogen breakdown product is glucose 6-phosphate in which the glucose molecule binds with one phosphate group. It may be used for energy in a muscle under either aerobic or anaerobic conditions, utilized via the liver as glucose elsewhere in the body or converted into ribose and NADPH for use in various tissues.

Adipose tissue is the body’s main long-term energy storage system. It consists of connective tissue cells and vascular endothelial cells. Fat cells contain a lipid droplet consisting of triglycerides and glycerol. Adipose tissue is located under the skin, in bone marrow, between muscles, around internal organs (visceral fat) and breast tissue. Adipose tissue is also hormonally active, as it produces leptin, adiponectin, and resistin that regulate the energy metabolism and body weight. In lipolysis, adipose tissue is oxidized by lipase and triglyceride lipase into free fatty acids and glycerol. Fatty acids are used for energy in the muscles, liver, and heart; glycerol is mainly used in the liver.

Insulin inhibits lipolysis. If the body’s stored insulin levels are consistently elevated, the fatty acids circulating in the blood are stored in the adipose tissue. This is called lipogenesis.

Burn Notes

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.

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.