Energy Systems

The Metabolic Engine

Every contraction of every muscle, every thought your brain produces, every immune cell that patrols your bloodstream, every breath you take while reading this, all of it runs on energy. Specifically, it runs on adenosine triphosphate (ATP), the molecule your cells use as their universal energy currency. Your body cycles through approximately your bodyweight in ATP each day, not by storing it (the human body holds roughly 250 grams of ATP at any moment) but by continuously regenerating it from food, oxygen, and stored fuel.

How you produce that ATP, what fuels you burn to produce it, how much energy you have available for any given task, and how your body distributes that energy across competing demands (movement, immune function, reproduction, digestion, thinking) is the central question of energy metabolism. It’s also, as Herman Pontzer’s research at Duke has demonstrated over the past two decades, the central question of human exercise biology. Your daily energy budget isn’t unlimited. Where you spend it shapes everything from how hard you can train to how often you get sick to how well you recover.

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. Regulated by hormones, growth factors, vitamins, minerals, and the autonomic nervous system.

Examples: the breakdown of carbohydrates, proteins, and fats into energy (the citric acid cycle); the removal of superfluous ammonia through urine (urea cycle); the breakdown and transfer of various chemicals across tissues. The first metabolic pathway ever fully described was glycolysis, where glucose is broken down into pyruvate, supplying small amounts of ATP and NADH to cells. From there, depending on whether oxygen is available, the energy production process branches into the two main systems we’ll cover next.

The Aerobic Energy System

Otherwise known as cellular respiration. The processes involved are glycolysis, pyruvate oxidation, the citric acid cycle (also called the Krebs cycle), and the electron transport chain. Glucose and oxygen are combined inside mitochondria to create ATP. The main byproducts are carbon dioxide and water, which is why you exhale CO2 and produce metabolic water you’ll lose through breath, sweat, and urine.

The full process broken down:

• Aerobic glycolysis is the first phase, occurring in the cytoplasm of the cell. A glucose molecule is broken down into pyruvate, producing 2 ATP and 2 NADH along the way. Glycolysis can also take place under anaerobic conditions, but the result there is lactate rather than the further processing that aerobic conditions allow.
• The citric acid cycle (Krebs cycle) takes place inside the mitochondria. The primary metabolic compound entering the cycle is acetyl coenzyme A (acetyl-CoA), produced from carbohydrates (via pyruvate), fatty acids (via beta-oxidation), and, to a lesser extent, amino acids. Hydrogen ions and electrons are transferred to the inner mitochondrial membrane for oxidative phosphorylation and the electron transport chain. The reaction releases NADH and small amounts of ATP and CO2. The cycle involves 10 enzymatic steps, each affected by B vitamins and certain minerals such as magnesium and iron, plus the liver’s main antioxidant glutathione. The reactions are inhibited by heavy metals such as mercury, arsenic, and aluminium, which is part of why heavy metal toxicity produces fatigue as a primary symptom.
• Oxidative phosphorylation consists of two parts: the electron transport chain and ATP synthase. This is where most of the energy in aerobic conditions gets captured as ATP. In the electron transport chain, hydrogen ions (H+) are pumped into the mitochondrial intermembrane space, creating an electrochemical gradient. ATP synthase then uses the energy from H+ flowing back across the membrane to convert ADP into ATP. Ubiquinone (Coenzyme Q10) acts as an electron carrier in this chain; statin medications, which block the same enzymatic pathway that produces both cholesterol and CoQ10, have been found to be a contributing factor to CoQ10 deficiency, which is part of the mechanism behind statin-induced fatigue and muscle pain.
• Oxaloacetate acts as a compound used to fulfil a sudden need to produce energy (for example, in the brain or muscles during cognitive or physical demand). Taking an oxaloacetate supplement may help boost regeneration of mitochondria in the brain, reduce silent inflammation, and increase nerve cell numbers, though the evidence base for supplementation is modest compared with the substantial mechanistic literature.
• Fatty acid oxidation (beta-oxidation) is the parallel pathway for energy production from fats. Fatty acids broken down in the digestive system are transported through the bloodstream and into the mitochondria of cells. Inside the mitochondrion, fatty acids are activated by binding to coenzyme A, producing acetyl-CoA, which then enters the citric acid cycle. The oxidation of long-chain fatty acids requires carnitine acyl transferases to transport the fatty acids from the cytoplasm into the mitochondrion; short and medium-chain fatty acids move there by diffusion without requiring carnitine.

The bottom line on the aerobic system: it’s slower to ramp up but produces dramatically more ATP per unit of fuel than the anaerobic alternatives, and it can run on multiple fuel sources (glucose, fatty acids, ketones, occasionally amino acids). It’s the system you’re using right now, sitting still, breathing normally. It’s also the system that long-duration endurance work primarily trains.

The Anaerobic Energy System

Typically used during high-intensity activities where ATP demand outpaces what aerobic processes can supply. ATP is produced by breaking down glycogen stored in muscles and the liver, and by utilising the free ATP molecules and creatine phosphate immediately available in the muscle cells.

• Anaerobic glycolysis: glucose is broken down into pyruvate without the involvement of oxygen, producing a small amount of ATP very quickly. The pyruvate is then converted into lactate (lactic acid) rather than entering the mitochondria, allowing glycolysis to continue producing ATP without waiting for oxidative metabolism to catch up.
• The creatine phosphate system is one of the main energy systems for short, explosive muscular work. Approximately 95% of the body’s creatine is located in the skeletal muscles. Creatine phosphate (phosphocreatine) is synthesised in the liver from creatine and the phosphate group of ATP. Red meat is the main dietary source of creatine, but it can also be synthesised endogenously from the amino acids arginine and glycine.
  • Creatine phosphate significantly increases force generation in skeletal muscles. It’s formed and recycled in the creatine phosphate shuttle, which transports high-energy phosphate groups from mitochondria to myofibrils, forming phosphocreatine through the enzyme creatine kinase. This phosphocreatine is then used for fast muscle contraction. Unused creatine is transported via the same shuttle back into mitochondria, where it’s resynthesised into creatine phosphate. Used phosphocreatine forms creatinine, which exits the body in urine via the kidneys. Blood creatinine levels are routinely measured to determine kidney filtering capacity; higher muscle mass produces higher creatinine secretion, which is why athletes can be flagged as having unusual kidney function on standard tests when the actual finding is just elevated muscle turnover.
  • Creatine supplementation (typically 3-5g of creatine monohydrate daily) has one of the best evidence bases of any sports supplement, with documented benefits for power output, anaerobic capacity, lean mass, recovery between high-intensity efforts, and cognitive function. We cover this in more detail in Training Specificity and Testing.

The Body’s Main Energy Storage Systems

Glycogen is a large branched molecule formed of several glucose molecules linked together. It’s stored in the liver (about 10% of liver weight in a well-fed state), muscle cells (about 2% of muscle weight), and to a lesser extent in red blood cells. Glycogen binds three times its weight in water, which is why a person’s body weight may fluctuate by several kilograms within 24 hours purely from glycogen-bound water shifts. This is the same reason why low-carbohydrate diets produce rapid initial “weight loss” that’s largely water, and why glycogen-depleted athletes can sometimes drop weight rapidly before a competition.

• The glycogen in the liver acts as an energy reserve for the entire body’s energy production needs, particularly those of the central nervous system. The brain runs primarily on glucose (with ketones as a secondary option once metabolically adapted), and liver glycogen is the buffer that maintains blood glucose between meals.
• The amount of glycogen present is determined by physical exercise, the basal metabolic rate, and eating habits. Glycogen stores are useful for the regulation of blood sugar between meals and during intensive exercise. Glucose can be used for energy under either aerobic or anaerobic conditions, whereas fatty acids are broken down 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 muscle under aerobic or anaerobic conditions, utilised 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 primarily of triglycerides: three fatty acid chains attached to a glycerol backbone. Adipose tissue is located under the skin (subcutaneous), in bone marrow, between muscles, around internal organs (visceral fat), and in breast tissue. The visceral fat is metabolically distinct from subcutaneous fat and is much more strongly associated with metabolic disease.

• Adipose tissue is hormonally active, producing leptin (which signals long-term energy adequacy to the brain), adiponectin (which improves insulin sensitivity), and resistin (which contributes to insulin resistance), along with various inflammatory cytokines when adipose tissue becomes dysfunctional. The recognition over the past two decades that adipose tissue is effectively an endocrine organ has reshaped how the field thinks about obesity, metabolic syndrome, and insulin resistance.
• In lipolysis, adipose tissue is broken down by lipase enzymes (hormone-sensitive lipase and adipose 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 for gluconeogenesis (the production of new glucose from non-carbohydrate sources).
• Insulin inhibits lipolysis. If the body’s circulating insulin levels are consistently elevated (the metabolic state produced by frequent eating, particularly of refined carbohydrates and refined fats), the fatty acids in the blood are preferentially stored in adipose tissue rather than burned for energy. This is called lipogenesis. The implications for body composition, blood sugar regulation, and metabolic flexibility are covered in more detail in Macronutrient & Hydration Basics and The “Natural” Diet.

Mitochondria as the Hub

A short detour worth taking because mitochondria will come up repeatedly across the rest of this section, and a clear mental model of what they are makes the subsequent material easier to follow.

Mitochondria are organelles inside almost every cell of your body. Their job is energy production. Each one is essentially a small bioreactor that takes in fuel (glucose, fatty acids, ketones, amino acids) and oxygen, runs the citric acid cycle and electron transport chain, and produces ATP plus carbon dioxide and water. A liver cell might contain 1,000-2,000 mitochondria. A heart cell, around 5,000. A muscle cell varies widely depending on the muscle’s training state, ranging from a few hundred in unconditioned tissue to several thousand in highly trained endurance muscle.

Mitochondria have their own DNA (separate from the DNA in the cell nucleus), inherited entirely from your mother. This is a remnant of their evolutionary origin: mitochondria were once free-living bacteria that, roughly 1.5-2 billion years ago, were absorbed into a host cell in an endosymbiotic relationship that became permanent. The cell got reliable energy production; the bacterium got a protected environment. Every cell in every plant and animal on Earth is the descendant of this original merger.

A few practical implications:

• Mitochondrial density is trainable: Endurance training increases the number of mitochondria in muscle cells (a process called mitochondrial biogenesis), increases the size of those mitochondria, and improves the efficiency of their internal machinery. This is part of why endurance-trained athletes can sustain higher work outputs for longer than untrained people. They have substantially more cellular energy-producing capacity. The signalling pathway involved (PGC-1α and downstream targets) responds to endurance training, high-intensity intervals, and several lifestyle inputs, including caloric restriction, cold exposure, and certain compounds (resveratrol, possibly NAD+ precursors).
• Mitochondrial function declines with age: The mitochondria in your 60-year-old cells produce ATP less efficiently than the mitochondria in your 20-year-old cells, partly because of accumulated damage to mitochondrial DNA (which has weaker repair mechanisms than nuclear DNA), partly because of declining levels of key cofactors like NAD+ and coenzyme Q10, and partly because of reduced mitochondrial biogenesis. Mark Tarnopolsky’s group at McMaster demonstrated in a landmark 2011 PNAS paper that 6 months of endurance training in older adults reversed a substantial portion of the age-related mitochondrial gene expression changes. Roughly 40 years’ worth, in their analysis. This finding has been replicated and extended; mitochondrial dysfunction is one of the few age-related decline patterns that exercise reliably reverses.
• Mitochondrial dysfunction underlies a lot of diseases: Chronic fatigue, fibromyalgia, neurodegeneration (Alzheimer’s, Parkinson’s, ALS), heart failure, metabolic syndrome, and many cancers, all of which directly involve mitochondrial dysfunction as part of their pathophysiology. Whether mitochondrial dysfunction is causal or downstream varies by condition, but the consistency of the involvement has produced substantial research interest in “mitochondrial medicine” approaches.

The implications for training and longevity: Building and maintaining mitochondrial density and function through regular endurance and high-intensity training is one of the most reliable interventions available for both performance and long-term health. This is part of the rationale for The Longevity Program framework.

Fuel Selection by Intensity

The fuel your body burns depends substantially on how hard you’re working.

At rest and during low-intensity activity (walking, easy cycling, gardening), the body burns predominantly fat. Fat oxidation is slow but yields large amounts of ATP per molecule of fuel, and your fat stores are essentially unlimited compared with your glycogen stores (a lean adult carries 50,000-100,000+ calories of fat versus 1,500-2,000 calories of glycogen). At low intensity, you can keep going for a very long time on fat alone.

As intensity climbs, the mix shifts toward glucose. Glucose can be oxidised faster than fat, providing ATP at the rate higher-intensity work demands. The crossover point, where fuel use shifts from predominantly fat to predominantly glucose, varies substantially between individuals but typically sits somewhere around 60-75% of VO2 max in untrained people, and shifts higher (more reliance on fat at higher intensities) in trained endurance athletes.

At the highest intensities, glucose is the dominant fuel, and once aerobic glycolysis can’t keep up, anaerobic glycolysis adds to the mix, producing lactate. The phosphocreatine system covers the very shortest, most explosive efforts (sprinting, lifting, jumping) where neither aerobic nor anaerobic glycolysis can ramp up fast enough.

The training-specific implications:

Zone 2 training (roughly 60-75% of maximum heart rate, conversational pace, predominantly fat-burning) has been substantially reframed in the past decade thanks to the work of researchers and practitioners like Phil Maffetone (the MAF method, “180 minus age” heart rate ceiling), Stephen Seiler (Norwegian endurance sport science, polarized training framework), and Iñigo San Millán (University of Colorado, mitochondrial metabolism in elite cyclists). The core finding from this research community: elite endurance athletes spend roughly 80% of their training volume in zone 1-2 (predominantly fat-burning, easy aerobic) and roughly 20% in zone 4-5 (high-intensity, VO2 max work), with very little in the moderate-intensity middle zone.

• This is the opposite of how most recreational athletes train. The typical recreational pattern is “mostly moderate”, running, cycling, or training at an effort level that feels productive but is too hard to be truly aerobic development and too easy to drive high-intensity adaptation. Greenfield popularised the term “Black Hole Training” for this pattern (covered in detail in The Longevity Program). Moderate-intensity continuous training produces measurable adaptations but is less efficient than the polarised approach, more taxing to recover from, and more associated with chronic injury and overtraining.
• The polarised framework practical recommendation: most of your aerobic work should be easy enough to hold a nasal-only breathing pattern and a conversational pace. A smaller portion (perhaps once or twice a week) should be genuinely hard. High-intensity intervals at or near VO2 max. The middle zone, where most people spend most of their cardio time, is the zone to minimise.

The metabolic flexibility framework: Your ability to switch efficiently between fuel sources (burning fat at rest and low intensity, burning glucose at high intensity, accessing ketones during fasting, returning to fat after a meal) is what the research community calls metabolic flexibility. It’s trainable through a combination of zone 2 work, fasting, low-carbohydrate eating cycles, and high-intensity training. People with poor metabolic flexibility (often correlated with insulin resistance and metabolic syndrome) get “stuck” on glucose even at low intensity, which is part of why they fatigue quickly, get hungry frequently, and struggle to maintain stable energy across the day. The full discussion on metabolic flexibility lives in Fasting in Part II, but the relevant point here is that your training mix substantially shapes which fuels your body becomes good at burning.

Burn Notes: The Pontzer Framework

The fact that our closest evolutionary cousins (chimpanzees, bonobos, gorillas) don’t need to be active to stay healthy tells us something important. Exercise isn’t like water or oxygen, a required element that all animals need. Our need to exercise is peculiar, and Herman Pontzer’s research at Duke explains why.

As our hominin ancestors evolved into hunter-gatherers, the body adapted to the incredible physical demands the lifestyle 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 a high-octane survival strategy. Humans burn more calories per day, controlled for body size, than any other great ape. We have larger babies, longer lives, bigger brains, and faster metabolisms than our nearest relatives. The trade-off is that we need the activity our biology was selected for, and modern life doesn’t provide it.

Exercise Gets Everywhere

Men who can do more than ten push-ups in one go reduce their risk of a heart attack by more than 60 percent compared with men who can’t. Aerobic fitness is associated with better cardiometabolic health and longer, healthier lives. The benefits of staying strong are particularly important as we age. One standard measure of fitness for older adults is the 6-minute walk test, where 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 with those who can’t make 950.

Vigorous exercise (above 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: brisk walking, easy cycling, gardening) is good for you too. It helps with the trafficking of glucose out of the blood and into cells, improves mood and stress regulation, and can even help treat depression (covered in the SMILE trial findings in Exercise 101).

A Different Way of Thinking About Exercise Energetics

The conventional model of exercise energetics is additive: your basal metabolic rate burns X calories per day, and every activity you add (walking, climbing stairs, exercise) adds Y calories on top. Burn more than you eat, lose weight. The calories-in-calories-out math seems straightforward.

The Pontzer data, gathered using doubly labelled water methodology in populations ranging from highly sedentary office workers to hunter-gatherers to elite athletes, shows something different. Total daily energy expenditure is constrained, not additive. When activity increases, the body compensates by reducing energy spent on other tasks. As you increase the amount of energy burned on physical activity, the energy available for other tasks is diminished.

With a fixed energy budget, everything becomes a trade-off. Instead of adding to the calories you burn each day, exercise tends to reduce the energy spent on other activities.

In a long-lived species like ours, the evolved metabolic strategy is different from what the conventional model assumes. Sam Urlacher’s work with Shuar children in Ecuador (a Pontzer collaborator) showed that children fighting an infection increase the energy spent on immune defence while reducing their growth. 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, the same prioritisation happens. Activities that aren’t essential get 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 health implications.

The Trade-offs: What Exercise Suppresses

Inflammation

When your body is under attack from bacteria, viruses, or parasites, the body’s first line of defence is inflammation. Immune cells are sent to the site of infection, signalling 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 harmless particles rather than true threats. Depending on the tissues involved, inflammation can lead to anything 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 diseases. A constrained daily energy budget helps explain why exercise is so effective at reducing inflammation. When a large portion of the daily budget is spent on physical activity, 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 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 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, the 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, exactly as the constrained daily energy model would predict.

Another study of college-age women with moderate depression found that when they exercised regularly, their bodies produced 30 percent less adrenaline and cortisol each day. Their depression improved alongside the hormonal changes.

Reproduction

With more energy spent on physical activity, less is available for reproduction. Anthony Hackney’s research comparing testosterone levels in endurance runners to age-matched sedentary men found about a 10 percent drop in testosterone, on average, among men who had been training for one year, a 15 percent drop for those training for two years, and about a 30 percent drop for those training five years or more. The body takes years to fully adjust to different levels of exercise.

Suppressing the reproductive system might sound bad, 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 (breast and prostate cancer, particularly), in part because it keeps reproductive hormone levels in check. Reproductive hormone levels in the sedentary industrialised 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 United States mean their bodies can put more energy into reproduction and can recover from pregnancy sooner than Hadza mothers can.

Taken to the extreme, exercise can begin to cut into normal reproductive function. At unhealthy workloads, ovulatory cycles can stop completely (a condition called amenorrhea, particularly common in endurance athletes and dancers), libido can evaporate, and sperm count can plummet. The “female athlete triad” (low energy availability, menstrual dysfunction, low bone density) and its broader formulation Relative Energy Deficiency in Sport (RED-S) are increasingly recognised as significant health concerns in both female and male athletes who push energy expenditure too far above intake.

The Dark Side of Extreme Training

At the workloads most of us are likely to experience, the suppressive effects of exercise are good for us. They help keep inflammation, stress response, and reproductive hormones at healthy levels. At extreme workloads, exercise cuts deeper. Tour de France cyclists like Floyd Landis burn over 6,000 kcal each day on cycling, and the race lasts nearly a month. They’re pushing their bodies to the brink. Their bodies shut down other functions, cutting into 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 competitive edge, crashes. Unless, of course, they can artificially elevate it with discreet injections.

A 2014 study by Karolina Lagowska and colleagues provided food supplements to thirty-one women endurance athletes (rowers, swimmers, 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. Body 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 luteinising 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 the logical extension of the same energetic trade-offs that make moderate exercise so good for us.

Of Apes and Athletes

Professional 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.

The encouraging finding from large epidemiological studies is that the dose required for substantial health benefit is much lower than the elite athlete workload. 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 in a US study covering 150,000 adults. A similar Australian study 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 plays a role here, too. 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 chronic inflammation brought on by inactivity may have similar effects in the human brain.

The second caveat in the relationship between activity and weight is that exercise can be useful for maintaining weight loss once achieved. Exercise is a poor tool for achieving weight loss (the constrained energy model explains why), but it appears 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. This is why the “calories in, calories out” framing is technically true but practically incomplete. Your hunger and satiety regulation aren’t free variables you can simply override with willpower indefinitely.

Pushing the Limits

By telling the public the truth about exercise not contributing meaningfully to weight loss, public health officials worry people won’t do it. Longevity isn’t as strong a motivator as vanity. The danger in selling exercise as a way to lose weight is that it doesn’t work, and eventually, people notice the results don’t match the sales pitch. Some keep with it anyway, hooked by the many other benefits of exercise (improved mood, clearer mind, stronger body) and willing to overlook the bait and switch. But there would be more happy customers if those of us interested 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: Why You Quit Before You Run Out of Fuel

A person who feels completely exhausted still has plenty of fuel on board. Even when we feel 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. Our bodies don’t just stop when we run out of fuel. Intensity matters more than absolute fuel availability.

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.

One reason 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 our primary fuel. As exercise intensity picks up, more glucose is added to the mix. Some of this additional glucose is supplied from circulating blood sugar; some is pulled from glycogen stores in the muscle. Compared with fat, glucose is easier and faster to burn (even when 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 and converts them to ATP via the mitochondria has reached its limit. It simply can’t supply energy any faster.

With 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 would normally jump into the mitochondria, transform into acetyl-CoA, and feed the Krebs cycle. There’s a traffic jam getting into the mitochondria, and the excess pyruvate is diverted and converted to lactate.

Your muscles start to burn. 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 races without blowing up before the finish.

The neural component: A finding the field has gradually absorbed – neural control of fatigue helps explain the strange effects of mood and perception on performance. Your willingness to keep going is shaped by signals from your brain (particularly the locus coeruleus and prefrontal cortex) that can be modulated by perception, motivation, and conscious focus. The “central governor” model proposed by Tim Noakes and others holds that fatigue is fundamentally a protective neural mechanism, not a peripheral muscle failure. Your brain shuts down effort before your muscles actually fail, to keep you alive. This is why elite athletes can sometimes find another gear when the situation demands it, and why psychological factors (visualisation, music, competition, stakes) measurably affect endurance performance.

Endurance Over Days, Weeks, and Months: The Digestive Ceiling

One of Pontzer’s most striking findings concerns what limits human endurance over very long durations.

He tracked down credible estimates of daily expenditure during ultra-long-distance events, from the maximum distance run in 24 hours to the 46-day record for the 2,200-mile Appalachian Trail. The longest-lasting high-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 he plotted the daily energy expenditures of human endurance events against their duration, the result was an elegant curve sweeping down from the high expenditures of the shortest events to the lower expenditures of the longest. Those points marked the boundaries of human endurance. Adding every other high-endurance study he could find, from military endeavours to athletes in training, every one 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 metabolic capacity. Expecting mothers were pushing the same metabolic limits as Tour de France cyclists.

Pregnancy is the ultimate ultramarathon.

Endurance Takes Guts

When Pontzer’s group put the daily energy expenditure measurements together with data on weight loss in long-duration endurance events, a pattern emerged. Every athlete and pregnant mother in their dataset was taking in the same amount of energy per day. Across the board, from 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 2.5x 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: regardless of the event or circumstances, the maximum amount of energy the human body can absorb is around 4,000 to 5,000 kcal per day. Beyond that, you’ll be in negative energy balance, burning more fat and glycogen than you can replenish, and slowly melting away.

The bottleneck isn’t your muscles. It isn’t your willpower. It isn’t your training. Your digestive system simply can’t 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 have historically circumvented this limitation by injecting intravenous lipids and glucose at night between race stages, allowing absorption to bypass the digestive ceiling. The practice has been associated with several historic doping scandals; the underlying physiological reason it works is the digestive ceiling itself.

Athletes Everywhere

For mothers, the limits on energy absorption may prevent pregnancies from lasting too long. Throughout gestation, for the fetus to grow, the mother has to take in more energy than she burns. 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, mum won’t be able to bring in enough calories to sustain them both. The signals of metabolic stress released as mothers approach their metabolic limits may help trigger the birth process.

If the neonate is even slightly too large, serious and often life-threatening complications arise. How do babies get too big? They get too much energy from mum, either by annexing a greater share of the nutrients in her 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 is available for the fetus to commandeer. Their babies are unlikely to get too big before mum’s metabolic limits trigger birth. In the United States and other sedentary industrialised populations, mothers are awash in easy calories, and the fetus doesn’t need to compete with the energy demands of physical activity. This may contribute to babies being born slightly later and slightly larger, not by much, but enough to cause trouble. The rates of delivery by cesarean section have skyrocketed over the past half-century, alongside the same 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. Around the globe, when researchers look at daily energy expenditures during normal life for the hundreds of populations who have been measured, from Holland to the Hadza, everyone lives their lives well under the 2.5x BMR ceiling. In physically active populations like the Hadza, the body adjusts to keep daily expenditures at a sustainable level by suppressing other functions, exactly as the constrained model predicts.

What’s So Special About Michael Phelps?

The story goes that Phelps ate 12,000 kcal per day during peak training. Pontzer’s analysis suggests this is exaggerated; the real figure is probably closer to 7,000 kcal. Phelps is 6’4 and 194 pounds. His BMR should be about 1,900 kcal, give or take 200. Adding his above-average lean mass raises his BMR to about 2,100 kcal.

If Phelps ate 7,000 kcal per day, he’d absorb about 6,650 kcal for his body to burn (the remainder lost in the bathroom). At 6,650 kcal per day, Phelps would be absorbing a little over three times his BMR. That places him in the very upper range of energy absorption in the human endurance dataset, pushing the digestive ceiling rather than breaking it.

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

Evolved to Break the Rules

Our metabolic machinery provides a clean demonstration of the body’s physiological interconnections. The same machinery that limits our athletic endurance also shapes gestation and pregnancy and constrains daily energy expenditure. All of these aspects of our metabolism are enhanced compared with 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 to support the high-octane survival strategy that defines our species.

The Practical Synthesis

The energy systems content above can feel dense. The practical implications for training and daily life come down to a few principles:

• Build the aerobic base: The majority of your training time should be low-intensity, predominantly fat-burning, conversational-pace work. This builds mitochondrial density, capillary networks, fat oxidation capacity, and the metabolic flexibility that makes everything else possible.
• Add high-intensity work sparingly: One or two sessions per week of genuine high-intensity intervals (above lactate threshold, often at or near VO2 max) produce adaptations that the easy work doesn’t. Don’t mistake this for daily moderate-intensity grinding; that’s the Black Hole.
• Train mitochondrial function deliberately: Endurance work, high-intensity intervals, occasional fasting, cold exposure, and adequate sleep all contribute to mitochondrial biogenesis and function. The compound effect of training the cellular energy machinery is one of the most reliable health interventions available.
• Respect the constrained energy budget: Adding more training doesn’t add unlimited capacity. At some point, more training cuts into other essential functions (immune, reproductive, cognitive, and repair). The relationship between training volume and overall health is U-shaped, not linear. The optimal volume for longevity is much lower than the volume that elite athletes train at.
• Eat to fuel the work, not to chase weight loss through exercise: Exercise is a poor weight-loss tool but a powerful health and longevity tool. Eat well, train consistently, and get the substantial benefits without expecting one to substitute for the other.