Narrator: Imagine a hunter-gatherer in the Tanzanian savanna. He walks for miles every day, hunts with a bow and arrow, and forages for wild plants and honey. Now, picture an office worker in New York City who sits at a desk for eight hours, commutes by subway, and orders takeout for dinner. Who burns more calories? The answer seems obvious, but it is surprisingly, fundamentally wrong. This paradox is at the heart of our misunderstanding of the human body’s most essential function: metabolism.

In his groundbreaking book, Burn: The Misunderstood Science of Metabolism, anthropologist Herman Pontzer dismantles the myths we have been told about diet, exercise, and energy. He takes us on a journey from the plains of Africa to labs studying apes in zoos, revealing that the story of our metabolism is not one of simple mechanics, but of deep evolutionary adaptation.

Narrator: The common view of metabolism is that of a simple engine: food is fuel, exercise burns that fuel, and any excess is stored as fat. But Pontzer argues this model is deeply flawed because our bodies are not products of engineering; they are products of millions of years of evolution. To illustrate this, he points to his surprising research on orangutans.

For a long time, scientists believed it was nearly impossible to measure the daily energy expenditure of a great ape. The consensus was that primates had unremarkable metabolisms, similar to other mammals. But when Pontzer and his team finally managed to measure the daily calorie burn of orangutans in a sanctuary, the results were stunning. A 250-pound male orangutan named Azy burned only 2,050 calories a day—the same as a 65-pound human child. Their metabolic rates were in the lowest one percent of all placental mammals, on par with sloths. This slow metabolism was not a flaw; it was a brilliant evolutionary adaptation. In their native rainforests, orangutans face unpredictable food shortages. Their slow-burning engines allow them to conserve fuel, survive lean times, and support their slow-paced life history of late maturity and long birth spacing. This discovery proved that metabolism is not a fixed, universal engine but a malleable trait that evolution tunes to fit a species' ecological strategy.

Narrator: While the primate family tree is generally characterized by a slow metabolism, the human lineage took a sharp evolutionary turn. Pontzer’s research revealed that humans evolved a supercharged metabolic rate compared to our closest relatives. When comparing humans to chimpanzees, bonobos, gorillas, and orangutans in zoos—all living similar sedentary lifestyles—the difference was clear. Humans burn about 20 percent more calories than chimps and bonobos, 40 percent more than gorillas, and a staggering 60 percent more than orangutans.

This "Metabolic Revolution" was a pivotal moment in our history. Around 2.5 million years ago, our ancestors began hunting, gathering, and, most importantly, sharing food. This new, cooperative strategy provided a more reliable and energy-rich diet, which in turn fueled profound physiological changes. The extra energy supported larger, more expensive brains, longer childhoods for learning, and higher reproductive rates. It also explains another uniquely human trait: our high body fat percentage. Humans carry twice as much body fat as other apes, an evolved adaptation to buffer our high-speed metabolism against the risk of starvation. This faster engine, and the fat to fuel it, is what made us human.

Narrator: The most revolutionary finding in Burn comes from Pontzer's work with the Hadza, a modern hunter-gatherer population in Tanzania. The Hadza live a life of incredible physical activity, walking miles each day to hunt and forage. The logical assumption was that they would burn far more calories than sedentary people in industrialized nations. But when Pontzer and his team used the gold-standard doubly labeled water method to measure their daily energy expenditure, the results were shocking. The Hadza burned the same number of calories each day, after adjusting for body size, as adults in the United States and Europe.

This discovery led to the theory of "constrained daily energy expenditure." The body, it seems, is not a simple adding machine where calories burned from exercise are tacked on top of our basal metabolic rate. Instead, the body adapts to higher physical activity by reducing the energy it spends on other, less visible tasks. When we exercise more, our bodies compensate by dialing down energy for processes like inflammation, stress reactivity, and cellular repair. Our total daily energy expenditure is maintained within a surprisingly narrow range, regardless of lifestyle. This finding has been replicated in other active, non-industrialized populations and even in lab animals, suggesting it is a deeply rooted evolutionary strategy.

Narrator: The constrained energy model has profound implications for how we view exercise. If becoming more active does not significantly increase the total calories we burn, it means exercise is a poor tool for weight loss. Studies confirm this. The Midwest Exercise Trials, for example, put overweight adults on strenuous, sixteen-month exercise programs. The men lost only ten pounds, and the women lost nothing. The body simply compensates for the extra activity.

However, this does not mean exercise is pointless. In fact, Pontzer argues it is more important than we ever realized. The metabolic trade-offs the body makes are the key to its benefits. By forcing the body to divert energy to our muscles, exercise starves other processes. It reduces chronic inflammation, a known driver of cardiovascular disease and other illnesses. It dials down our stress response, lowering levels of hormones like cortisol and adrenaline. It even reduces the production of reproductive hormones, which is linked to a lower risk of cancers like breast and prostate cancer. Exercise does not change the number of calories we burn, but it fundamentally changes how we spend them, making our bodies run more efficiently and healthily.

Narrator: If energy expenditure is constrained, then the modern obesity epidemic must be a problem of energy intake. Pontzer explains that our brains, specifically the hypothalamus, evolved to manage our energy budget with incredible precision. For millennia, this system worked perfectly. Hunter-gatherers like the Hadza maintain stable body weights throughout their lives.

The problem is that our Paleolithic brains are now living in a space-age food environment. Modern processed foods have been engineered to be overeaten. They are stripped of fiber and protein, which promote satiety, and loaded with refined sugar, fat, and salt to maximize their reward value. Furthermore, the sheer variety of foods available today sabotages our brain’s ability to feel full. This phenomenon, known as sensory-specific satiety, means that even if we are full from a savory meal, the reward circuits for sweet foods are still primed and ready. This modern diet overwhelms our hypothalamus, causing a small but persistent overconsumption of calories—just 5 to 10 extra calories a day is enough to cause steady weight gain over a lifetime.

Narrator: While daily energy expenditure is constrained under normal circumstances, humans can push their metabolic engines to incredible extremes. Athletes in events like the Tour de France and the Race Across the USA can sustain energy expenditures far above the normal range. Pontzer’s research on these endurance athletes revealed another fundamental limit: the digestive system.

The body can only absorb about 2.5 times its basal metabolic rate in calories each day. For a typical adult, this is about 4,000 to 5,000 calories. Any energy burned beyond that must come from the body’s own fat and muscle stores. This digestive ceiling is the ultimate constraint on long-term endurance. Intriguingly, Pontzer discovered that another human experience pushes this exact same metabolic limit: pregnancy. Over nine months, a pregnant woman’s energy expenditure ramps up until, in the final trimester, she is operating at the same metabolic ceiling as an elite ultramarathoner. This suggests that the metabolic signals of approaching this limit may be what helps trigger the birth process.

Narrator: The single most important takeaway from Burn is that our metabolism is an evolved, adaptive system that actively manages a constrained energy budget. It is not a simple calculator, and we cannot "boost" it or outrun a bad diet. Our bodies are designed to work the same way, whether we are hunting on the savanna or working in a cubicle.

The challenge we face is that our ancient metabolic machinery is now operating in a world it was not designed for. The solution is not to fight our biology but to change our environment. We must build a better "human zoo" by re-engineering our food landscape to prioritize whole, satiating foods and redesigning our communities to make physical activity an effortless part of daily life. The question is not whether we can change our metabolism, but whether we have the wisdom to change our world to better suit the remarkable, evolved bodies we inhabit.