Burn: The Misunderstood Science of Metabolism

by Herman Pontzer

The Hadza know better. They understand intuitively that food and the energy it holds are the fundamental stuff of life. Each day they confront an ancient and unforgiving arithmetic: acquire more energy than you burn or go hungry.

where does the fat go when we “burn it off” with exercise? Think it turns into heat? sweat? muscle? Wrong, wrong, wrong. You breathe most of it out as carbon dioxide, and turn a small fraction of it into water (but not necessarily sweat).

Our bodies don’t work like simple fuel-burning machines because they aren’t products of engineering, they’re products of evolution.

metabolism is the unseen foundation underlying everything, slowly shifting and shaping our lives.

As Charles Darwin himself observed, drawing on the writings of Thomas Malthus, there is always a struggle for resources among the denizens of the natural world. There’s never enough to go around. Consequently, all species evolve under conditions of scarcity. You can’t eat your cake and have it, too.

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

our drive for food is ferocious, as we can see with the Hadza. If our evolved appetites can push us into a pride of hungry lions for breakfast, how can we keep ourselves out of the fridge?

The sooner we move beyond the simplistic armchair engineer’s view of metabolism and embrace a Darwinian perspective, the better chance we’ll have.

Since work and energy are two sides of the same coin, we can think about all the work that our cells do and all the energy they consume as two ways of measuring the same thing. We can use “metabolism” and “energy expenditure” interchangeably.

All the work our bodies do is powered by microscopic alien life-forms called 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.

We are all walking chimeras, part human and part other, performing the ordinary miracle of turning dead food into living people every day without a moment’s thought.

Humans, like other animals, have evolved hard limits on the amount of glycogen our bodies can hold. Once those buckets are full, blood sugar has to go somewhere else. And the only place left to go is fat.

We tend to think of oxygen as a good thing, the sustainer of life, but its true chemical nature is devastating. It steals electrons and binds to other molecules, altering them completely and often tearing them apart. Oxygen is Shiva the destroyer, obliterating everything it touches either slowly (rust) or violently (fire).

Evolution is a heartless accountant: the only thing that matters, at the end of a life, is how many surviving offspring an organism has produced. Organisms that spend their calories unwisely, in the eyes of natural selection, will reproduce less.

regardless of how fast you run, you’ll burn the same number of calories per mile.

to consider growth and reproduction solely as costs misses a fundamental point: those calories aren’t simply spent, they’re invested. As far as evolution is concerned, life is a game of turning energy into offspring.

The bottom line is that your daily activity level has almost no bearing on the number of calories you burn each day.

People and populations vary in the energy they burn every day, but the simple arithmetic of the factorial method doesn’t capture what our bodies are doing.

To be Hadza is to give. Everyone shares with everyone, all the time. That’s the rule.

Sharing is the glue that binds hunter-gatherer communities together and provides the fuel that makes them run. It radically changed the hominin metabolic strategy.

Sharing, smarts, and stamina, the key ingredients of hominin cooperative foraging, were a potent combination.

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.

Our metabolic engines shift and change to make room for increased activity costs, ultimately keeping daily energy expenditure within a narrow window. As a result, physically active people—whether it’s hunter-gatherers living today or in our collective past, or people in the industrialized world who exercise regularly—burn the same amount of energy as people who are much more sedentary.

Great insight and very much contrarian

durable, meaningful changes in daily energy expenditure are extremely difficult to achieve through exercise.

Exercise will keep you healthy and alive. It just won’t do much for your weight.

There are 37 trillion employees, the cells, hard at work each day doing their part. Calories are the currency for all transactions. Energy comes in with the food we eat and is allocated to each of the support systems and their employees as needed. If there’s any extra, it’s deposited into a checking account for quick access (glycogen), or into savings (fat).

Our metabolism doesn’t dictate energy balance, it responds to energy balance.

every gram of tissue in your body, fat and lean, is made from food you ate, and nothing else. Every calorie of fat you carry is a calorie you ate and didn’t burn

Constrained energy expenditure and metabolic compensation make exercise a poor tool for weight loss (Figure 5.3), but nearly every other aspect of our health relies on regular activity.

These wild tubers are the more fibrous cousins of the potatoes, yams, and other domesticated root vegetables in your supermarket. They are the caloric cornerstone of the Hadza diet: energy rich, plentiful, and available year-round. Between the tubers and the honey (and aside from a few larvae), it was an all-carb day. Hadn’t they read Gary Taubes?

Humans eat whatever’s available, which is almost always a mix of plants and animals (and honey).

Just because we didn’t eat a certain way in the past doesn’t necessarily mean we shouldn’t. We didn’t evolve with indoor plumbing, modern medicine, vaccines, or literature, but they undoubtedly make our lives better.

If you eat more calories than you burn, you gain weight. Eat less than you burn, and you lose weight.

Simple and key takeaway of this book

As a general rule, we ought to seek out foods higher in fiber and protein that fill us up, and avoid processed foods with added sugars and fats that push our food reward systems over the edge.

The diet that works for you is the one that allows you to achieve and maintain a healthy weight without feeling like you’re starving.

while exercise doesn’t do much to change the number of calories we burn each day, it does change the way those calories are spent, and that can mean the difference between health and disease.

our bodies are built to move. In our modern, industrialized world, free of the daily demands of foraging for our food, we need to exercise for our bodies to function properly. It’s a legacy of our hunter-gatherer past.

line—that exercise helps us burn more calories—is wrong. Sadly, a lot of people, when they find out that exercise doesn’t have a big effect on daily energy expenditure or a durable impact on weight, assume that exercise isn’t important. That’s precisely the wrong message to take home!

Exercise doesn’t change the number of calories you burn each day, but it does change how you spend them—and that makes all the difference.

We’ve known for decades that regular exercise is an effective way to lower chronic inflammation,15 and that lower inflammation means less risk of heart disease, diabetes, and other metabolic disease.

As is true with sex, water, bluegrass music, beer, and all other wonderful things, there is such a thing as too much exercise.

Despite all the fascinating similarities that bind us to our ape relatives, our metabolic engines are fundamentally different. When we act like apes, we get sick.

Exercise is a poor tool for achieving weight loss, but it does seem to help people maintain weight loss.

Exercise won’t keep you thin, but it will keep you alive.

Key takeaway

We’ve managed to turn one of the fundamental principles of ecology on its head. In nature, energy-rich foods like honey, prey, or fruit are always less abundant and harder to get than low-calorie foods like leaves. In a modern supermarket, it’s just the opposite. Highly processed items like oils, sweeteners, and junk food pack more calories per ounce and cost less per calorie.