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Home >Opinion >Columns >A new argument enters the loud world of fitness debates

When I hear the words, “what we are evolved for is…", I know some nonsense is coming. Some phrases foreshadow scholarly blather. Like, “hunter-gatherer" and “Palaeolithic". I think Western anthropology is like a great novel that wants to be bad science. It has a naive obsession with “what we are evolved for" (“to pick berries" is a favourite answer). All this when all of human history offers colossal evidence in plain sight that points to what we are actually “evolved for". As billions of people of all races across aeons have shown, by simply living, what we are evolved for is whatever it is that we are doing right now, including those of us who are anthropologists with a mysterious fondness for gathering berries.

But now and then the field asserts its right as a proper science. That is the rebuke I felt when I read Burn: The Misunderstood Science of Metabolism, which released last week. In this illuminating work, the evolutionary anthropologist Herman Pontzer partly supports his theories through his painstaking study of the Hadza people in northern Tanzania. They are an ancient tribe of “hunter-gatherers" who do pick berries. But still.

Burn is a portrait of humans according to metabolism. And its central hypothesis is at first glance disturbing to the hardworking, embarrassing to those who have heard too many Silicon Valley podcasts on fitness, and fated to be misunderstood by the lazy. The hypothesis is this: There is a limit to how many calories you can burn in a day, no matter what you do. Even if you run a marathon every day for over a hundred days, and there are people who do that.

The more you exert yourself, the more your body conserves itself by turning off the other ways in which it spends energy—such as nursing a grouse, feeling mental stress, creating inflammations, fighting infection, nurturing sustainable libidos, preparing for reproduction, and taking care of your skin. Thus, there is a predestined cap on the number of calories your body will burn in a day.

This does not mean you should stop exercising. This means the most valuable thing about exercise is not weight loss; it does something more important. Moderate to hard exercise may push the body to suppress some of its trivial and harmful activities, like creating needless inflammations, wallowing in excessive psychological distress and holding up a placard with a moral message on the wayside.

But, as physical exertion grows more strenuous, for instance when a person rows across the Atlantic for a month, the body begins to switch off some of its essential functions. Inside the body, the cost of something lighting up is at the expense of something else being shut down. The brain of children, Pontzer says, takes up so much energy, “it actually slows down growth in the rest of the body".

A resting person who weighs 70kg will burn 70 kilocalories in an hour. According to The Compendium of Physical Activity, a dynamic compilation of calorific costs, sitting burns 1.3 times more calories as resting; and standing “on two legs" burns 1.8 times, which is, oddly, the same as “sexual activity, general, moderate effort". Maybe the Compendium should study more passionate people. Other forms of a high-intensity work-out, like “Navy Seal training", burn 10 to 13 times more calories than resting. Even though the brain takes up a lot of energy, thinking itself is believed to burn very little. About 4 kilocalories combust when a chess player plays a tough opponent. But learning, Pontzer says, burns a lot more calories. What the body can burn through activity, however, is meagre compared to the energy that food can store. This is why your diet has become the key to modern fitness.

One of the surprising claims Pontzer makes is that all diets are equal. There is no particular advantage in the famed low-carb diets. He challenges the theories of low-carb evangelists like Gary Taubes, whose Why We Get Fat is a persuasive book that considers sugar the primary villain of our times and fat a victim of the sugar lobby. Taubes is of the view that just the thought of sugar (which, from the point of view of chemistry, includes rice and maida) makes the body secrete the hormone insulin. And when we eat sugar, the body releases more insulin. And insulin, according to Taubes, prevents the human body from burning fat.

But, according to Pontzer, calories from sugar are not particularly more evil than calories from fat. No matter what you eat, if you eat less than what you burn, you will lose weight; and if you eat more than what you burn, you will gain weight. That is all there is to it. And the extra weight you feel after a sugary day is merely water stored in glycogen, which is excess glucose.

But Pontzer does concede that sugar is dangerous, and that is because it is tasty. Fast-burning carbohydrates like sugar and maida have confused the modern human’s hypothalamus, which is the seat in the brain that gives you the feeling of satiety and asks you to please stop eating. The sheer variety of modern sugar-infused food has exhausted the capacity of our brain to tell us when to stop, with all its consequences.

So how, then, must we live? What we can glean from the metabolic biography of our species is the banal truth that what is banal is exactly what will make us endure. The ideal way to be is what you had always suspected—on most days, eat austere, and on almost all days, give yourself a work-out that is a bit more than moderate and a little less than Olympic-training level.

A new book on the science of metabolism debunks many things you may have heard on this subject.

Manu Joseph is a journalist, and a novelist, most recently of ‘Miss Laila, Armed And Dangerous’

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