That’s because your body has evolved to have a fixed idea of what your daily energy expenditure should be, and that expenditure returns to around where it started after those 10 months, no matter what you are doing.

Your metabolic engines – mainly the hypothalamus – work hard to keep your energy expenditure within a narrow band, whether you are exercising or not, and your body compensates for anything you do to influence it.

Although it sounds counterintuitive, when you’re pushing hard on the treadmill, after a few months you’re actually not burning up much more energy than when you were avoiding the gym and watching TV on the couch instead.

They’ve aged well – and they’re on social media to help others do the same

This has important ramifications for weight-loss strategies, Pontzer says.

If energy going out – the energy that you expend during exercise – does not affect your weight, it must be the energy going in that’s causing you to put the pounds on. What’s going in is food. So the only way to lose weight is to take control of what you eat, he says.

“It’s a long-held opinion in public health that one of the long-standing changes in modern life is reduced physicality,” Pontzer says in a lecture called ‘Healthy as a Hunter-Gatherer – Insights From Small-Scale Societies’, organised by the UK’s New Scientist magazine.

“That is, the modern industrialised population is less active than their ancestral counterparts so they burn less calories, they burn less energy each day. As the calorie reduction is less, we burn fewer fat calories and therefore gain weight. Because of that, we have obesity,” he says – describing conventional wisdom.

The problem with this theory, Pontzer says, is that his research with the Hadza, a physically active hunter-gatherer society in northern Tanzania, showed that their energy expenditure is no different from the energy expenditure of their sedentary counterparts in the US.

In spite of a rigorous lifestyle, from hunting wild game and foraging for plants – five to 10 times more physical activity every day than most women in the US or Europe – the Hadza surprisingly burn the same amount of calories as their more sedentary counterparts in the West.

The difference is the tribe expends their calories more on activity, and less on other, unseen tasks in the body – such as fighting inflammation, reacting to stress, and other things that make us sick.

“[The research showed that] there is no difference in total daily energy expenditure in calories burned per day between really physically active folks compared to people in the US,” Pontzer says.

While the energy expenditure is roughly the same in the two groups, obesity is rare in societies like the Hadza. The key factor, he concludes, must be their diet.

Pontzer’s work as an evolutionary biologist explores how our species’ past shapes our health and physiology today, and he investigates the physiology of humans and other primates to understand how ecology, lifestyle, diet and evolutionary history affect metabolism and health.

One of Pontzer’s primary concerns is the obesity epidemic.

“I study how our bodies evolved, and how evolution got us to the bodies that we have today, looking at metabolism, exercise, activity and diet from an evolutionary perspective,” he says.

Obesity among Hong Kong children increased during Covid-19 pandemic: survey

The background to Pontzer’s research lies 2.5 million years in the past, when our species, Homo sapiens, became hunter gatherers, foraging for wild plants and hunting animals for food.

This was successful, and shaped our evolution – it defines our species, he says. Our bodies evolved in a certain way to support our hunter-gatherer lifestyles for 2.5 million years.

Then came the industrial revolution in the 18th and 19th centuries, followed by a more sedentary lifestyle – and supermarkets. Our bodies have not yet evolved to fit this recent – in evolutionary terms – lifestyle change. And they are consequently confused.

The way we manufacture our food today injects a lot of energy, that is calories, into it, and it’s also less filling, so we eat more of it – calorie bombs that cause us to overconsume, he says. This could be at the root of the obesity problem.

“The radical change in our diets only happened in the last 200 years or so, with industrialisation,” he notes. “Once we started processing our food and making it in factories, we started pouring all sorts of energy into it. Processed food makes you gain weight.”

The hunters shared their spoils with the gatherers, so everybody ate a bit of everything, he explains.

Fibre is important, not least because it fills you up, so you eat less. The Hadza, he notes, chew on stringy fibrous roots, and the vegetables they eat tend to be fibrous rather than fleshy like supermarket versions.

But the availability of plants in most regions meant that they did eat more plants than meat. Hunter-gatherer diets feature a huge variety of plants and berries, he says, noting that this is not energy-rich food, so it will not lead to weight gain.

He stresses: “Exercise will keep you alive.”