Can’t lose weight? It probably has nothing to do with exercise


So why, I ask Pontzer when we speak via Skype, have we been barking up the wrong tree for so long? The answer, he believes, is that it’s hard to measure metabolism and, until recently, we lacked the scientific techniques to do so.

“In the absence of good methods, people have been doing the best they can, which is making good guesses. The guesses have become the dogma,” says Pontzer, who grew up in rural Pennsylvania. “There’s this disconnect between the easy story and the more complex evolutionary story.”

“All the research we’ve done in the last 10 years, not just my lab but other people, too, points to diet as being the culprit here for obesity. It’s not sloth, it’s the food.”

And when it comes to challenging the “easy story” the stakes could not be higher.

As Pontzer writes: “Public health strategies stubbornly cling to the simplistic armchair engineer’s view of metabolism, hurting efforts to combat obesity, diabetes, heart disease, cancer and the other diseases that are most likely to kill us.”

Those 10,000 daily steps you’ve been trying to achieve? A typical 70-kilogram adult burns about 250 calories while doing them, he explains. This is roughly equivalent to half a Big Mac. Climbing one flight of stairs burns about 3.5 calories – less energy than you’ll get from a single M&M.

It’s enough to send you back to the sofa, defeated. But it shouldn’t because, as Pontzer makes clear, exercise is vitally important for your health – it just won’t make you thin. “If you start a new exercise plan tomorrow and stick to it religiously, you will most probably weigh nearly the same in two years as you do right now,” he writes. “You should still do it! You’ll be happier, healthier and live longer. Just don’t expect any meaningful weight change in the long term from exercise alone.”

So if squeezing in an extra trip to the gym each week won’t stop us piling on the pounds, what will? I put it to Pontzer that the supervillain in Burn is the ultra-processed, highly flavoured foods in Western diets. While the Hadza have stuck to a plain diet of wholefoods, we have been seduced by an ever greater array of engineered delights. These won’t nourish us or fill us up, so we eat more of them.

“I think that’s right, if you had to point your finger at one thing,” he says. “All the research we’ve done in the last 10 years, not just my lab but other people, too, points to diet as being the culprit here for obesity. It’s not sloth, it’s the food.

“What particularly is it about the food? Is it sugar? No. Is it fats? No. It’s the fact we engineer our foods in labs and focus group test them to make sure you eat too much. That’s literally the point of these big industries: to make sure you buy as much as you can. That’s how they make money. Obesity has come up right alongside the availability and engineering of processed foods.”

Some of us eat more than others, of course, and some eat more ultra-processed food than others, for various reasons. Genes are part of the picture – though perhaps not in the way you’d expect.

You may think you have a fast or slow metabolism, but Pontzer suggests this perception usually translates as “I feel I can eat whatever I want and I never feel tempted or hungry outside of that”, or “I feel hungry all the time and if I don’t work really hard I will overeat”.

He says: “We know that is your brain’s management of your hunger and fullness and satiety, and we know people are wired differently and the genes that contribute to the variation in BMI [body mass index], they’re active in your brain, not in your fat or muscle cells. So it’s how you’re wired, it seems, that’s going to affect how fast you feel your metabolism is.”

There are also external factors that influence how we eat. One is stress. “It makes our brains take decisions about food that probably aren’t the healthiest,” says Pontzer. “Comfort-eating and stress-eating are real. I suspect if COVID is having an effect on people’s waistlines, it’s as much about that [as anything].”

Lowering the emotional and psychological stress in our lives – as well as physical stress caused by sleep deprivation – could help tackle overeating. Though Pontzer would be the first to acknowledge that in busy, modern lives revolving around working long hours in sedentary jobs, it’s easier said than done.

Obesity, then, stems from a complex interplay of biology and socioeconomic factors. And once you’ve become overweight or obese, our highly evolved metabolisms make it incredibly difficult to shed the pounds gained.

“What particularly is it about the food? Is it sugar? No. Is it fats? No. It’s the fact we engineer our foods in labs and focus group test them to make sure you eat too much.”

Pontzer points to research such as the landmark study conducted during the 2010s on obese people who went to weight-loss bootcamps for the reality TV show The Biggest Loser. After 30 weeks of calorie reduction and exercise, the contestants had all lost weight, but tests showed that their metabolic rates had slowed down dramatically – they were in starvation mode, where cells burn energy more slowly as the body works to conserve calories. When researchers checked in with 14 of the contestants six years after the programme, their basic metabolic rates were still lower than expected and all but one had regained a considerable amount of weight.

It’s perverse – and depressing – but, writes Pontzer, “from an evolutionary perspective, it makes all the sense in the world”. It’s time, then, for a rethink of how our metabolic engines work.

But what do we do, meanwhile, to keep the weight off? Although he’s amused by the modern obsession with eating like our ancestors did, in the form of so-called Paleo diets, Pontzer says we can learn from the Hadza.

“They stay thin because they eat a diet that doesn’t have these processed foods in it. I think 90 per cent of it is that simple.”

We frequently complicate it with fad diets because we like to believe in the narratives around them, he says. It’s appealing to think there might be a magic bullet. But there isn’t.

“Every diet that works, works because it cuts calories,” says Pontzer. “There are different ways to do that. There’s no magic. Every diet works if you stick to it.”

The diet that works best for you depends on “your particular reward system and the variety of foods that satisfy you most on the fewest calories,” Pontzer writes. On a personal level, we can keep tempting treats out of reach. At a societal level, we will only tackle obesity by changing our food environment, he believes. Extra taxes on ultra-processed food might be one way. Making wholefoods cheaper and easier to come by, another.


Physical activity remains vital – for regulating our metabolism, including our feelings of hunger and fullness, protecting us against every major disease, helping us live longer and maintaining weight loss.

When it comes to understanding metabolism better, Pontzer is optimistic the tide is turning. And when public health guidelines are rewritten to clarify that exercise isn’t the key to weight loss, he’ll know the penny has finally dropped.

The Telegraph, London

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