If you want to lose weight, exercise doesn’t really matter.
That doesn’t sound right, does it? After all, for decades we’ve been told that the way to burn off excess calories is simple: move more. Have a slice of cake? No problem, just make up for it at the gym.
But a major new study challenges that long-standing belief. Collating data on more than 4,200 people across 34 different countries, researchers found that people who exercise more don’t burn more calories than those who sit around all day.
In fact, the research found that, matched pound-for-pound, a hunter-gatherer from Tanzania’s Hadza community burns about the same number of calories each day as a typical US office worker – even though the Hadza are more active in a single day than most Americans are in an entire week.
There were some small differences in calorie burn between individuals, but overall they were minimal – and only around 10 per cent could be explained by exercise.
“We previously thought that the amount of activity could change the number of calories we burn per day. But what we’ve learned over the past decade or so is that it’s not anywhere near that simple,” says Duke University evolutionary anthropologist Prof Herman Pontzer – one of the 68 scientists behind the new research.
So, is it time we all chuck out our running shoes and cancel gym memberships? Spoiler: sadly not. While this research suggests ‘how much you move’ has very little bearing on ‘how much you weigh’, giving up on exercise isn't the answer either.
Metabolism myths
If, as the new study suggests, only a sliver of your daily calorie burn comes from exercise, what’s using up the rest? In short: everything your body does behind the scenes to keep you alive.
Your lungs drawing in air, your heart pumping blood, your immune system fighting infections, your gut digesting food, your brain firing with thoughts – all of these processes need energy. A lot of it. In fact, this background work, known as your basal metabolic rate (BMR), accounts for the overwhelming majority of calories you use each day.
And here’s the crucial point: the more cells and mass you have, the more energy you’ll need overall, and the larger your basal metabolic rate will be. It makes sense in theory, but the real-world results are pretty surprising.
Namely, thanks to their larger size, the overweight US office worker will generally have a higher basal metabolic rate than the Hadza hunter-gatherer.
“People in the developed world burn more calories every day than people in farming, foraging and traditional groups,” Pontzer says. “They burn more, because people in the developed world tend to be bigger, and being larger generally means you’re going to burn more calories.”
It all means that once you account for body size and composition, there’s “hardly any difference” in total energy use between populations around the world, says Pontzer, referring to the study.
Or, to put another way, pound-for-pound, the office worker tied to their desk will use a similar number of calories per day as somebody far more active.
The calorie equation
How could it possibly be that we all burn about the same amount of energy for our body size? According to Pontzer, it’s because when we move more (or less), our bodies don’t simply burn extra calories; our metabolism adapts, conserving energy elsewhere to balance things out.
Simply put, our bodies want to stay in balance (a process biologists call homeostasis). This means that, Pontzer says, if you suddenly decided to cartwheel across a meadow every morning, your body would adapt, shifting its energy resources away from some of those background tasks to accommodate your new exercise habit.
“Our bodies are able to adjust to our different lifestyles,” he explains. “If we’re more physically active, our bodies seem to adjust to spend less energy on other stuff. The total calories burned per day is not different, even though we’re more or less physically active.”
Spending fewer calories on other bodily functions may sound alarming, but Pontzer says it’s usually not something to worry about – in fact, it’s desirable.
“Rejuggling how calories are spent is a really good thing. Most of us spend excess calories on things like background inflammation,” he says.
By redirecting energy away from those unnecessary background processes, exercise helps the body run more efficiently – reducing inflammation and keeping us healthier overall. In this way, Pontzer says that while it might not be the key to shedding weight, it’s crucial for maintaining a healthy, balanced body.
Of course, this effect only goes so far. Suddenly ramping up your activity levels can put strain on your body, causing your immune or reproductive systems to take a hit. You might find yourself getting ill more often, your libido dropping, or your periods stopping altogether.
“It’s called overtraining syndrome,” says Pontzer. “But, unless you’re a professional-level athlete, you’re probably not going to see that.”
The same thing happens in reverse when you suddenly stop exercising. If you give up your cartwheeling, for instance, your metabolism shifts gears – redirecting that spare energy into background processes like inflammation instead.
“Regardless of how active you are, the body adapts dynamically to keep expenditures within a narrow range,” explains Pontzer.
The foods that fuel fat
In short, according to Pontzer, the real factor associated with body fat percentage isn’t exercise – it’s food.
“All the weight that we carry is the calories that we ate and never burned off,” Pontzer says. “And so, if our energy expenditures aren’t different, then it must be diet.”
However, the researchers found that one type of food stood out as the main culprit behind weight gain. It wasn’t how much meat or other main food groups people ate that explained differences in obesity – it was the amount of ultra-processed food in their diets.
These packaged products are made in factories, using industrial ingredients and techniques, and sold in shops. They might include supermarket bread, biscuits, sweets, snacks, colourful cereal, bottled sauces, ready meals, flavoured yoghurts – and more.
“What we saw is that, sure enough, across all these groups, the amount of ultra-processed food that these populations are eating is a good predictor of how much body fat they have,” says Pontzer.
“It’s correlational, but I think it’s very suggestive that if you eat more ultra-processed food, you’re more likely to be overweight and obese.”
While the findings are compelling, it’s important to note that the researchers didn’t directly measure what participants ate. There were no food diaries, calorie logs or nutrient breakdowns for either the US office workers or the Hadza members. Instead, the team drew on population-level data from national dietary surveys and global food consumption databases to estimate participants' typical diets.
That might sound like nitpicking, but it’s a crucial limitation. Because we can’t see the link between ultra-processed foods, body fat and energy expenditure on an individual level, we can’t be sure why ultra-processed foods may lead to obesity.
It could be that, calorie for calorie, ultra-processed foods have particularly harmful effects. Or – as Dr Adam Collins, associate professor of nutrition at the University of Surrey, argues – it might simply be that these foods make it far easier to overconsume calories.
“It’s not the ultra-processing itself,” he says. “It’s the properties of these foods that make them hyper-palatable, easy to overconsume – as well as the fact that they’re usually energy dense but not very nutritionally dense.”
By definition, ultra-processed foods are designed to be as delicious as possible, which means they tend to be high in salt, fat and sugar, low in fibre and nutrients, and easy to eat quickly and in large portions.
“When people say they’re going to cut out all their ultra-processed food, what happens?” says Collins. “People end up eating less. People are more mindful of what they’re eating. They end up losing weight and becoming healthier.”
He adds: “It's not that we've got to cut out all ultra-processed foods, or that every ultra-processed food is bad.” Instead, we might benefit from consuming them more mindfully and in moderation.
Chewing the fat
The new study, however, isn’t without its critics – Collins among them. In particular, he argues that exercise does play more of a role in weight loss than it’s given credit for.
“It’s true that exercise might not burn off many calories directly. And tracking ‘calories in versus calories out’ – this thinking of ‘I’ll just run off that chocolate bar’ – is outdated,” he says.
“But exercise can help regulate appetite and change body composition, reducing body fat and increasing muscle mass.”
As Collins points out, muscle cells are more metabolically active than fat cells, meaning that building more muscle subtly raises your daily energy needs and overall calorie burn.
Pontzer agrees that exercise can change body composition, which in turn affects metabolism – but he says these are “really small, marginal changes,” and that exercise programmes relying on them for weight loss generally don’t work.
Collins, however, remains unconvinced. He calls Pontzer’s model “a neat idea,” but says it’s still unproven – and that long-term studies following individuals over their lifetimes are needed to truly test it.
Why you still need exercise
Despite disagreements over how effective exercise is for fat loss, one thing everyone agrees on is that it’s incredibly good for us. It strengthens the heart, protects bones, keeps metabolism in check and boosts mental wellbeing – helping us live longer, healthier lives.
Pontzer is hoping that health advice about weight will change – but he’s not advocating that people abandon exercise.
“Rather than blaming both diet and physical activity equally, which is how it’s all framed right now, I hope that we can focus entirely on diet for the obesity crisis, because that seems to be the main driver here,” he says.
“It doesn’t mean that we should stop exercising or stop telling people to exercise. Those are important goals as well – but we really need to focus our efforts on diet.”
