In a world of calorie-counting fitness trackers and workouts promising to 'shred your belly fat', we’ve grown used to the idea that exercise equals weight loss. The more miles on the treadmill, the more calories you burn, the more weight you stand to lose… right?
Not necessarily.
A growing body of evidence suggests that exercise, on its own, has far less of an impact on your weight than you might think. In fact, scientific studies have even shown that people exercising five days a week with no change in diet may lose only a few kilos a year, and many – particularly women – will lose none at all.
Even more mysteriously, studies have shown that highly active people, and even ultra-endurance athletes, simply do not burn as much energy as would be expected given their levels of physical activity.
Why? The theory is that their bodies seem to adapt to high levels of exercise, limiting their total daily energy use to prevent them losing weight.
Exercise is, of course, crucially important to our overall health. It has positive effects on everything from our bone density to our mental wellbeing. It can reduce the likelihood of a range of health problems including diabetes, heart disease, cancer and dementia.
But when it comes to shrinking belly fat, some data suggests creating an energy deficit through diet is far more effective than exercise.
So, what’s really going on here? Are all those freezing park runs, punishing functional fitness classes and expensive exercise machines helping us lose weight or not? Here’s the surprising science of how exercise really changes your body.
The calorie paradox
Ideas about the relationship between exercise and weight loss began to shift around a decade ago, following the surprising results of work by an American evolutionary anthropologist named Prof Herman Pontzer.
The professor, from Duke University in the US, had been living with Tanzania’s indigenous Hadza tribe, one of the last remaining populations of traditional hunter-gatherers in the world.
With no electricity, running water, agriculture or machinery, the Hadza live what most of us would consider to be an extremely tough and physically active life. They spend many hours in the Sun hunting large animals, felling timber, climbing for berries or honey, fetching water and digging up wild root vegetables.
Everyone, including Pontzer himself, assumed that these hardened hunter-gatherers would be using far more energy each day than those of us with modern and relatively sedentary lifestyles. After all, the only hunting and gathering most of us do involves tracking down the best supermarket deals.
Pontzer gave some of the Hadza specially treated water to drink that allowed him to monitor their daily metabolic energy use with great accuracy.
The findings were bizarre. Hadza people, when the data was adjusted for their differences in weight, used up a broadly similar amount of energy on a typical day to pretty much all the other populations whose daily energy demands had been studied.
People walking dozens of kilometres a day to track, kill and haul giraffes back to camp apparently burnt a similar amount of energy on a typical day to Americans with cars, well-stocked fridges and office jobs.
Pontzer explained his surprising results through what has become known as the ‘constrained energy expenditure hypothesis.’ The theory suggests that humans, and indeed many other animals, may have a built-in system to keep our overall energy expenditure within a fixed range, no matter how much physical activity we do.
In other words, Pontzer suggests your body generally has a set amount of energy to use each day.
Dedicate calories to a bout of exercise and, according to the theory, you won’t burn extra calories, but spend fewer in other areas, such as your immune system. While this initially sounds alarming, giving your immune system a reprieve may stop unnecessary inflammation.
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Your personal energy cap
While it sounds odd, it does make evolutionary sense. After all, most organisms don't want to burn more energy than they can take on board, as doing this regularly would increase the risk that they’ll use up their fuel reserves and starve.
If you are having a tough week physically – perhaps involving several long and failed chases to catch food, and some long journeys to flee predators – it’s preferable to have a metabolism that can reduce your other, non-physical energy usage to compensate. Your body doesn’t want your hard-won energy reserves being burnt away to nothing.
It wasn’t just the Hadza people that seemed to underpin this theory. Pontzer found similar results in other highly active populations, including indigenous communities in South America and other parts of Africa.
Deciding he needed to find some even more extreme examples of physical lifestyles, he performed the same set of tests on athletes. And not just any athletes: these were competing in the brutal ‘Race Across America’ event, an almost 5,000km (3,000-mile) ultra-endurance road cycling race from California to Washington DC.
He also studied cyclists doing the Tour de France (3,600km, or about 2,200 miles), as well people doing other gruelling physical tests, from running 258km (160-mile) ultramarathons to trekking in the Arctic.
Again, his studies found that people completing these extraordinary feats didn't burn the additional calories such levels of physical activity would seem to require.
While all of them did have extremely high energy expenditure initially, the daily energy use of those competing in longer events gradually fell, and stabilised at around 4,000 calories per day. That’s fewer than would be expected for people running, cycling or hauling equipment virtually all day.
The data suggested that for an average-sized human, 4,000 calories might be the ‘hard limit’ of how much energy the body is prepared to burn in a given day, in the long term. It also provided more evidence that the body seems to adapt to high activity levels to ensure energy use is not excessive.
“I don't think I was the first person to show this sort of result,” says Pontzer of his work. “But I think I was maybe the first person to say ‘Hey, I think this is a phenomenon that deserves attention, and here's how I think it works: that the body's adjusting to keep energy expenditures in check.’”
Indeed, other studies have led to similar findings in the past. A study of a cohort of elderly people in Vermont in the 1990s found that regular endurance training – in other words, ‘cardio’ exercise – did not seem to increase their total daily energy use at all.
The authors at the time suggested the participants may be compensating for the exercise by resting more before and after.
Other research has also shown that without a change in diet, exercise doesn’t seem to have the impact one might expect. In a key research project known as the Midwest Exercise Trials, hundreds of overweight or obese people took part in an exercise programme that involved five calorie-burning sessions a week for 10 months.
Crucially, participants were asked to maintain their normal diets so scientists could assess the impact of exercise alone.
The study found that the exercise had virtually no effect on the weight of participants who were women – some even gained weight. Men did lose weight, but the average amount of weight lost was around 4kg (8.8lb) – not a particularly pleasing amount for overweight people after more than 200 exercise sessions.
“Generally speaking, people lose less than half of the weight that you'd expect them to lose from the exercise they're doing,” says Pontzer.
“Within a couple of months of somebody doing the exercise, we see the body adjusting and spending less energy than it should be. In terms of being able to create an energy deficit to lose weight, diet is by far the bigger lever.”
Burn, baby! Burn?
Not all experts agree with the constrained energy hypothesis – some believe that there could be other explanations for Pontzer’s results.
For example, the Hadza people may just be extremely well adapted to that lifestyle. Their bodies could be much more efficient at doing those particular activities than other populations would be – and therefore they use fewer calories than we expect them to.
There’s also no solid evidence to suggest how their bodies might lower other energy usage to compensate for higher activity levels.
But regardless, evidence quite clearly shows that diet is far more important than exercise for those hoping to lose weight. And there are a number of other things that we tend to believe about exercise that don’t quite match up to the reality of what is happening in our bodies.
Firstly, we tend to overestimate the difference that a long run or heavy session in the gym will make to the energy we use in a day: it’s actually quite a small proportion of our total energy use.
The vast majority of our body’s energy demand comes from the background metabolic activity that keeps all our organs and trillions of cells alive and functioning. The liver and brain are particularly energy hungry, and the calories burned by our bodies just while sleeping can make up almost half our total energy expenditure in a 24-hour period.
Small but constant muscle movements – standing up, walking around, even reaching for a doughnut – also add up to more energy use than one typical exercise session such as a jog, cycle or swim.
“A 30-minute session of exercise might burn a couple of hundred calories,” says Javier Gonzalez, professor of nutrition and metabolism at the University of Bath.
“But just moving around, staying upright, fidgeting – movements we are hardly aware of – can add up to quite a lot over a whole day. It can be over 1,000 calories, especially if you have an active job.”
There are other factors that mean our energy use is not as simple as the numbers our fitness trackers tell us, says Gonzalez. The first is the tendency for people to compensate post-exercise by resting more than they might have.
In other words, deciding you ‘deserve’ a lazy evening on the sofa because you went to the gym earlier.
Secondly, the exercise might also have been done during a time when you would be physically active anyway, meaning you might have only burnt a few hundred calories more than you would if you were, say, walking about or cleaning.
“Let's say you plan to do 500 calories of energy use in the gym,” Gonzalez explains.
“Whether it's an additional 500 calories burnt depends on what you would have been doing otherwise. If you'd have been doing absolutely nothing, then yes, you’ve burnt an additional 500 calories. But that's rarely the case – you might have been walking to the shops, going up and down the stairs or doing other things.”
On top of this, we are also notoriously poor at estimating how many calories we’ve consumed, and extremely high-calorie food is readily available everywhere for most of us, says Gonzalez.
The net result is that it is far easier to consume a few hundred calories more than we need than it is to create a deficit of a few hundred calories through exercise.
“The amount of energy that a typical person can burn in an hour of exercise is pretty modest compared to the amount of energy you can eat in an hour,” says Gonzalez.
Fit for life
For those of us who are currently slogging through a gruelling weight-loss regime with a personal trainer, the information above might seem pretty dispiriting. But experts across the board, including Pontzer, agree that the worst thing to take away from all this is that exercise is pointless.
“I’ve never said exercise doesn’t matter,” says Pontzer. “And it’s deeply frustrating when I hear people take that away, because it’s not what I’m saying.”
Dr Adam Collins, an associate professor of nutrition at the University of Surrey, says that exercise has several important roles in maintaining a healthy weight, even if it doesn’t have a direct impact as you might think.
“If you're just looking at exercise purely in terms of how many calories you're burning, then you're on a road to misery – that's not really the benefit of it. You are exercising to become fitter, to improve how your body and its metabolism functions.”
During dieting, some people’s metabolisms slow down as the body attempts to prevent weight loss for the same reasons it might limit energy use: survival. Your body is essentially trying to protect itself.
But there’s evidence exercise can actually help counteract this slowdown – and regulate the changes in appetite that follow a diet.
“One of the things that compromises people's ability to sustain weight after a diet is the fact that appetite comes back with a vengeance, and exercise can help attenuate that appetite response,” he says.
A study of contestants from the US reality TV show The Biggest Loser – in which they competed to shed the most pounds – seemed to show just this.
True, it demonstrated there was little relationship between competitors’ physical activity and success in initially losing weight. But exercise seemed to be a key factor in keeping the weight off.
In fact, six years after the show, only those who had continued regular daily exercise had maintained their lower weight.
As well as a host of other benefits to everything from the immune system to mood, exercise also makes your body more efficient, meaning it uses energy more effectively.
“It has important effects on how you use fuel – how you can cope with what you're eating,” says Collins. Resistance exercise also helps maintain or grow muscle mass, which helps you burn more calories.
“Your muscles are among your biggest consumers of the fuel swimming round in your body,” says Collins.
“So, if your muscles are not using it, or are not able to use it as effectively, then it ends up being stored in body fat or being kicked around elsewhere in the circulation. Many health problems like diabetes and cardiovascular disease are basically a product of the mismanagement of fuel in your body.”
While Collins is not wholly convinced by the evidence for the constrained energy hypothesis, he is pleased that people are starting to think about exercise in a different way.
“It has at least highlighted the importance of exercise beyond just the number of calories that you're burning, beyond trying to hit a certain number of steps or calories burned at the gym. That's not really what exercise is doing for you.”
So while academics continue to work out the fine details of how the body responds to high levels of activity and exercise, their advice on how to remain healthy and happy, and maintain a sustainable weight is largely the same as it always was: a sustainable, healthy diet, and plenty of exercise.
It’s also increasingly clear that the relationship between exercise and weight loss is not as clear cut as weight-loss influencers and cardio equipment manufacturers would have you think. While your smartwatch tells you that the more exercise you do, the more calories you burn, it’s just not that simple.
It’s diet that is the key to weight loss, not exercise. Or, as researchers wrote famously in The British Medical Journal: “You cannot outrun a bad diet.”
About our experts
Prof Herman Pontzer is a professor of evolutionary anthropology and global health at Duke University. He has been published in a number of journals including The Journal of Experimental Biology, American Journal of Human Biology and Journal of Human Evolution.
Javier Gonzalez is a professor of nutrition and metabolism at the University of Bath. His work is published in journals such as American Journal of Clinical Nutrition, International Journal of Sport Nutrition and Exercise Metabolism and European Journal of Nutrition.
Dr Adam Collins is an associate professor of nutrition at the University of Surrey. He is published in Applied Physiology, Nutrition, and Metabolism, British Journal of Nutrition and Research in Endocrinology to name a few journals.
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