Move more, eat less is a well-worn mantra for weight control – but does exercise really help when it comes to getting slim?
New US research suggests probably not – and that humans may in fact have evolved a trait which means the number of calories you burn stays pretty constant, whether you are spending hours on the treadmill, or sitting at a screen all day.
Scientists at Duke University in North Carolina found evidence for a calorie-burning ceiling when they crunched data from 14 different studies, involving more than 400 people, who had been given set amounts of exercise to do each week.
The results, published in Current Biology, showed those who exercised more did not lose as much weight as expected. ‘It seems that when we spend more calories on activity, our bodies compensate by burning less energy for other things, such as hormone production and sleeping energy expenditure [for instance, tossing and turning in sleep burns calories],’ says Herman Pontzer, a professor of evolutionary anthropology and global health, who co-led the study.
He believes evolution may have created a ‘ceiling’ of energy expenditure to avoid ‘massive fluctuations in energy output’ – an idea known as compensation theory.
In fact, studies have consistently found that this ceiling is around 2.5 times the energy needed to drive someone’s basal metabolic rate (BMR), which maintains essential life-sustaining functions, such as breathing and circulation.
This ratio applies to everyone, from couch potatoes to elite athletes – and when calorie burning gets close to our ceiling, our metabolism goes into energy-saving mode, Professor Pontzer told Good Health.
Scientists at Duke University in North Carolina found evidence for a calorie-burning ceiling: people who exercised more did not lose as much weight as expected
‘The basal metabolic rate usually accounts for a little over half of energy expended and then another 30 per cent is spent on activity such as exercise.’
The rest is used up by being alert and attentive, undergoing a stressful event or maintaining libido or optimum fertility – all of which are ‘expendable’ if energy needs have to be diverted to exercise.
As a rule of thumb, an average person uses 50 calories to walk a mile, or 100 calories to run a mile. But over time, this calorie expenditure will reduce.
Professor Pontzer explains: ‘We know, for example, that athletes who exercise too hard start to get decreased levels of sex hormones circulating in their bodies, which is the energy-saving mode being turned on. For men, this may lead to loss of libido and women may stop their monthly menstrual cycles. This is the body saving energy to keep its energy output constant.’
Herman Pontzer, professor of evolutionary anthropology and global health
Over-exercise, where people push their bodies beyond their ability to recover, can also result in a lowered immune system and more susceptibility to diseases. Essentially, when you are exercising too hard, the immune system gets less fuel than it needs to work effectively.
The new study echoes earlier research showing the amount of calories being used up is not as dependent on activity levels as was thought. For instance, in a 2025 study in the journal Proceedings of the National Academy of Sciences (PNAS), researchers compared different global populations, including hunter gatherers in North Tanzania, herdsmen, and people living in cities.
They found that even people who hunt their own food and live a hunter-gatherer lifestyle, spending many hours being active each day, expend roughly the same energy as sedentary Westerners.
But other studies contradict the idea of compensation theory. Scientists at Virginia Polytechnic Institute and State University reported last year in the PNAS that being more active does in fact burn up more calories.
Whatever its role in weight loss, no one is denying that sustained exercise is crucial for good health, adds Professor Ziyad Al-Aly, a clinical epidemiologist at Washington University in St Louis.
Working with weights builds muscle, which burns calories and therefore increases the basal metabolic rate and ‘raises the ceiling’, so more calories are burned
‘Exercise may not be the best way to lose weight but it has multiple health benefits, ranging from boosting the cardiovascular system and reducing systemic inflammation to improving mood and cognition and building muscle and bone mass,’ he told Good Health.
Indeed, a study published in January in the journal BMJ Medicine showed a combination of exercises can even extend life.
Scientists at Harvard University tracked more than 111,000 people over more than 30 years, examining the impact of activities including walking, running, weight training and tennis. Taking part in any one of these reduced the risk of premature death, but the greatest benefit was seen with combined workouts.
Researchers found people doing those workouts had a 19 per cent lower risk of premature death compared to those who engaged in just one or two activities.
So why is a variety of exercises best? ‘It reduces injury caused by overuse of particular muscles and tendons,’ says Sammy Margo, a physiotherapist based in London.
‘Different activities work out different areas. Some improve coordination and balance, such as yoga, while others work out the heart and improve the cardiovascular system, such as swimming.’
She adds: ‘Variety also promotes anti-inflammatory recovery between sessions, whereas chronic single-sport training can maintain low-grade inflammation.’
But if you want to lose weight you should focus more on resistance training – which includes weight training – rather than running or cycling, says Professor Al-Aly.
This is because it builds muscle, which burns calories, increasing the basal metabolic rate and ‘raising the ceiling’, so more calories are burned.