What’s the Cap on Human Energy Expenditure? Elite Athletes Reveal ‘Metabolic Ceiling

When athletes devote large amounts of energy to running or cycling, they unconsciously reduce energy output elsewhere.

Ultra-runners face a limit on how much energy they can expend.

The human body has a ‘metabolic ceiling’ that even extreme athletes cannot surpass. A recent study in the journal Current Biology finds that over 30 weeks or more, the ceiling is about 2.4 times an athlete’s basal metabolic rate (BMR), the minimum amount of energy the body needs daily for essential tasks, such as breathing.

People can burn up to about ten times their BMR for short periods. Researchers have previously proposed that the body has a metabolic limit of around 2.5 times BMR over extended periods. Still, it has never been tested adequately until now, says Andrew Best, a biological anthropologist at Massachusetts College of Liberal Arts in North Adams and a co-author of the work.

Best and his colleagues recruited 14 high-endurance athletes — ultra-runners, cyclists and triathletes. Participants drank doubly labelled water containing deuterium and oxygen-18, heavy hydrogen and oxygen isotopes. Once ingested, both deuterium and oxygen-18 will be lost from the body in the form of water, through urine and sweat. However, some oxygen-18 will also exit the body through exhaled carbon dioxide.

By tracing the quantity of these molecules flushed out in urine, the scientists were able to calculate the amount of CO₂ exhaled, and, from this, estimate the number of calories the athletes burned. According to Best, this allowed the team to trace the athletes’ energy output as they competed in real time, rather than measuring them on treadmills in a laboratory.

Hitting the ceiling

During multi-day competitions, some athletes burn around 9,000 calories a day; however, over more extended periods — 30 and 52 weeks — their energy expenditure averaged about 2.4 times their BMR. The results show that even the most extreme athletes reach a metabolic ceiling that is challenging to exceed.

The researchers also found that when athletes devote more energy to running, cycling, and swimming, they unconsciously cut back on using energy elsewhere, such as walking or fidgeting.

Nigel Turner, a metabolic researcher at the University of New South Wales in Sydney, Australia, says that doubly labelled water, the gold-standard method for measuring energy expenditure, allowed the team to get accurate estimates of energy output on athletes’ real activities. Best says doubly labelled water has only recently become affordable, making the study possible.

Turner says one factor that probably limits prolonged exertion is nutrient intake, because that also requires energy. Past research has suggested that humans can only metabolise the caloric equivalent of about 2.5 times their BMR.

If energy expenditure exceeds 2.5 times BMR for a prolonged period, the energy intake of athletes wouldn’t be able to keep up, and they would have to start using body stores, which could include muscle breakdown…This would, in turn, potentially compromise performance.[Turner]

doi: https://doi.org/10.1038/d41586-025-03389-7

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