How far can the human body be pushed? It's a question you might ask yourself on a 5K run, or an early morning gym session, but a new study says there is a definite limit to human endurance – beyond which our bodies start breaking down.
For long-term levels of exertion, that limit is burning calories at 2.5 times our resting metabolic rate, no matter your level of fitness or training. Beyond that, our bodies actually start feeding off their own tissue and fat reserves to keep the engines running.
To figure out our limits, a team of researchers analysed data from a variety of different endurance challenges and life events – from a 4,957 km (3,080 mile) coast-to-coast ultramarathon in the United States to the demands of pregnancy on the human body.
"This defines the realm of what's possible for humans," says one of the team, evolutionary anthropologist Herman Pontzer from Duke University in North Carolina.
"There's just a limit to how many calories our guts can effectively absorb per day."
Part of the new study involved tracking the energy expenditure of six competitors in the Race Across the USA: a gruelling five-month event that saw runners completing six marathons per week. The researchers also looked at previously published data on the Tour de France, Arctic expeditions, and other events.
The same L-shaped curve was noticeable across all the different types of exertion. While we can expend energy at many times our resting metabolic rate in short bursts – up to 15.6x for a single marathon, 4.9x for a pro cycling event, or 2.2x for a pregnancy, for example – eventually that 2.5x hard limit kicks in.
"You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back," Pontzer told James Gallagher at the BBC. "Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance."
"It's very cool data," evolutionary biologist Daniel Lieberman from Harvard University, who wasn't involved in the study, told Michael Price at Science. "It makes a very convincing case that at the extremes of human endurance, there's a hard limit."
It appears that the human body can actually downshift its calorie burning in order to stay at a sustainable limit, where we're not using up energy faster than it can be replenished. That is possible in the short term, the study shows, but eventually our bodies decide to put the brakes on.
The digestive system is one of the biological bottlenecks, the researchers suggest: at some point our bodies just can't process food into fuel any faster.
For athletes training for high endurance events, these new findings could help them plan how to best work underneath this 2.5x ceiling. However, the researchers haven't ruled out the possibility that it could be broken through by certain people.
"So I guess it's a challenge to elite endurance athletes," says Pontzer. "Science works when you're proven wrong. Maybe someone will break through that ceiling some day and show us what we're missing."
The research has been published in Science Advances.