Scientists have worked out the ultimate limit of human endurance by analysing the energy expenditure during some of the world's longest, most gruelling sporting events such as the Tour de France.
The study, published in the journal Science Advances, suggests that no matter what the activity, everyone hits the same metabolic limit -- a maximum possible level of exertion that humans can sustain in the long term.
When it comes to physical activities lasting days, weeks and months, the researchers found, humans can only burn calories at 2.5 times their resting metabolic rate.
Not even the world's fastest ultra-marathoners managed to surpass that limit, the researchers found.
"This defines the realm of what's possible for humans," said Herman Pontzer, an associate professor at Duke University in the US.
Beyond the threshold of 2.5 times a person's resting metabolic rate, researchers found, the body starts to break down its own tissues to make up for the caloric deficit.
One explanation for this limit may be the digestive tract's ability to break down food, said John Speakman from Scotland's University of Aberdeen and the Chinese Academy of Sciences.
"There's just a limit to how many calories our guts can effectively absorb per day," Pontzer said.
The team measured daily calories burned by a group of athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA, a 4828-kilometres race from California to Washington.
The team also considered other feats of human endurance, including punishing 160-km trail races and pregnancy.
When they plotted the data over time, they found an L-shaped curve.
The athletes' energy expenditure started out relatively high, but inevitably plunged and flattened out at 2.5 times their basal metabolic rate for the remainder of the event.
Co-author Caitlin Thurber analysed urine samples collected during the first and final legs of Race Across the USA.
After 20 weeks of running back-to-back marathons, the athletes were burning 600 fewer calories a day than expected based on their mileage.
The findings suggest that the body can "downshift" its metabolism to help stay within sustainable levels.
"It's a great example of constrained energy expenditure, where the body is limited in its ability to maintain extremely high levels of energy expenditure for an extended period of time," Thurber said.
"You can sprint for 100 metres, but you can jog for miles, right? That's also true here," Pontzer said.
The endurance events followed the same L-shaped curve, whether the athletes were hauling 500-pound sleds across Antarctica for days in sub-freezing temperatures, or cycling the Tour de France in summer.
That finding challenges the idea, proposed by previous researchers, that human endurance is linked to the ability to regulate body temperature.