Ultimate limit of human endurance found

Scientists have worked out the ultimate limit of human endurance by analysing the energy expenditure during some of the worlds 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.