Is there a limit to human endurance? Science says yes

From the Ironman triathlon to the Tour de France, some competitions test the limits of the toughest endurance athletes. Today, a new study on energy expenditure during some of the world's longest and most exhausting sporting events suggests that whatever the activity, everyone meets the same metabolic limit: a maximum possible level of effort that humans can endure in the long run.

The researchers found that when it was physical activity for days, weeks or months, humans can burn calories only 2.5 times their resting metabolic rate.

The researchers found that even the fastest ultra-marathon runners in the world could not exceed this limit.

"This sets the realm of what is possible for humans," said Herman Pontzer, co-author of the study, associate professor of anthropology of evolution at the University of Duke.

The researchers found that beyond the threshold of 2.5 times a person's resting metabolic rate, the body begins to break down its own tissues to compensate for the caloric deficit.

One explanation for this limitation could be the ability of the digestive tract to degrade food, said team leaders Pontzer and John Speakman of the Scottish University of Aberdeen and the Chinese Academy of Sciences.

In other words, eating more will not necessarily help anyone to write the Iditarod story. "There is just a limit to the number of calories that our guts can effectively absorb per day," said Pontzer.

The results will be posted online on June 5 in the journal Progress of science.

For the study, the team measured the daily calories burned by a group of athletes who ran six marathons a week for five months as part of the 2015 Across the United States race, a 3,000-kilometer run. from California to Washington, DC. exploits of human endurance, including punishing 100-mile racing and pregnancy.

When they plotted the data in time, they found an L-shaped curve. The energy expenditure of the athletes was relatively high at first, but inevitably dropped and flattened to 2.5 times their basic metabolic rate until the end of the competition.

Co-author Caitlin Thurber analyzed urine samples collected during the first and last stages of Race Across the USA. After 20 weeks of consecutive marathons, athletes burned 600 fewer calories a day than expected based on their mileage. The results suggest that the body can "downgrade" its metabolism to help stay in sustainable levels.

"This is an excellent example of limited energy expenditure, in which the body is limited in its ability to maintain extremely high levels of energy expenditure for a long time," Thurber said.

"You can sprint 100 meters, but you can run for miles, is not it? It's also true here," said Pontzer.

All endurance events followed the same L-shaped curve, carrying 500-pound sleds through Antarctica for days under freezing temperatures, or performing the Tour de France in the summer. This discovery challenges the idea, proposed by previous researchers, that human endurance is linked to the ability to regulate body temperature.

Researchers have discovered that one of the factors limiting endurance events, lies in the process of digestion: the body's ability to transform food and absorb the calories and nutrients needed to fuel bodily processes .

It is interesting to note that the maximum sustainable energy expenditure observed in endurance athletes was only slightly higher than the metabolic rates that women experience during pregnancy. This suggests that the same physiological limitations that keep Ironman triathletes from breaking speed records, for example, can also limit other aspects of life, such as the size of babies who can grow up in the belly.

To the researchers' knowledge, no one has ever maintained levels above this limit. "So, I guess it's a challenge for elite athletes," Pontzer said. "Science works when we prove that we are wrong, maybe someone will exceed that ceiling one day and show us what we are missing."