Washington (AFP)

American researchers have estimated an absolute physiological limit to human endurance after studying the performance of athletes from various sports, including the most extreme race runners on the planet, 5,000 km in the United States for five months.

The calculations are a little technical but the study, published Wednesday by the journal Science Advances, puts the finger on a unique quality of humans among primates: their extraordinary endurance, likely legacy of hunter-gatherers for two million years. No monkey can compete with a human for a long time.

The limit of endurance is measured in terms of the basal metabolism, which is the minimum energy (in calories) that the body expends to run the body every minute.

This limit is about 2.5 times this basic metabolism, concludes the team of scientists led by Herman Pontzer, of Duke University (North Carolina).

In "short" sports performances - races ranging from a few hours to a few weeks, such as a triathlon, a marathon or the Tour de France - athletes can increase their metabolism to five or ten times their basic metabolism.

But after a while, the performance will inevitably go down to a limit of about three, explain the researchers. It is impossible for humans to maintain a higher diet beyond a few weeks.

To reach this figure, they followed five runners and one extreme runner, participants of the Race Across the USA from January to June 2015: 4.957 km between Los Angeles and the capital Washington, the equivalent of a marathon per day, six days a week for 20 weeks.

"Even these runners end up spending more calories than they can get back into their bodies," says Herman Pontzer to AFP. "All these people lose weight, they burn more fuel than they can get back."

"What is the point at which we can absorb as many calories as we lose each day?" It's 2.5 times the basal metabolism, about 4,000 calories, "says the researcher.

- Fools of humans -

The fools of the "Race Across the United States" consumed eight liters of water a day in the first week, and 6,000 calories a day.

But they continued to lose weight until the end, never finding balance.

"You can not absorb more than 4,000 calories a day," says Herman Pontzer. "You can eat more than that, but you will lose weight every day, so it will not last forever, you can do it for a few days, a few weeks, but not forever."

According to him and his colleagues, the limit is related to the digestive functions of humans, not the muscles or the vascular system, because it is ultimately the lowest common denominator of all activities studied: cycling, running and triathlon do appeal to different muscles, but obviously the same digestive system.

If humans are the best primates, other animals are candidates for higher endurances, such as migratory birds. But it is not so well studied.

And Herman Pontzer notes a difference between humans and animals: "Other animals are too smart to do things as crazy as humans do."

? 2019 AFP