Washington (AFP) - US scientists who studied the performance of myriad athletes -- including adventurous souls who spent five long months running across the United States -- have come up with an estimate of the absolute physiological boundary of human endurance.

The conclusions are pretty technical but the study published Wednesday in the journal Science Advances highlights one feature in particular that makes people unique among primates: their physical endurance is indeed extraordinary.

It is probably something we inherited from hunter-gatherers over the course of two million years, the researchers say. In a nutshell, when it comes to pushing one's body to the limit, humans run circles around monkeys.

The limit to human endurance is measured in multiples of something called basal metabolism, which is the minimum energy, counted in calories, that is expended by the body to keep itself going for one minute.

And the limit to human endurance is about 2.5 times this basal metabolism, says the team, led by Herman Pontzer, a professor of evolutionary anthropology at Duke University.

In athletic events defined as short -- races that last anywhere from hours to weeks, such as a triathlon, a marathon or the Tour de France -- people can crank their metabolism up to five or even 10 times their resting metabolic rate.

But there comes a point in which performance will fall back down to about three times that rate, these scientists say. It is simply impossible for humans to stay above that level for more than a few weeks.

To arrive at this figure the team followed five men and one woman who are considered extreme runners.

From January to June of 2015 they took part in the Race Across the USA -- 4,957 km (3,080 miles) from Los Angeles to Washington, DC. That's the equivalent of doing a marathon a day, six days a week, for 20 weeks.

"All of those people are losing weight. They're burning away their fuel faster than they can put it back in," Pontzer said.

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"What point do I hit where I can finally put as many calories back in as I burn each day?"

The answer, Pontzer said, is 2.5 BMR -- which stands for basal total metabolic rate.

"That's about 4,000 calories a day," he said.

- Crazy humans -

Die-hards who ran across America drank eight liters of water a day in the first week and burned up 6,000 calories per day.

But they kept losing weight all the way through to the end of the race and never reached a state of equilibrium.

"You cannot really take in more than 4,000 calories a day," said Pontzer.

"You can burn more than that, but you're losing weight every day. So that's not really sustainable forever. You can do it for a couple of days, a couple of weeks, but you can't do it forever."

His team says the limit to human endurance is probably linked to people's digestive activity, rather than their muscles or heart, because this was the least common denominator in all of the sports that were studied.

To wit: cycling, running and triathlons cause people to use different muscles. But they all have the same stomach.

While humans stand out among primates in their ability to perform physically, other animals are also good at extreme endurance, such as migratory birds. But this part of the picture has not been studied as thoroughly.

Pontzer notes an important difference between people and animals. "The other animals are too smart to do all these crazy things that people do," he said.