The human mind and body are capable of great things, but even the best can hit a wall, new research suggests.

In endurance challenges such as the Ironman triathlon or the Tour de France bicycle race, everyone has the same maximum level of exertion they can sustain over the long haul, researchers found.

They concluded that in grueling physical activities that last for days, weeks and months, humans can only burn calories at 2.5 times their resting metabolic rate. Not even the fastest ultra-marathoners can best that limit.

"This defines the realm of what's possible for humans," said researcher Herman Pontzer, an associate professor of evolutionary anthropology at Duke University in Durham, N.C.

No one has ever sustained levels above this limit, Pontzer said. "So I guess it's a challenge to elite endurance athletes," he added in a university news release. "Maybe someone will break through that ceiling someday and show us what we're missing."

Above the limit of 2.5 times a person's resting metabolic rate, the body begins to feed on itself to obtain the additional calories needed to function.

The limit may have to do with digestive capability, the researchers speculated. That's why just eating more won't improve endurance. "There's just a limit to how many calories our guts can effectively absorb per day," Pontzer noted.

For the study, the researchers measured daily calories burned by athletes who ran six marathons a week for five months as part of the 2015 Race Across the USA, which stretched 3,000 miles from California to Washington, D.C. They also looked at other feats of endurance, such as 100-mile trail races and pregnancy.

The investigators found that the mega-marathoners burned 600 fewer calories a day than expected. This suggests that the body can power down its metabolism to keep the body going.

They also found that the maximum sustainable energy expenditure in endurance athletes was only slightly more than women's metabolic rates during pregnancy.

This might mean that the same limits that prevent Ironman triathletes from shattering speed records may also keep babies from growing too big in the womb, the researchers said.

Also, all endurance events revealed the same patterns whether in freezing cold or hot weather temperatures. This belies the notion that endurance is tied to the ability to regulate body temperature, the authors noted.

The report was published June 5 in the journal Science Advances.

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