Human endurance study finds pregnant women are pushing the limits

Rodiano Bonacci
Giugno 6, 2019

From the Ironman triathlon to the Tour de France, some competitions test the limits of even the toughest endurance athletes.

The study found a pattern between the length of a sporting event and energy expenditure; and that while running a marathon may be beyond many, it is nowhere near the limit of human endurance.

According to Pontzer and John Speakman, of Scotland's University of Aberdeen and the Chinese Academy of Sciences, one explanation may be the digestive tract's ability to break down food.

Not even the world's fastest ultra-marathoners can surpass the 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.

"Secondly, we're talking about endurance over days and weeks and months, so it is most applicable to training regimens and thinking whether they fit with the long-term metabolic limits of the body". "There's just a limit to how many calories our guts can effectively absorb per day", Pontzer noted.

The results of the study were published Wednesday in the journal Science Advances.

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 U.S., 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.

When tracked over time, the data revealed that although athletes' energy expenditure started out high, it eventually plunged and leveled out to burn calories at 2.5 times their resting metabolic rate.

Co-author Caitlin Thurber analyzed urine samples collected during the first and final legs of Race Across the USA. Which indicates that the body can actually "downshift" its metabolism to stay within sustainable levels.

They found the body can not digest, absorb and process enough calories and nutrients to sustain a higher level of energy use.

"You can sprint for 100 meters, but you can jog for miles, right? That's also true here", Pontzer said. During the 23-day long Tour de France, for example, cyclists burned calories at 4.9 times their RMR, and a 95-day trek through Antarctica had participants burning calories at 3.5 times their RMR.

And you don't need to be participating in 100-mile trail runs to reach maximum sustainable energy expenditure: Pregnant and breastfeeding women sustain similar levels of output.

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. "Science works when you're proven wrong. Maybe someone will break through that ceiling some day and show us what we're missing".

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