Human endurance has a metabolic ceiling

Henrietta Strickland
June 8, 2019

The researchers measured this by starting with an average person's Basal Metabolic Rate - the number of calories burned during a day of rest - and judging how many calories from food an athlete's body can convert into energy each day, over the course of many days of progressive exercise. The human body has limitations to maintain high levels of energy for an extended period of time. 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.

Beyond that threshold of 2.5 times a person's metabolic rate, they found that the body starts to break down its own tissues to make up for the calorie deficit.

Furthermore, the maximum sustainable energy expenditure among endurance athletes are only a little bit higher than the metabolic rates sustained during pregnancy. The 2.5 times RMR is the hard limit endurance of the body, and it is related to the human body digestion and energy.

Scientists have dispelled the myth that the human body can be limitless.

A study of how much energy athletes expend as they undertake some of the world's longest and most grueling sporting events has found that there is an absolute ceiling to how many calories we can burn, no matter how much we eat.

"This defines the realm of what's possible for humans", says one of the team, evolutionary anthropologist Herman Pontzer from Duke University in North Carolina.

"There's just a limit to how many calories our guts can effectively absorb per day", Pontzer explained in an article outlining the research.

Carrying a baby during pregnancy and performing in extreme endurance sports hit the same mark in pushing the human body to its limit, suggests a study.

Resting metabolic rate - the calories the body burns through when it is relaxing - was recorded before and during the race.

"You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back", said Pontzer of the findings, which could be especially useful in endurance races such as the Tour de France. Pontzer and colleagues reported their findings in Science Advances, in a paper titled, "Extreme events reveal an alimentary limit on sustained maximal energy expenditure". "They're burning away their fuel faster than they can put it back in", Pontzer said.

This challenges the idea proposed by previous researchthat it was humans' ability to regulate our body temperature that curtailed our endurance.

Finally, they have mapped all the barriers of human endurance after collecting every data from every event and measurements.

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 United States of America, 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.

'Science works when you're proven wrong.

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