Mind and Body

Study pinpoints the limit of human endurance -- and it's impressively high

"The limits of endurance appear to be non-negotiable."

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As limitless as we like to believe human resilience to be, biology really does impose a hard stop on how hard you can go. Which bigs the question: Where does the limit lie? In 2017, a brave group of endurance athletes looked the challenge square in the face. The data they generated has allowed scientists to pinpoint just where that breaking point is.

The results, published in June 2019 in Science Advances, found that when the athletes burned about 2.5 times their resting metabolic rate (the number of calories consumed per day), their bodies couldn’t keep up with the energy demands. (For an average person, this would be around 4,000 calories per day.)

This is #8 on Inverse’s list of 25 striking lessons from 2019 for humans to help maximize our potential.

Back in 2017, six runners attempted to run back-to-back marathons from coast-to-coast across the United States for 20 weeks. Only three finished on schedule; one dropped out, and two decided to go off-route. The athletes had a team of scientists along with them, observing to see what happened to their bodies during what worked out as a grueling six marathons-a-week schedule.

They were searching for the body’s “alimentary limit.” That is, the point at which it is impossible to take in enough calories to keep your body functioning.

After running 20 weeks of running marathons for six days per week, the scientists were able to identify an "alimentary limit" of human endurance. Shutterstock 

“The limits of endurance appear to be non-negotiable,” Herman Pontzer, the study’s senior author, told Inverse at the time.

“They impose a hard limit on what humans can do.” 

The idea of an alimentary limit comes in useful if you’re someone like an Arctic trekker attempting to cross the frozen tundra. To avoid dying from exposure, you’ll want to go as fast as possible, which means you have to expend more energy. To do that, you’ll have to fuel your body with the requisite amounts of fats and carbohydrates.

If you’re expending calories below the rate of 2.5 times your resting metabolic rate, you’ll probably be able to make it while maintaining your breakneck pace, Pontzer said. But if you hit that ceiling, your body simply won’t be able to replenish the resources lost. Even as the body starts to tap into fat stores and muscle tissues to compensate, they won’t sustain you for long.

““If [athletes] tried to keep at that pace indefinitely, their bodies would shut down,” he said.

The good news is that it takes an impressive feat of athleticism to come anywhere even close to this alimentary limit. The subjects in this experiment ran marathons six days per week for months before this limit became clear to scientists. In fact, the runners’ bodies seemed to automatically shy away from going over that limit.

As the races progressed, the athletes began burning fewer and fewer calories. By the end, they were burning about 600 fewer kilocalories per day than they were at the beginning of the six-week period. That suggests that their bodies made energy sacrifices elsewhere to keep overall levels from going beyond that unsustainable limit.

Although the study does impose limits on human athletic potential, it also opens up avenues for surpassing them. Now that scientists know just how much our bodies can take, there’s a chance that we may one day find ways to expand those limits.

As 2019 draws to a close, Inverse is revisiting 25 striking lessons for humans to help maximize our potential. This is #8. Some are awe-inspiring, some offer practical tips, and some give a glimpse of the future. Read the original article here.

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