Scientists Finally Answer Whether There is a Limit to Human Endurance

No longer just a cliche, whether there is a limit to human endurance has finally been answered by science.

Scientists have not only shown that there is a limit to human endurance, but they have also even been able to put a number to it.

Limit of Human Endurance Found in Science

Scientists have finally put to rest the question of whether there is a limit to human endurance and have even been able to assign a value of the ceiling.

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A new report from BBC News details the work of scientists to study some of the most extreme athletes in the world to determine what, if any, limit there was on human endurance and reveals that they did in fact conclude that there is an ultimate limit to what the human body could do.

The research, conducted by Duke University scientists, looked at athletes competing in a 140-day 3,080 mile run across the United States and looked at the effect the event was having on their bodies. The scientists measured the resting metabolic rate (RMR), or the number of calories the body burns when it isn't active, both before and during the race and also measured the number of calories burned during the event itself.

The study, published in Science Advances, found that the calories burned started high before falling over time and leveling off around 2.5 times the competitor's RMR.

"You can do really intense stuff for a couple of days," Duke University's Herman Pontzer told BBC News, "but if you want to last longer then you have to dial it back."

For example, running a marathon, a little more than 26 miles, used up 15.6 times as many calories as were burned by a runner's RMR, but such events usually last only a day. The longer the time frame, the closer to the 2.5 times the RMR you get. 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.

Essentially, the longer the body is called upon to perform, the closer you close in on the 2.5 times RMR.

"Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance," Pontzer said. "Nobody we know of has ever pushed through it."

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What makes the 2.5 times RMR the hard limit might be related to the way the human body digests nutrients and converts food into energy. The researchers found that the human body simply cannot digest and process enough food to generate more than the 2.5 times RMR in caloric intake over the long term. The body can take advantage of other energy stores initially, giving that initial burst of calories burned, but once you burn up all the fat and excess muscle in the body, all that's left is caloric intake, which is ultimately the limit of what the human body can do, energy-wise.

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