What running ultramarathons and giving birth tell us about the limits of human endurance... We can all only go so far.

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▼ Runners on the 3,080 mile Race Across the USA in 2015. A group of researchers studied people who ran across the United States over a five-month period to understand the limits of human endurance.
Bryce Carlson
Sure, the human body can do amazing things: run hundreds of miles, scale mountains, even grow a new human. But there might be a limit for how long one can go—even for a world-class endurance athlete. A new study suggests that the length of time even highly-trained humans can sustain themselves might be a simple mathematical equation involving the length of the endurance event, and how many calories it’s physically possible to consume.
A group of researchers studied people who ran across the United States over a five-month period and used those results alongside conclusions from other studies on humans engaged in a variety of endurance activities, from races like the Tour de France to pregnancy. By comparing all the data, they found that the maximum amount of energy a human can expend indefinitely is roughly equal to 2.5 times the number of calories they burn at their basal metabolic rate (BMR), or the amount of energy they use while at rest. For the average human included in this new study, that equated to about 4000 calories of energy. The bodies of athletes adjusted over time to function on this amount of calories: In runners engaged in a five-month race across the United States, for instance, the researchers found their metabolisms actually slowed in order to keep within this limit.
The studyis an attempt to map a limit for human endurance, says study author Herman Pontzer, an evolutionary anthropologist at Duke University. “We also found that the limits of how long and how hard you can push your body forever, without ever backing down, are a lot lower than we used to think.”
Until recently, he says, physiologists thought that the limits of exertion weren’t so limited—in other words,  (▪ ▪ ▪)

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