The human body is capable of amazing things. Some of us can run for miles, climb the tallest mountain, and even grow an entire human being inside us. But even all the cool things we can do have a limit, and scientists think they may have just pinpointed it.

A new study indicates that even the most highly-trained among us are capable of carrying out physically demanding tasks up to a certain time limit. Apparently, it’s all down to mathematical equation involving how many calories it’s possible for a person to eat.

A group of researchers from Duke University in the US studied endurance runners that ran across the country over a five-month period. They used their findings there to correlate it to other kinds of endurance activities, everything from the Tour de France to giving birth. They found that the maximum energy a human can expend is roughly 2.5 times the number of calories they burn at rest, called the basal metabolic rate (BMR).

For the average person, that limit is 4000 calories of energy. Endurance athletes, through their training, actually condition their bodies to better use this limit, slowing down their metabolism in order to prolong the activity.

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Until recently, that energy limit wasn’t considered to be so hard and fast. This new data suggests however that, if a person expends more energy than 2.5 times their BMR in a day, their body actually begins to consume itself, despite eating the maximum amount of calories.

Once that happens, your body is actually starting to burn your critical energy reserves, usually in the form of stored fat. And that’s the emergency supply, so it can only go on for so long.

“If you are burning more calories than you can put back every day that’s obviously not sustainable forever,” says Dr Herman Pontzer, “because you’ll disappear. But if you’re at least at that breakeven point, then at least theoretically, you could do it forever. You might not want to, and you probably wouldn’t, but you could.”

Reuters

Though the athletes the team studied started out expending higher than 2.5 times their BMR, that level gradually went down over time. Pregnant and breastfeeding women also max out at 2.2 times their BMR, but the researchers suspect there might be a skew in the results, thanks to the weight gain from the growing fetus. But just the fact that the energy expenditure required to have a baby is so high, almost as much as a marathon runner, shows just how physically taxing it is.

So, you know, don’t knock it when people say childbirth is hard. There’s concrete proof now.

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