Carrying an 8-pound being inside your body during the entirety of waking hours is a super-human feat, probably as physically draining as running 5,000 marathons over a terrain of hot coals and quick sand. Now research more or less backs this up: A new study published in Science Advances found, per Quartz, that the “upper limit [of]...energy that human bodies can expend consistently over time, is one pushed by endurance athletes....and people who are pregnant and lactating.”
In the study, published in Science Advances, a group of anthropologists, evolutionary biologists, and public health researchers from around the world studied the metabolic rates of runners who competed in Race Across the USA, a 14-week race spanning more than 3,000 miles, in which competitors cover roughly a marathon per day, six days a week. The study found that metabolic rates in these ultra-long distance runners slowed to nearly the same rate of pregnant people.
Those of us who are not pushing our bodies to such extremes tend to burn a consistent number of calories on a daily basis. Previous studies have shown that triathletes and marathon runners burn calories at a much higher rate during competition, triathletes hitting up to 10 times their normal rate. What this study shows, however, is that when the body is pushed even farther, as in the case of the Race Across the USA runners, while there’s the same initial spike, it eventually finds similar ways to store energy.
“We were able to show that in the face of running a marathon a day, your body finds a way to save calories,” Herman Pontzer, an evolutionary anthropologist at Duke and co-author of the paper, told Quartz.
In other words, when the body is subjected to extreme activity for an extended period of time, after a point, it will adapt, and one of the most extreme things a human body can go through is, apparently, running a 3,000-mile race—or being pregnant. Pontzer hypothesizes on the findings in Quartz:
Humans may have evolved to be capable of incredible endurance feats, which had the side effect of allowing us to have large babies. The reverse could also be true: evolving to have large babies gave us the capabilities to carry out extreme sports over long periods of time. “There’s no reason it can’t be both,” Pontzer says.