Athletes who can run the equivalent of 117 marathons in just months might seem unstoppable. The biggest obstacle, it turns out, is their own bodies. A new study quantifies for the first time an unsurpassable “ceiling” for endurance activities such as long-distance running and biking—and it also finds that pregnancy’s metabolic toll resembles that of an ultramarathon.
“It’s very cool data,” says Harvard University evolutionary biologist Daniel Lieberman, who wasn’t involved with work. “It makes a very convincing case that at the extremes of human endurance, there’s a hard limit.”
Physiologists and athletes alike have long been interested in just how far the human body can push itself. When exercising over a few hours, a wealth of evidence suggests most people—and mammals—max out at about five times their basal metabolic rate (BMR), or the amount of energy they expend while they’re at rest. How humans use energy during longer endurance activities is another question entirely, says Herman Pontzer, an evolutionary anthropologist at Duke University in Durham, North Carolina.
Pontzer saw an opportunity to answer that question when Bryce Carlson, an endurance athlete and former anthropologist at Purdue University in West Lafayette, Indiana, organized the Race Across the USA in 2015. Runners covered 4957 kilometers over the course of 20 weeks in a series of marathons stretching from Los Angeles, California, to Washington, D.C.
To find out how many calories the athletes in the study burned, Pontzer, Carlson, and colleagues replaced the normal hydrogen and oxygen in their drinking water with harmless, uncommon isotopes of those elements—deuterium and oxygen-18. By chemically tracing how these isotopes flush out in urine, sweat, and exhaled breath, scientists can calculate how much carbon dioxide an athlete produces—a measure that directly relates to how many calories they burn.
Pontzer’s team measured the initial BMRs of six runners, five men and one woman. Then they collected energy expenditure data over the course of the race to see how many calories they burned per day. The researchers plotted those data over time and analyzed them along with previously collected metabolic data from other endurance events, including triathlons, 160-kilometer ultramarathons, long-distance cycling races like the Tour de France, and Arctic expeditions.
They found that no matter the event, energy expenditure sharply leveled off after about 20 days, eventually plateauing at about 2.5 times an athlete’s BMR. At that point, the body is burning calories more quickly than it can absorb food and convert it into energy, representing a biologically determined ceiling on human performance, the researchers report today in Science Advances. After an athlete hits this ceiling, their body must dip into fat reserves for energy. “It was just one of those beautiful moments of discovery that as a scientist you just live for,” Pontzer says. “We ended up plotting out the very limits of human endurance, the envelope for what humans can do.”
Brent Ruby, an exercise physiologist at the University of Montana in Missoula who wasn’t involved in the study, says the new findings demonstrate how ultraendurance athletes can expend energy over long periods without losing body weight.
In a second finding, the authors report that human pregnancy—the energy expenditure of which has been measured in earlier studies—demands about the same level of energy as long athletic endurance events. It is also governed by the same metabolic constraints. “To think about pregnancy in the same terms that we think about Tour de France cyclists and triathletes makes you realize how incredibly demanding pregnancy is on the body,” Pontzer says.
Some researchers, including Lieberman, have hypothesized that humans evolved bodies that can run long distances in order to hunt down large, calorie-rich animals, and that those same metabolic adaptations could have allowed human mothers to birth larger babies with bigger brains. Given that pregnancy and endurance activities operate under the same metabolic rules, it could have been the other way around, Pontzer argues: Perhaps humans evolved to have bigger-brained babies, which then afforded our species more endurance.
On that point, Lieberman isn’t convinced. “That’s a pretty big leap to make and would need a lot more evidence to support it,” he says. “Let’s take it one step at a time—just like a marathon.”
*Correction, 7 June, 12:20 p.m.: The original version of this article incorrectly noted that Brent Ruby implied that athletes should load up on fat before endurance events. He actually said that the findings show that athletes’ bodies adapt to endurance events so they don’t need to dip into their fat stores.