Athletes work hard to push their limits, especially in endurance events like ultramarathons or long-distance cycling races. There are always new records to break and better PRs to achieve, but a new study followed top-tier endurance athletes and found there is, indeed, “a maximum level of exertion humans can sustain in the long term,” according to a Duke University blog post, upending the idea that barriers to athletic achievement are more mental than physical.
The study, which was published in the journal Science Advances earlier this month, assessed energy exertion during some of the most grueling endurance events, and researchers found that all athletes hit the same limit: 2.5 times their resting metabolic rate. Beyond that, their bodies start to break down their own tissues to gain more calories.
“This defines the realm of what’s possible for humans,” Herman Pontzer, associate professor of evolutionary anthropology at Duke and one of the study’s co-authors said.
To probe the limits of stamina, Pontzer and his team tracked the daily calories burned by athletes participating in the 2015 Race Across the USA, a 3,000-mile running race from California to Washington, D.C. It’s a grueling event—the runners had to complete six marathons a week for five months to finish the race. The researchers also studied caloric data from runners competing in 100-mile trail races and other endurance events.
Remarkably, across all the races, the athletes’ energy expenditures followed the same L-shaped curve, with energy output starting out high, then dropping and leveling off at 2.5 times their resting metabolic rate. The data illustrates that the human body is constrained in how much energy it can expend for long periods of time. It’s possible for athletes to achieve a high energy output in short bursts, but not indefinitely.
“You can sprint for 100 meters, but you can jog for miles, right? That’s also true here,” Pontzer said.
But what causes this particular limit? It’s related to the body’s ability to process food and absorb the necessary calories and nutrients. Researchers found that a key factor was the digestive tract’s ability to break down food.
“There’s just a limit to how many calories our guts can effectively absorb per day,” Pontzer said.
Ultrarunners and pro cyclists aren’t the only ones who bump up against this constraint, either. The researchers also compared endurance athletes’ energy output to that of pregnant women, and found that the expecting mothers had only a slightly lower maximum metabolic rate. That suggests that the same bodily limits that keep a runner from sprinting through an ultramarathon could affect a range of other functions, like the size of an unborn baby in the womb.
Although the data is convincing, the researchers concede that perhaps this metabolic benchmark is just another record waiting to be broken.
“I guess it’s a challenge to elite endurance athletes,” Pontzer said. “Maybe someone will break through that ceiling some day and show us what we’re missing.”