There’s a Limit to How Many Calories Humans Can Burn—Even Ultramarathoners

OMG, Debbie Downer! Really, I can't drill it for 10 hours every day?

The author doing his daily jog to the peak of Mount Everest.

There’s a Limit to How Many Calories Humans Can Burn—Even Ultramarathoners

Even most elite endurance athletes have metabolic ceiling, research suggests

By Aylin Woodward, WSJ

Jan. 13, 2026 12:00 pm ET

Lots of Americans resolved to hit the gym in the new year. But no matter how fit you are, eventually you hit a limit on the number of calories you can burn without resting, research suggests—even if you’re an elite ultramarathoner.

Ultramarathoners—who typically race distances longer than a marathon—can burn more than 10,000 calories in a single day, but that number isn’t sustainable over months of training. After about six months, these athletes hit a metabolic wall, according to a recent study published in the journal Current Biology.

That long-term limit averages out to about 2.5 times a person’s basal metabolic rate, which is the minimum number of calories the body needs at rest to perform essential functions like breathing, said Andrew Best, a Massachusetts College of Liberal Arts biological anthropologist and co-author of the recent work.

Most of us, regular people, don’t get close to that 2.5 level, at least not on any consistent basis. For a 150-pound person (who needs around 1,700 calories to fuel their body at rest, according to Best), that rate would involve burning some 4,000 calories a day for months. Most of us aren’t fit enough to do that much physical exertion.

The study followed 14 endurance athletes, two women and a dozen men, for about a year of training and competition. The researchers had the athletes drink water containing traceable forms of oxygen and hydrogen, then measured how quickly these molecules were metabolized in urine.

Both the recent study and previous research found athletes can push past that wall for short-term bursts, like a multiday race, burning around seven times their basal metabolic rate, or BMR, a day. But staying at that elevated level for the long term is unsustainable: Athletes start to draw on their fat and protein reserves, so they’re losing body mass, Best said.

“You can absolutely exceed it for hours, days, weeks, even months—but that debt eventually has to get paid back,” Best said.

One study participant completed the roughly 2,200-mile Appalachian Trail in less than two months, exerting about four times his basal metabolic rate for the duration, Best said. But then the athlete had to spend four weeks doing almost nothing, giving his muscles time to recover and regaining some of the weight he lost.

The findings are especially relevant to athletes with long seasons or training arcs, like NFL players heading into the playoffs, or Winter Olympians, according to Herman Pontzer, a professor of evolutionary anthropology and global health at Duke University and study co-author.

“You might have a training program that works great for two months, but it’s actually too high to sustain for five months,” Pontzer said.

Write to Aylin Woodward at aylin.woodward@wsj.com