October 20, 2025
3 min read
Is There a Fundamental Limit to Human Metabolism? New Research Suggests Yes
A new study finds that even elite endurance athletes run up against a hard metabolic ceiling
Scientists have long hypothesized about a theoretical limit to human energy use. Yet elite endurance athletes are constantly pushing these biological boundaries. Now a new study that followed ultramarathon runners for up to a year suggests an answer.
The research, published today in Current Biology, showed that ultramarathoners can burn an astounding 11,000 calories per day during competition. But they can’t keep up these efforts for long—and they pay a price for doing so. The findings home in on a biological limit that even the world’s fittest athletes seem unable to break.
“I get the sense a lot of people were surprised” by the findings, says study co-author Andrew Best, a biological anthropologist at Massachusetts College of Liberal Arts, who is himself a marathon runner. “I was surprised, too…. We had a guy who ran 4,500 miles a year on trails—pretty rugged stuff—and he was well under the limit.”
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The study builds on decades of research aimed at determining the human body’s metabolic ceiling—the maximum sustained rate of calories our bodies can tolerate burning. Foundational research from the 1980s and 1990s based on the 23-day Tour de France bike race set that limit at four to five times a person’s basal metabolic rate (BMR), defined as the energy required to maintain the body at rest. But subsequent studies from shorter races revealed that endurance athletes reach 9.4 times BMR in 11-hour Ironman triathlons and 8.5 times BMR in 25-hour ultramarathons. Scientists proposed in 2019 that the metabolic ceiling depends on the durationof exertion: extremely high-calorie burn is possible for short bursts, but metabolic rate cannot exceed 2.5 times BMR when averaged over periods of training and competition lasting 28 weeks or longer.
Average people and hobby joggers will never approach that apparent limit, which amounts to burning roughly 3,750 calories per day for a lean, 150-pound person. “This work was really meant to test this 2.5-times hypothesis in a population of people who should be uniquely suited to potentially break that ceiling,” Best says.
In the study, the researchers followed 12 male and two female ultraendurance athletes over the course of a year—the longest duration for any study of its kind. Most participants were professional athletes. Ten competed primarily in ultramarathons and several others specialized in Ironmans, multiday triathlons and cycling races.
The researchers used a state-of-the-art method that involves having athletes drink doubly labeled water, or water containing heavier elemental forms of hydrogen and oxygen, and measuring how quickly the body metabolizes it through urine tests. They found that athletes reached up to seven times their baseline rate during races, but metabolic rates always fell to around 2.5 times BMR or below after 30 weeks.
Sustaining exercise near the metabolic ceiling comes at a cost, says Amanda McGrosky, an evolutionary anthropologist at Elon University, who wasn’t involved in the study. The specific functions the body sacrifices during intense physical activity are still under investigation, but McGrosky cites early research indicating that it compensates by slowing digestion, weakening immune responses and temporarily shrinking brain tissue. And there’s some evidence that people invest less energy in sexual arousal and reproduction after races.
Experts caution that the study sample was small. If the proposed metabolic ceiling does represent a true human limit, then athletes could benefit from understanding how close to that limit they get during training and competitions, says Bryce Carlson, a former anthropology assistant professor and world-record-holding ultraendurance athlete, who wasn’t involved in the new research. He says the most useful next step would be to identify the mechanism behind this limit and determine whether athletes can train their bodies to more closely approach it.
It is possible that this energetic limit is higher than what researchers observed in this study. Carlson and others hypothesize that the metabolic ceiling fundamentally reflects a limit in digestion and nutrient absorption, which fuel the body’s muscles. Yet advances in sports nutrition mean athletes can burn more calories than ever—some of today’s best ultramarathoners have been known to consume a whopping 500 calories per hour for 15 hours.
For now, scientists can only speculate. “Is it an absolute human limit that we will never be able to surpass?” Carlson asks. “Or is it a historically contingent limit that we just haven’t passed yet?”
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