Even the most elite endurance athletes can't outrun biology. A new study finds that humans hit a metabolic ceiling at about 2.5 times their resting energy burn.
When ultra-runners take on races that last for hundreds of miles and continue for several days, they are not only challenging their mental endurance and physical strength. They are also exploring the outer boundaries of what the human body can sustain.
According to a study published in the Cell Press journal Current Biology, researchers discovered that even the most seasoned endurance athletes are unable to exceed a "metabolic ceiling" averaging 2.5 times their basal metabolic rate (BMR), which is the body's rate of energy use while at rest.
This metabolic ceiling defines the upper limit of how many calories the body can continuously burn. Earlier studies had proposed that humans might temporarily reach up to 10 times their BMR, but only for brief, intense periods before the body begins to slow down.
"Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question," says lead author and anthropologist Andrew Best of the Massachusetts College of Liberal Arts, who is also an endurance athlete.
"To find out, we asked, if we get a group of really competitive ultra-athletes, can they break this proposed metabolic ceiling?"
Testing Extreme Athletes
The researchers recruited 14 ultra-runners, cyclists, and triathletes and tracked them during competitions and training periods. To allow the researchers to measure energy expenditure, participants drank water containing deuterium and oxygen-18—slightly heavier versions of hydrogen and oxygen. By tracing these molecules when flushed out in urine, the scientists were able to calculate the amount of carbon dioxide an athlete exhales and the number of calories burned.
During multi-day races, some athletes burned six to seven times their BMR, around 7,000 to 8,000 calories a day. But when the team calculated the athletes' caloric burn over longer periods—30 and 52 weeks—their burn rates mostly returned to the predicted ceiling, averaging around 2.4 times their BMR. These results show that even the most extreme athletes reach a metabolic ceiling, and exceeding the limit proves exceptionally difficult, say the researchers.
"If you go over the ceiling for short periods, that's fine. You can make up for it later," says Best. "But long term, it's unsustainable because your body will start to break down its tissue, and you'll shrink."
How the Body Adapts
The study also revealed how the body copes with these extreme endurance activities. As athletes devoted more energy to running, cycling, and swimming, they unconsciously cut back on using energy elsewhere.
"Your brain has a really powerful influence on how much you fidget, how much you want to move, and how encouraged you are to take a nap," says Best. "All these fatigues we feel save calories."
The team noted that the results depended heavily on the individual bodies of the athletes who were recruited. Some exceptional individuals capable of exceeding the ceiling may have been missed. While their findings may have implications for an athlete's performance, they also encourage researchers to investigate how the body's energy cap can shape other essential functions.
"For most of us, we're never going to reach this metabolic ceiling," says Best. "It takes running about 11 miles on average a day for a year to achieve 2.5 times BMR. Most people, including me, would get injured before any sort of energetic limit comes into play."
Reference: "Ultra-endurance athletes and the metabolic ceiling" by Andrew Best, Srishti Sadhir, Emily Hyatt and Herman Pontzer, 20 October 2025, Current Biology.
DOI: 10.1016/j.cub.2025.08.063
This work was supported by funding from Duke University and a Massachusetts College of Liberal Arts Faculty Incentive Award.
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