Even essentially the most elite endurance athletes cannot outrun biology. A brand new examine finds that people hit a metabolic ceiling at about 2.5 occasions their resting vitality burn.
When ultra-runners tackle races that final for tons of of miles and proceed for a number of days, they aren’t solely difficult their psychological endurance and bodily energy. They’re additionally exploring the outer boundaries of what the human physique can maintain.
In accordance with a examine printed within the Cell Press journal Present Biology, researchers found that even essentially the most seasoned endurance athletes are unable to exceed a “metabolic ceiling” averaging 2.5 occasions their basal metabolic charge (BMR), which is the physique’s charge of vitality use whereas at relaxation.
This metabolic ceiling defines the higher restrict of what number of energy the physique can repeatedly burn. Earlier research had proposed that people would possibly quickly attain as much as 10 occasions their BMR, however just for transient, intense intervals earlier than the physique begins to decelerate.
“Each dwelling factor has a metabolic ceiling, however precisely what that quantity is, and what constrains it, is the query,” says lead writer and anthropologist Andrew Better of the Massachusetts Faculty of Liberal Arts, who can be an endurance athlete.
“To seek out out, we requested, if we get a gaggle of actually aggressive ultra-athletes, can they break this proposed metabolic ceiling?”
Testing Excessive Athletes
The researchers recruited 14 ultra-runners, cyclists, and triathletes and tracked them throughout competitions and coaching intervals. To permit the researchers to measure vitality expenditure, contributors drank water containing deuterium and oxygen-18—barely heavier variations of hydrogen and oxygen. By tracing these molecules when flushed out in urine, the scientists have been capable of calculate the quantity of carbon dioxide an athlete exhales and the variety of energy burned.
Throughout multi-day races, some athletes burned six to seven occasions their BMR, round 7,000 to eight,000 energy a day. However when the crew calculated the athletes’ caloric burn over longer intervals—30 and 52 weeks—their burn charges largely returned to the expected ceiling, averaging round 2.4 occasions their BMR. These outcomes present that even essentially the most excessive athletes attain a metabolic ceiling, and exceeding the restrict proves exceptionally troublesome, say the researchers.
“For those who go over the ceiling for brief intervals, that is fantastic. You can also make up for it later,” says Greatest. “However long run, it is unsustainable as a result of your physique will begin to break down its tissue, and you may shrink.”
How the Physique Adapts
The examine additionally revealed how the physique copes with these excessive endurance actions. As athletes devoted extra vitality to working, biking, and swimming, they unconsciously reduce on utilizing vitality elsewhere.
“Your mind has a very highly effective affect on how a lot you fidget, how a lot you wish to transfer, and the way inspired you’re to take a nap,” says Greatest. “All these fatigues we really feel save energy.”
The crew famous that the outcomes depended closely on the person our bodies of the athletes who have been recruited. Some distinctive people able to exceeding the ceiling might have been missed. Whereas their findings might have implications for an athlete’s efficiency, additionally they encourage researchers to research how the physique’s vitality cap can form different important capabilities.
“For many of us, we’re by no means going to succeed in this metabolic ceiling,” says Greatest. “It takes working about 11 miles on common a day for a yr to realize 2.5 occasions BMR. Most individuals, together with me, would get injured earlier than any form of energetic restrict comes into play.”
Reference: “Extremely-endurance athletes and the metabolic ceiling” by Andrew Greatest, Srishti Sadhir, Emily Hyatt and Herman Pontzer, 20 October 2025, Present Biology.
DOI: 10.1016/j.cub.2025.08.063
This work was supported by funding from Duke College and a Massachusetts Faculty of Liberal Arts School Incentive Award.
