Scientists uncover a hidden limit inside human endurance
November 13, 2025
Ultra-endurance athletes can push their bodies to extraordinary extremes, but even they run into a hard biological wall. Researchers tracked ultra-runners, cyclists, and triathletes over weeks and months, discovering that no matter how intense the effort, the human body maxes out at about 2.5 times its basal metabolic rate when measured long-term. Short bursts of six or seven times BMR are possible, but the body quickly pulls energy away from other functions to compensate, nudging athletes back toward the ceiling.
When ultra-runners prepare for races that span hundreds of miles and last for days, they are not only challenging their determination and physical power. They are also exploring how far human physiology can be pushed. In a study published October 20 in the Cell Press journal Current Biology, researchers reported that even elite endurance athletes cannot consistently exceed an average “metabolic ceiling” equal to 2.5 times their basal metabolic rate (BMR) in daily energy use.
The metabolic ceiling refers to the upper limit of calories a person can burn in a sustained way. Earlier studies suggested that people might reach up to 10 times their BMR, which is the minimum amount of energy needed while resting, but only for short, intense periods.
“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. Science Daily
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