Researchers have identified a hard biological limit on sustained human energy expenditure, capping elite ultra-endurance athletes at about 2.5 times their basal metabolic rate. The study, published in Current Biology, tracked runners, cyclists, and triathletes over extended periods. This ceiling explains why even extreme efforts cannot be maintained indefinitely without physiological costs.
In a study published on October 20 in the journal Current Biology, anthropologists and physiologists revealed that ultra-endurance athletes face a metabolic ceiling of approximately 2.5 times their basal metabolic rate (BMR) for long-term energy use. BMR represents the minimum calories needed at rest, and the ceiling denotes the sustained maximum burnable calories. Lead author Andrew Best, an anthropologist at the Massachusetts College of Liberal Arts and an endurance athlete himself, explained, "Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question."
To investigate, the team monitored 14 elite ultra-runners, cyclists, and triathletes during races and training over 30 to 52 weeks. Participants ingested water enriched with deuterium and oxygen-18 isotopes, allowing researchers to measure carbon dioxide exhalation and total calorie expenditure via urine samples. During multi-day events, some athletes briefly hit six to seven times their BMR, burning 7,000 to 8,000 calories daily. However, averages over longer periods consistently hovered around 2.4 times BMR, indicating the body's inability to sustain higher outputs.
Best noted the consequences of exceeding this limit: "If you go over the ceiling for short periods, that's fine. You can make up for it later. But long term, it's unsustainable because your body will start to break down its tissue, and you'll shrink." The body compensates by reallocating energy, reducing non-essential activities. "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," Best said. "All these fatigues we feel save calories."
The findings, while focused on these athletes, suggest broader implications for human physiology. Best emphasized accessibility: "For most of us, we're never going to reach this metabolic ceiling. 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." The research was supported by Duke University and a Massachusetts College of Liberal Arts Faculty Incentive Award. Journal reference: Andrew Best et al., Current Biology, 2025; DOI: 10.1016/j.cub.2025.08.063.