Lead author Andrew Best, PhD, a biological anthropologist at the Massachusetts College of Liberal Arts, explains the central issue: “Every living thing has a metabolic ceiling, but exactly what that number is, and what constrains it, is the question.”
Methods: Measuring Calorie Burn in Extreme Athletes
Researchers recruited 14 ultramarathoners, cyclists, and triathletes to monitor training and competition periods. To estimate energy expenditure, participants consumed water enriched with deuterium and oxygen-18, heavier isotopes that are excreted in urine. Tracking these isotopes allowed scientists to infer carbon dioxide output and, consequently, calories burned during exertion.
Findings showed that some athletes burned six to seven times their basal metabolic rate (BMR) during multi-day races, equating to roughly 7,000–8,000 calories per day. Basal metabolic rate represents the minimum energy required for essential bodily functions at rest, such as breathing and thermoregulation.
For reference, the study notes typical BMR values: about 1,696 calories per day for the average man and about 1,410 calories per day for the average woman. However, sustained ultra-endurance efforts at 7,000–8,000 calories daily are not sustainable long-term.
Long-Term Burn Rates and Ceiling Estimates
When the researchers revisited calorie burn over longer periods—30 and 52 weeks—the athletes’ average burn aligned more closely with 2.5 times their BMR, a far lower figure than the 10× BMR ceiling suggested by some prior work. This confirms that while extreme athletes can temporarily push metabolism higher, there remains a physiological ceiling that cannot be maintained indefinitely.
Rutgers University sports medicine specialist Jill Kropa, MD, explains: “The body has ways to compensate to prevent this expenditure from being sustained for too long, thereby capping the overall long-term expenditure.”
Practical Implications for Everyday Fitness
For the typical gym-goer, reaching a metabolic ceiling is unlikely. The scenario would require sustained, extreme endurance that most people cannot or should not attempt. To contextualize the effort needed: running about 11 miles a day for a year would be necessary to reach roughly 2.5× BMR—a level that most individuals would be unlikely to sustain without injury.
Best cautions that while pushing metabolic limits might be scientifically interesting, most individuals should not chase such extreme expenditure. Prolonged high energy output can amplify the risk of musculoskeletal damage and prolong recovery times.
According to Kropa, the ceiling serves a protective purpose: exceeding it long-term can lead to tissue breakdown and systemic fatigue. Best adds that the brain also responds by reducing activity—altering movement, motivation, and even sleep—to conserve energy.
Strategies for Safe, Sustainable Improvement
For most non-professional athletes, sustained, consistent physical activity provides the best metabolic and health benefits. A single high-intensity workout is unlikely to produce lasting metabolic gains. Instead, experts recommend a balanced approach that supports durability and long-term adherence.
Specific guidance from the experts includes ensuring adequate energy intake, particularly carbohydrates, to fuel training. Once energy availability is sufficient, increasing training volume and durability becomes feasible and can support gradual improvements in metabolic efficiency without risking overtraining.
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