Ultra-endurance athletes and the metabolic ceiling.

Endurance limits, measured as the maximum sustainable rate of caloric expenditure, constrain human performance from athletics to manual labor. Early work on the metabolic limits of humans and other species focused on establishing a single estimate for maximal sustained expenditure, often expressed as a multiple of basal metabolic rate (BMR; kcal/day)^1,2,3,4 and known as "metabolic scope" (MS). More recently, Thurber and colleagues⁵ proposed a duration-dependent metabolic ceiling for humans, in which the limit of sustained expenditure falls in a semi-log manner from ∼10× BMR for events lasting 1 day and reaches an asymptote of ∼2.5× BMR at approximately 28 weeks. We tested this metabolic ceiling model in 14 highly trained, elite, and world class⁶ ultra-endurance athletes (12 males and 2 females; 37.2 ± 7.0 years). We measured MS (total energy expenditure [TEE]/BMR) using doubly labeled water during ultra-endurance competitions spanning ∼24 h to ∼13 days, and lower- and higher-workload training weeks, to generate predictive equations for MS as a function of exercise volume. We then applied these equations to participants' training records to calculate their maximum MS for 1-, 3-, 6-, 12-, 30-, and 52-week periods. Maximum MS for competitions and training periods up to 12 weeks approached, but never exceeded, the proposed metabolic ceiling. Four athletes exceeded 2.5× BMR at 30 and 52 weeks (maximum: 2.74), but mean MS for the sample at 30 weeks (2.43 ± 0.17; TEE: 4,085 ± 642 kcal/day) and 52 weeks (2.39 ± 0.17; TEE: 4,020 ± 641 kcal/day) did not exceed the proposed limit. These results suggest that exceeding the hypothesized duration-dependent limit of energy expenditure is rare.

Duke Scholars

Author Herman Pontzer Evolutionary Anthropology