Total Energy Expenditure and Water Turnover During a 322 Km Mountain Ultramarathon

High doubly labeled water (DLW) derived total energy expenditure (TEE) and water turnover (rH₂O) occur during 161km (100-mile) ultramarathons. TEE and rH₂O during races exceeding 161km and requiring multiple days to complete constitute an anomalous physiological stressor to further elucidate duration-dependent limits of human endurance. PURPOSE: To quantify TEE and rH₂O during a multi-day mountain ultramarathon. METHODS: Two runners (female [R1], male [R2]), completed 322km following DLW (²H₂¹⁸O) consumption. TEE and rH₂O were calculated from urinary isotopic elimination rates. Race-provided GPS monitors measured duration, moving time, and non-activity time. RESULTS: R1 finished in 73.6h at a pace of 1.3 m·s^-1 with non-activity/rest time totaling 32% of the race (50.3h moving, 23.3h resting). In-race TEE for R1 was 123.9 MJ (29,376 kcals) and 35.5 MJ·d^-1 (8,408 kcals·d^-1, 5.6 times the basal metabolic rate [×BMR]) when expressed per 24h. In-race rH₂O for R1 was 35.2 L (10.1 L·d-1). R2 finished in 89.9h at a pace of 1.0 m·s^-1 with non-activity/rest time totaling 52% of the race (43.0h moving, 46.9h resting). In-race TEE for R2 was 164.2 MJ (38,909 kcals) and 41.1 MJ·d^-1 (9,737 kcals·d^-1, 5.0 ×BMR) when expressed per 24h. In-race rH₂O for R2 was 47.5 L (11.9 L·d^-1). CONCLUSIONS: Akin to 161km events, multi-day events are exceptional human endurance feats, demonstrating large absolute TEE and rH₂O values. However, the daily TEE range of multi-day events appears diluted by unique non-activity time needs (rest, sleep). Thus, the human endurance metabolic ceiling will be largely contingent upon event duration.