![]() Water throughput for the five race finishers was high in both Week 1 (8.1 ± 2.2 liters/day) and the final week (7.7 ± 1.6 liters/day P = 0.32, paired one-tailed t test), underscoring the importance of water balance and fluid availability in maintaining performance ( 15). The two other subjects who finished the transcontinental run on an accelerated schedule (100 days) had slightly higher estimated average metabolic scopes (3.32 and 3.56), consistent with the shorter duration of their run (table S2 and fig. Metabolic scope averaged across the entire 140-day transcontinental run was 3.11 (range, 2.86 to 3.28 fig. 2), resulting in a decreased metabolic scope (mean, 2.81 range, 2.74 to 2.89). ![]() RMR for these three subjects was essentially unchanged ( P = 0.18, paired t test), trending ~6% greater (table S1 and Fig. At Week 20, their TEE decreased by 1224 kcal/day (range, 791 to 1606 kcal/day) for these three subjects, ~20% down from Week 1 values ( P = 0.02, paired t test) to 4906 ± 864 kcal/day. For the three subjects with RMR measurements, mean metabolic scope (i.e., TEE/RMR) increased from 1.76 (range, 1.59 to 1.92) Pre-Race to 3.76 (range, 3.08 to 4.13) during Week 1. Week 1 TEE measured during the first five consecutive days of marathon running (6202 ± 881 kcal/day) was elevated from Pre-Race, differing by only 2% from the increase predicted from running workload (table S2, Fig. Resting metabolic rate (RMR) was measured in three of the racers at Week 1 and Week 20 via respirometry. Using the doubly labeled water (DLW) method, we measured TEE in six RAUSA athletes (table S2) immediately before the start of the race (Pre-Race), during the first week of the race (Week 1), and during the final week of the race. RAUSA athletes ran approximately one marathon (42.2 km) per day, 6 days per week, for 14 to 20 weeks from the Pacific (Huntington Beach, CA) to the Atlantic coast (Washington, DC), making it the longest event to date with associated metabolic measures. Metabolic data from RAUSA athletes provide a detailed examination of energy expenditure over the course of an extreme endurance event. The limits and timing of this metabolic compensation are unclear, particularly under extreme workloads. Complicating the relationship between event duration and SusMS, individuals respond to sustained increases in physical activity by reducing total energy expenditure (TEE) ( 17, 18). The estimated SusMS of ~4× to 5× BMR came from the 23-day Tour de France, but athletes in 11-hour triathlons and 25-hour ultramarathons exhibit metabolic scopes of 9.4× and 8.5× BMR, respectively ( 15), and a metabolic scope of 6.6× BMR was measured over a 10-day period of arctic trekking ( 16). However, analogous to the relationship for short-duration events between maximum running speed and race distance ( 14), evidence suggests that SusMS may decrease over longer time periods. The effect of event duration on SusMS has been hypothesized ( 3) but has never been quantified and has often been ignored. Seminal studies have suggested that maximum sustained metabolic scope (SusMS) is capped at ~4× to 5× BMR in humans and other endotherms ( 1– 3), approaching ~7× BMR for some rodents under cold conditions ( 4, 13). Transcontinental race data suggest that humans can partially reduce TEE during long events to extend endurance.Ĭonsiderable interest has focused on the physiological limits on energy expenditure, which is often expressed as a multiple of basal metabolic rate (BMR), also called “metabolic scope,” to account for differences in body size ( 1– 13). Incorporating data from overfeeding studies, we find evidence for an alimentary energy supply limit in humans of ~2.5× BMR greater expenditure requires drawing down the body’s energy stores. This relationship differs from that of shorter events (e.g., marathons). For events lasting 0.5 to 250+ days, SusMS decreases curvilinearly with event duration, plateauing below 3× BMR. We compiled measurements of total energy expenditure (TEE) and basal metabolic rate (BMR) from human endurance events and added new data from adults running ~250 km/week for 20 weeks in a transcontinental race. ![]() ![]() Here, we show that sustained expenditure in humans, measured as maximum sustained metabolic scope (SusMS), is a function of event duration. The limits on maximum sustained energy expenditure are unclear but are of interest because they constrain reproduction, thermoregulation, and physical activity.
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