Energy and fluid balance during a 214-km winter ultraendurance race: a case study

The upper limits for total energy expenditure (TEE) and water turnover (rH₂O) in humans have been reported during several continuous single-day ultraendurance races (running, cycling, and triathlon). Currently, the upper limits for TEE and rH₂O during continuous single-day activity in cold weather (<0 °C) remain unknown. The Arrowhead Ultra is one of the coldest ultraendurance races in North America and provides a unique opportunity to answer these questions. Racers select a bicycle, cross-country skis, or foot travel to traverse a 214-km snow-covered trail (altitude range: 345-426 m; 2,030-m elevation gain). Historically, approximately one-half of the racers complete the event. In this case study, we assessed TEE and rH₂O from the racer [cyclist: age: 22 yr; height: 1.84 m; body mass: 87.7 kg; and maximal oxygen consumption (V̇o_2max): 5.0 LO₂·min^-1] who won the 2025 Arrowhead Ultra (17.9 h; -13 to -1 °C) using the doubly labeled water method. Total energy and fluid intake were recorded to assess energy and fluid balance. Mean heart rate was 141 beats·min^-1 (71% of maximum heart rate). TEE was 63.9 MJ (15,273 kcal; 9.6 times basal metabolic rate) while total energy intake was 33.2 MJ (7,941 kcal). Mean carbohydrate intake was 88 g·h^-1. Water turnover was 17.7 L, yielding a rH₂O/TEE ratio of 0.28 L·MJ^-1 for the race. The cyclist demonstrated high TEE and rH₂O that were comparable to values from other ultraendurance athletes competing in a range of temperatures (3-34°C). Notably, rH₂O from this cyclist was higher compared to athletes performing other ultratype endeavors in cold weather conditions (-25 to -19 °C). Our observations shed light on energy and fluid demands during continuous single-day activity in the cold and have endurance training and performance implications.NEW & NOTEWORTHY In this short report, we detail new data on the upper limits of human energy expenditure (9.6 times basal metabolic rate) and water turnover (10.5 L·12 h^-1) during continuous single-day exercise in the cold. Our results indicated elevated energy use, and impressively, water turnover comparable to exercise in warmer conditions. These findings have implications for training and endurance performance of strenuous exercise in cold weather environments.