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 (V^_ O_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.