Energy and Fluid Balance During a 214km Winter Ultra Endurance Race: A Case Study

The upper limits for total energy expenditure (TEE) and water turnover (rH 2 O) in humans have been reported during several continuous single-day ultra-endurance races (running, cycling, triathlon). Currently, the upper limits for TEE and rH 2 O during continuous single-day activity in cold weather (<0C) remain unknown. The Arrowhead Ultra is one of the coldest ultra-endurance 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 214km snow-covered trail (altitude range 345-426m, 2,030m elevation gain). Historically, ~1/2 of the racers complete the event. In this case study, we assessed TEE and rH 2 O from the racer (cyclist, age:22yrs, height:1.84m, body mass:87.7kg, VO 2max :5.0LO 2 .min - 1) who won the 2025 Arrowhead Ultra (17.9hr, - 13- - 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 141bpm (71% of maximum heart rate). TEE was 63.9MJ (15,273kcals, 9.6x basal metabolic rate) while total energy intake was 33.2MJ (7,941kcal). Mean carbohydrate intake was 88g.hr - 1. Water turnover was 17.7L yielding a rH 2 O/TEE ratio of 0.28L.MJ - 1 for the race. The cyclist demonstrated high TEE and rH 2 O that were comparable to values from other ultra-endurance athletes competing in a range of temperatures (3-34C). Notably, rH 2 O from this cyclist was higher compared to athletes performing other ultra-type endeavors in cold weather conditions ( - 25- - 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.