Body heat storage during physical activity is lower with hot fluid ingestion under conditions that permit full evaporation

Aim To assess whether, under conditions permitting full evaporation, body heat storage during physical activity measured by partitional calorimetry would be lower with warm relative to cold fluid ingestion because of a disproportionate increase in evaporative heat loss potential relative to internal heat transfer with the ingested fluid. Methods Nine males cycled at 50% VO 2max for 75 min at 23.6 ± 0.6 °C and 23 ± 11% RH while consuming water of either 1.5 °C, 10 °C, 37 °C or 50 °C in four 3.2 mL kg −1 boluses. The water was administered 5 min before and 15, 30 and 45 min following the onset of exercise. Results No differences in metabolic heat production, sensible or respiratory heat losses (all P  > 0.05) were observed between fluid temperatures. However, while the increased internal heat loss with cold fluid ingestion was paralleled by similar reductions in evaporative heat loss potential at the skin ( E sk ) with 10 °C ( P  = 0.08) and 1.5 °C ( P  = 0.55) fluid, the increased heat load with warm (50 °C) fluid ingestion was accompanied by a significantly greater E sk ( P  = 0.04). The resultant calorimetric heat storage was lower with 50 °C water ingestion in comparison to 1.5 °C, 10 °C and 37 °C (all P  < 0.05). In contrast, heat storage derived conventionally using thermometry yielded higher values following 50 °C fluid ingestion compared to 1.5 °C ( P  = 0.025). Conclusion Under conditions permitting full sweat evaporation, body heat storage is lower with warm water ingestion, likely because of disproportionate modulations in sweat output arising from warm‐sensitive thermosensors in the esophagus/stomach. Local temperature changes of the rectum following fluid ingestion exacerbate the previously identified error of thermometric heat storage estimations.