Temperature of Ingested Water during Exercise Does Not Affect Body Heat Storage

Purpose: The objective of this study was to examine the effect of ingested water temperature on heat balance during exercise as assessed by direct calorimetry. Methods: Ten healthy males (25 +/- 4 yr) cycled at 50% (V) over dotO(2peak) (equivalent rate of metabolic heat production (M-W) of 523 +/- 84 W) for 75 min under thermocomfortable conditions (25 degrees C, 25% relative humidity) while consuming either hot (50 degrees C) or cold (1.5 degrees C) water. Four 3.2 mL.kg(-1) boluses of hot or cold water were consumed 5 min before and at 15, 30, and 45 min after the onset of exercise. Total heat loss (H-L = evaporative heat loss (H-E) +/- dry heat exchange (H-D)) and M-W were measured by direct and indirect calorimetry, respectively. Change in body heat content (Delta H-b) was calculated as the temporal summation of M-W and H-L and adjusted for changes in heat transfer from the ingested fluid (H-fluid). Results: The absolute difference for H-L (209 +/- 81 kJ) was similar to the absolute difference of H-fluid (204 +/- 36 kJ) between conditions (P = 0.785). Furthermore, the difference in H-L was primarily explained by the corresponding changes in HE (hot: 1538 +/- 393 kJ; cold: 1358 +/- 330 kJ) because H-D was found to be similar between conditions (P = 0.220). Consequently, no difference in Delta H-b was observed between the hot (364 +/- 152 kJ) and cold (363 +/- 134 kJ) conditions (P = 0.971) during exercise. Conclusion: We show that ingestion of hot water elicits a greater H-L relative to cold water ingestion during exercise. However, this response was only compensated for the heat of the ingested fluid as evidenced by similar Delta H-b between conditions. Therefore, our findings indicate that relative to cold water ingestion, consuming hot water does not provide a thermoregulatory advantage. Both hot and cold water ingestion results in the same amount of heat stored during prolonged moderate-intensity exercise.