Fluid balance during hot-dry and warm-humid heat stress while adhering to occupational hydration recommendations.

PURPOSE: Current National Institute for Occupational Safety and Health (NIOSH) hydration recommendations advise consuming approximately 237 mL of water every 15 to 20 min during work in the heat to limit body mass loss (BML) to <2%. However, whether fluid balance differs between hot-dry (HD) and warm-humid (WH) environments when workers adhere to these recommendations remains unclear. Therefore, this study evaluated fluid balance during simulated work in HD and WH conditions while following the prescribed hydration strategy. METHODS: Twenty healthy adults (10 females; 24 ± 5 y) completed 2 randomized crossover trials consisting of 120 min of moderate-intensity intermittent walking (338 W) under HD (40.6 °C, 20% relative humidity [RH], wet-bulb globe temperature [WBGT] of 29.3 °C), and WH (30 °C, 80% RH, WBGT of 28.3 °C) conditions. Participants consumed 237 mL of cold water every 20 min. Outcomes included body mass change, whole-body sweat rate (WBSR), net fluid balance, hydration efficiency, hydration biomarkers, thirst sensation, rectal temperature, and heart rate. RESULTS: No participant exceeded the 2% BML threshold. WBSR was greater in HD than WH (0.53 ± 0.13 versus 0.35 ± 0.10 L/h, P < 0.001). Conversely, body mass gain (1.1 ± 0.5 versus 0.6 ± 0.4%), net fluid balance (0.70 ± 0.27 versus 0.40 ± 0.26 L), and hydration efficiency (48.9 ± 19.3 versus 28.2 ± 18.0%) were greater in WH (all P < 0.001). Hydration biomarkers and thirst sensation did not differ between conditions (all P > 0.05). CONCLUSIONS: Current NIOSH hydration recommendations prevented >2% BML during moderate-intensity work in both HD and WH environments; however, the prescribed fixed-volume hydration strategy resulted in positive fluid balance and body mass gain, indicating that it overprescribed fluid intake under the conditions examined. Future occupational hydration recommendations should consider environmental conditions and individual fluid requirements rather than relying solely on a universal fixed-volume strategy. Ad libitum drinking may also represent an acceptable alternative for maintaining hydration during occupational heat exposure.