Voluntary drinking prevents dehydration when following occupational heat stress recommendations

INTRODUCTION Federal and local entities issue guidance that aims to maintain core temperature (T C ) below 38.0°C in unacclimated workers exposed to heat. These heat stress recommendations prescribe work‐rest ratios as a function of wet bulb globe temperature (WBGT) and metabolic heat production (H prod ). Hydration recommendations are less nuanced and suggest drinking 237 mL of a cool sport drink every 15‐20 min for work in the heat exceeding 2 h. This drinking regimen is based on the premise that ad libitum drinking during heat exposure results in dehydration [defined as >2% body mass loss (BML)] due to inadequate drinking. To our knowledge, this hydration guidance has not been examined under heat stress recommendation compliant scenarios. Thus, it is unknown if ad libitum drinking results in dehydration during recommendation compliant work in the heat. PURPOSE Test the hypothesis that ad libitum drinking during heat stress recommendation compliant work in the heat will result in >2% BML. METHODS Six, healthy, unacclimated, euhydrated subjects (2 women; 27±5 y) completed four trials consisting of 4 h (half a workday) of exposure to WBGTs of 23.9±0.2°C (A), 26.3±0.9°C (B), 28.4±0.1°C (C), 29.2±0.7°C (D). Subjects walked on a treadmill evoking a H prod of 481±95 W, which is the average H prod for outdoor workers in the U.S. NIOSH‐compliant work‐rest ratios were prescribed as a function of WBGT and H prod [work:rest per hour (in min) ‐ A: 60:0, B: 45:15, C: 30:30, D: 15:45] and subjects were provided 237 mL of a cool (10±4°C) flavor preferred sport drink (Gatorade) every 15 min and drank ad libitum . BML was determined from changes in nude body mass and correction for fluid intake and urine loss provided an estimate of sweat rate. T C (telemetry pill) is reported as an average over the trial and as the percent of time T C exceeded 38.0°C (Time >38 ). Thirst perception (1 = not thirsty, 9 = very, very thirsty) is reported as an average over the trial. Data are presented as mean±SD (parametric) or median (interquartile range) (nonparametric). RESULTS Average T C was higher in Trial A (37.8±0.4°C; p=0.05) and Trial B (37.6±0.3°C; p=0.03) versus Trial D (37.3±0.4°C) but did not differ between the other trials (p≥0.20). Time >38 did not differ between trials (A: 24 (5,43)%, B: 6 (0,5)%, C: 0 (0,5)%, D: 0 (0,0)%; p=0.17). Sweat rate was higher in Trial A (0.5±0.1 L/h) versus Trial D (0.4±0.1 L/h; p=0.05) but did not differ between any other trials (p≥0.12). Thirst perception (A: 2±1 a.u., B: 2±0 a.u, C: 2±1 a.u, D: 2±0 a.u; p=0.20) and fluid intake (A: 1.6±0.6 L, B: 1.5±0.5 L, C: 1.7±0.5 L, D: 1.6±0.7 L; p=0.88) did not differ between trials. BML did not differ between trials (A: ‐0.9±0.9%, B: ‐0.8±0.6%, C: ‐0.5±0.4%, D: ‐0.3±0.6%; p=0.24). One subject in one trial lost >2% body mass (Trial A: ‐2.7%). CONCLUSION Ad libitum drinking during recommendation compliant work in the heat rarely resulted in dehydration. Modification of work‐rest ratios across a range of WBGTs while working at a fixed H prod mostly maintained T C <38.0°C when ad libitum drinking was permitted. Thus, absolute fluid prescription may not be necessary to prevent dehydration if adhering to occupational heat stress recommendations and cool fluids are readily available.