Creatinine clearance is maintained in a range of wet-bulb globe temperatures and work-rest ratios during simulated occupational heat stress.

We tested the hypothesis that compliance with the National Institute for Occupational Safety and Health (NIOSH) heat stress recommendations will prevent reductions in glomerular filtration rate (GFR) across a range of wet bulb globe temperatures (WBGTs) and work-rest ratios at a fixed work intensity. We also tested the hypothesis that non-compliance would result in a reduction in GFR compared to a work-rest matched compliant trial. Twelve healthy adults completed five trials (four NIOSH compliant, one non-compliant) that consisted of four hours of exposure to a range of WBGTs. Subjects walked on a treadmill (Hprod: ~430 W) and work-rest ratios (work per hour: 60, 45, 30, 15 min) were prescribed as a function of WBGT (24°C, 26.5°C, 28.5°C, 30°C, 36°C), and subjects drank a sport drink ad libitum. Peak core temperature (TC) and percentage change in body weight (%DBW) were measured. Creatinine clearance measured pre- and postexposure provided a primary marker of GFR. Peak TC did not differ among NIOSH compliant trials (p=0.065) but differed between compliant vs. non-compliant trials (p<0.001). %DBW did not differ among NIOSH compliant trials (p=0.131) or between compliant vs. non-compliant trials (p=0.185). Creatinine clearance did not change or differ among compliant trials (p³0.079). Creatinine clearance did not change or differ between compliant vs. non-compliant trials (p³0.661). Compliance with the NIOSH recommendations maintained GFR. Surprisingly, despite a greater heat strain in a non-compliant trial, GFR was maintained highlighting the potential relative importance of hydration.

Ad libitum drinking prevents dehydration during physical work in the heat when adhering to occupational heat stress recommendations.

Government entities issue recommendations that aim to maintain core temperature below 38.0°C and prevent dehydration [>2% body mass loss] in unacclimated workers exposed to heat. Hydration recommendations suggest drinking 237 mL of a cool sport drink every 15-20 min. This is based on the premise that ad libitum drinking results in dehydration due to inadequate fluid replacement, but this has never been examined in the background of recommendation compliant work in the heat. Therefore, we tested the hypothesis that ad libitum drinking results in >2% body mass loss during heat stress recommendation compliant work. Ten subjects completed four trials consisting of 4 hours of exposure to wet bulb globe temperatures (WBGT) of 24.1 ± 0.3°C (A), 26.6 ± 0.2°C (B), 28.5 ± 0.2°C (C), 29.3 ± 0.6°C (D). Subjects walked on a treadmill and work-rest ratios were prescribed as a function of WBGT [work:rest per hour - A: 60:0, B: 45:15, C: 30:30, D: 15:45] and were provided 237 mL of a cool sport drink every 15 min to drink ad libitum. Mean core temperature was higher in Trial A (37.8 ± 0.4°C; p = 0.03) and Trial B (37.6 ± 0.3°C; p = 0.01) versus Trial D (37.3 ± 0.3°C) but did not differ between the other trials (p ≥ 0.20). Body mass loss (A: -0.9 ± 0.7%, B: -0.7 ± 0.5%, C: -0.3 ± 0.5%, D: -0.4 ± 0.6%) was greater in Trial A compared to Trial D (p = 0.04) and was different from 2% body mass loss in all trials (p ≤ 0.01). Ad libitum drinking during recommendation compliant work in the heat rarely resulted in dehydration. Registered Clinical Trial (NCT04767347).