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

ABSTRACT

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)

KEYWORDS:

• Occupational heat stress
• hydration
• dehydration
• hypohydration
• heat strain
• hyperthermia
• recommendations

List of abbreviations

“BML” – body mass loss

“NIOSH” – National Institute for Occupational Safety and Health

“T_c” – core temperature

“USG” – urine specific gravity

“WBGT” – wet bulb globe temperature

Acknowledgments

We thank the subjects for participating in our study. Additionally, we thank the research staff and other students for assistance during data collection.

Disclosure statement

This article’s contents are solely the responsibility of the authors and do not necessarily represent the official views of the National Institute of Occupational Safety and Health. ZJS has received consultant fees from Otsuka Holdings Co., Ltd. No other conflicts of interest, financial or otherwise, are declared by the authors.

Author contributions

H.W.H. and Z.J.S. conceived and designed research; H.W.H., T.B.B., M.L.T., and Z.J.S. performed experiments; H.W.H. analyzed the data; H.W.H. and Z.J.S. interpreted results of experiments; H.W.H. prepared figures; H.W.H. and Z.J.S. drafted manuscript; H.W.H., T.B.B., M.L.T., D.H., and Z.J.S. edited and revised manuscript; H.W.H., T.B.B., M.L.T., D.H., and Z.J.S. approved final version of manuscript.

Additional information

Funding

This study was supported by awards from the National Institute of Occupational Safety and Health (R01OH011528; to Z.J.S.).