Ad libitum fluid intake during simulated occupational heat stress prevents dehydration in adults with obesity

Hydration recommendations for workers aim to limit dehydration to <2% body mass loss (BML). It has been previously demonstrated that dehydration is prevented with ad libitum drinking if adhering to occupational heat stress recommendations (i.e., work-rest ratios and/or work intensity [H prod ] is prescribed based on wet bulb globe temperature [WBGT] in adults without obesity. However, adults with obesity make up ~40 % of the U.S. workforce frequently exposed to occupational heat stress and it is unknown if ad libitum drinking will have the same outcome in this population. Purpose: To test the hypothesis that ad libitum fluid consumption does not prevent dehydration in adults with obesity during simulated occupational heat stress. Methods: Seventeen unacclimated adults with (n=5; 3 women; 37±4% body fat) or without obesity (n=12; 5 women; 21±6% body fat) completed a 4-hour (half workday) exposure to a fixed WBGT and work intensity. Work-rest ratio was 30 min of walking and 30 min of seated rest each hour. Work intensity (i.e., H prod ) was prescribed as a function of WBGT (27.6±0.4°C) and normalized to body mass (5.1±0.2 W/kg). All subjects were provided 237 mL of Gatorade every 15 min and drank ad libitum. Mean T C is reported for the 4-hour exposure. Percent changes in body mass provided an overall index of hydration status. Total fluid intake was quantified and sweat rate was calculated during the 4-hour exposure. Urine specific gravity (USG) was measured pre-, postexposure, and following a 1-hour recovery. Data are presented as mean±SD. Results: Mean T C differed between the obese (37.9±0.2°C) and non obese (37.3±0.3°C; p<0.01) groups. There was no difference in % body mass change between groups with (-0.6 ± 0.7 %) or without obesity (-0.5 ± 0.7 %; p=0.68) and both groups were less than 2% body mass loss (p<0.01). There was no difference in sweat rate between groups with (0.5 ± 0.1 L/hr) or without obesity (0.4 ± 0.1 L/hr; p=0.08) and total ad libitum consumption of sport drink was not different (obese: 1.8 ± 0.8 L; p=0.91, non obese: 1.7 ± 0.7 L). However, USG was elevated from pre-exposure following a 1-hour recovery in adults with obesity (+0.012 ± 0.005) compared to adults without obesity (0.000 ± 0.008); p=0.01). Conclusion: In contrast with our hypothesis, ad libitum drinking prevented dehydration in both adults with and without obesity during a four hour exposure. This finding is encouraging given the higher mean T c in adults with obesity. Sweating in adults with obesity was not different from non-obese adults, indicating insufficient evaporative heat loss and/or reduced convective heat loss due to the isolative effects of adipose tissue. Thus, adequate ad libitum fluid replacement likely prevented the compounding effects of heat stress and dehydration on T c in adults with and without obesity. These findings should be confirmed in longer exposures. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.