Short term heat acclimation reduces heat stress, but is not augmented by dehydration

Heat acclimation lowers physiological strain when exercising in the heat, and may be enhanced by promoting dehydration during acclimation. The purpose was to compare fluid intake during heat acclimation by promoting dehydration (DEH=0.5 mL kg^-1 15 min^-1, ~2.4% dehydration per acclimation session) compared to euhydration (EUH=2.0 mL kg^-1 15 min^-1, ~1.4% dehydration per acclimation session) following four heat acclimation bouts on thermal strain, and exercise performance. Thirteen males completed 90 min heat stress tests (HST) at 50% VO_2max (40 °C, 30%RH) before and after three 90 min heat acclimation trials, involving consecutive bouts with 4-fold less fluid (DEH) or EUH. DEH and EUH trials were separated by 48 h and assigned in a random crossover design separated by a 5 week washout. Wildland firefighter (WLFF) Nomex: shirt, pants, and a cotton T-shirt baselayer were worn. Peak core temperature (T_c) from the HST significantly decreased following both DEH (39.5 ± 0.1-39.0 ± 0.1 °C: P < 0.001) and EUH acclimation (39.5 ± 0.1-38.9 ± 0.1 °C: P < 0.001). HR, RPE, physiological strain index (PSI), and total work (J) completed in a graded exercise test to exhaustion were improved (P < 0.001) in effect for acclimation, but not different when comparing DEH or EUH fluid delivery. SBF was unchanged (P = 0.313). Sweat rate increased greater following DEH (1.52 ± 0.06-1.89 ± 0.09 L h^-1) compared to EUH acclimation (1.57 ± 0.06-1.79 ± 0.08 L h^-1: P = 0.015). Resting plasma volume increased in effect for acclimation (P = 0.002). Aldosterone decreased in effect for acclimation (P < 0.001) at rest and following exercise, and total protein was unaffected (P = 0.83). In conclusion, short-term heat acclimation (~360 min) attenuates heat stress, and improves exercise capacity in the heat, and was not impaired nor improved by promoting DEH during acclimation.