The Effect of Dehydration on Heat Acclimation

Minimizing heat-related illness for occupations in hot environments is essential. We hypothesize that short-term heat acclimation (STHA) in a state of hypohydration will improve cardiovascular and perceptual strain associated with heat stress, accelerating heat acclimatization. PURPOSE: To determine the impact of hydration status during STHA on heat adaptations. METHODS: Ten aerobically fit male subjects (23±1.00yrs, 82.1±2.14kg, 53.18±1.63mL•kg-1•min-1 VO2 max, 13.8%±1.31%BF) completed a heat stress test (HST) two days prior to, and following three acclimation bouts over six days. HST/acclimation bouts consisted of 90-minute treadmill walking at 50% VO2 max in hot conditions (40, 30%RH). Additionally, a performance test was conducted that the end of each HST. In a randomized crossover design, fluids were administered during STHA bouts to promote a dehydrated (DEH=0.5 mL/kg) or a euhydrated (EUH=2.0 mL/kg) state. A five-week washout period separated DEH from EUH trials. Participants wore standard wildland firefighter NoMex clothing with a cotton shirt. Peak heart rate (HR), peak rate of perceived exertion (RPE), percent dehydration (%DEH), sweat rate (SR), and peak core temperature (PCT) were analyzed by a 2(trt) X 2(trial)ANOVA. RESULTS: HR and RPE both decreased significantly from PRE to POST with no significant difference between treatments. SR showed a time and trtXtime interaction that nearly reached significance (p=0.054) with greater increases in the DEH trial. Conclusion: Our data suggests STHA reduces physiological and perceptual strain, regardless of hydration status. However controlled dehydration improved SR, suggesting the protocol may lead to greater heat adaptations during STHA