Effects of Heat Acclimation Following Heat Acclimatization on Whole Body Heat Exchange in Trained Endurance Athletes

The purpose of this study was to examine the changes in metabolic heat production (H(prod)), evaporative heat loss (H(evap)), and dry heat loss (H(dry)), following heat acclimatization (HAz) and heat acclimation (HA). Twenty-two male endurance athletes (mean ± standard deviation; age, 37 ± 12 y; body mass, 73.4 ± 8.7 kg; height, 178.7 ± 6.8 cm; and VO(2max), 57.1 ± 7.2 mL·kg(−1)·min(−1)) completed three trials (baseline; post-HAz; and post-HA), which consisted of 60 min steady state exercise at 59 ± 2% velocityVO(2max) in the heat (ambient temperature [T(amb)], 35.2 ± 0.6 °C; relative humidity [%rh] 47.5 ± 0.4%). During the trial, VO(2) and RER were collected to calculate H(prod), H(evap), and H(dry). Following the baseline trial, participants completed self-directed outdoor summer training followed by a post-HAz trial. Then, five days of HA were completed over eight days in the heat (T(amb), 38.7 ± 1.1 °C; %rh, 51.2 ± 2.3%). During the HA sessions, participants exercised to maintain hyperthermia (38.50 °C and 39.75 °C) for 60 min. Then, a post-HA trial was performed. There were no differences in H(prod) between the baseline (459 ± 59 W·m(−2)), post-HAz (460 ± 61 W·m(−2)), and post-HA (464 ± 55 W·m(−2), p = 0.866). However, H(evap) was significantly increased post-HA (385 ± 84 W·m(−2)) compared to post-HAz (342 ± 86 W·m(−2), p = 0.043) and the baseline (332 ± 77 W·m(−2), p = 0.037). Additionally, H(dry) was significantly lower at post-HAz (125 ± 8 W·m(−2), p = 0.013) and post-HA (121 ± 10 W·m(−2), p < 0.001) compared to the baseline (128 ± 7 W·m(−2)). H(dry) at post-HA was also lower than post-HAz (p = 0.049). H(prod) did not change following HAz and HA. While H(dry) was decreased following HA, the decrease in H(dry) was smaller than the increases in H(evap). Adaptations in body heat exchange can occur by HA following HAz.