Thermal and cardiovascular heat adaptations in active adolescents following summer

This study aimed to investigate seasonal heat acclimatization in active adolescents following summer. Fifteen (5 females) active adolescents (14.6 ± 1.0 y) completed a 45-min heat response test (HRT) walking at 60% $$ O_2peak in 40°C and 30% relative humidity before and after summer (i.e. November 2022 and March 2023). During the HRT, gastro-intestinal temperature (T_gi), skin temperature (T_sk), heart rate, local sweat rate (LSR) and whole-body sweat loss (WBSL) were recorded. Carbon monoxide rebreathing and dual-energy X-ray absorptiometry scans determined resting hematological measures and body composition. Participants completed physical activity (PA) diaries and wore an accelerometer for two one-week periods (pre- and post-summer). Daytime wet-bulb globe temperature (WBGT) was calculated for each summer day. Data are presented as posterior mean and 90% credible intervals. Participants reported 7 ± 4 h·wk^-1 of outdoor PA, and daytime WBGT was 21.2 ± 4.6°C. Following summer, resting T_gi and heart rate were reduced by 0.2°C [-0.3, -0.1; probability of direction = 99%] and 7 beats·min^-1 [-10, -3; 100%], respectively. During the HRT, there was an earlier onset of sweating (-0.2°C [-0.3, -0.0; 98%]), an attenuated rise of T_gi (0.2°C [-0.5, 0.0; 92%]) and mean T_sk changed by -0.2°C [-0.5, 0.1; 86%]. There was minimal evidence for heat adaptations in LSR or WBSL, hematological parameters or perceptual measures. This is the first study to demonstrate seasonal heat adaptations in active adolescents. Reductions in resting T_gi and exercising T_sk and a lower T_gi at the onset of sweating were associated with a smaller rise in T_gi during the HRT following summer.