Sauna exposure immediately prior to short-term heat acclimation accelerates phenotypic adaptation in females

Investigate whether a sauna exposure prior to short-term heat acclimation (HA) accelerates phenotypic adaptation in females.

Randomised, repeated measures, cross-over trial.

Nine females performed two 5-d HA interventions (controlled hyperthermia T_re≥38.5°C), separated by 7-wk, during the follicular phase of the menstrual cycle confirmed by plasma concentrations of 17-β estradiol and progesterone. Prior to each 90-min HA session participants sat for 20-min in either a temperate environment (20°C, 40% RH; HA_temp) wearing shorts and sports bra or a hot environment (50°C, 30% RH) wearing a sauna suit to replicate sauna conditions (HA_sauna). Participants performed a running heat tolerance test (RHTT) 24-h pre and 24-h post HA.

Mean heart rate (HR) (85±4 vs. 68±5 bpm, p≤0.001), sweat rate (0.4±0.2 vs. 0.0±0.0Lh^-1, p≤0.001), and thermal sensation (6±0 vs. 5±1, p=0.050) were higher during the sauna compared to temperate exposure. Resting rectal temperature (T_re) (-0.28±0.16°C), peak T_re (-0.42±0.22°C), resting HR (-10±4 bpm), peak HR (-12±7 bpm), T_re at sweating onset (-0.29±0.17°C) (p≤0.001), thermal sensation (-0.5±0.5; p=0.002), and perceived exertion (-3±2; p≤0.001) reduced during the RHTT, following HA_sauna; but not HA_temp. Plasma volume expansion was greater following HA_sauna (HA_sauna, 9±7%; HA_temp, 1±5%; p=0.013). Sweat rate (p≤0.001) increased and sweat NaCl (p=0.006) reduced during the RHTT following HA_sauna and HA_temp.

This novel strategy initiated HA with an attenuation of thermoregulatory, cardiovascular, and perceptual strain in females due to a measurably greater strain in the sauna compared to temperate exposure when adopted prior to STHA.