Individual baseline differences outweigh personal traits in short-term heat acclimation adaptations

Previous studies found individual variations during long-term heat acclimation, for short-term heat acclimation (STHA), this remains unclear. This study aimed to examine the impact of individual characteristics on thermophysiological adaptations during STHA. Forty-six service members participated in an STHA protocol in 35.1 ± 0.4 °C and 50 ± 4% RH. Gastrointestinal temperature (T_gi), mean skin temperature (T_sk), heart rate (HR), whole-body sweat loss, and subjective scores were recorded and analysed using mixed-effect modelling. Both end-of and change-during fixed work-rate T_gi, T_sk, and HR showed a decline from day 1 to 5 (p ≤ 0.011). Subjective scores improved (p ≤ 0.005). Fat percentage, body surface area-to-mass ratio, body mass, sex, and age showed relationships (p ≤ 0.05) with one or more outcome measures. STHA results in physiological and subjective benefits, although most of the variance remains unexplained by the recorded characteristics. No individual- or group-level time interactions were found during STHA, indicating the military population adapted uniformly to heat.