Sweat rate and sweat composition following active or passive heat re-acclimation: A pilot study

The purpose of this study was to investigate local sweat rate (LSR) and sweat composition before and after active or passive heat re-acclimation (HRA). Fifteen participants completed four standardized heat stress tests (HST): before and after ten days of controlled hyperthermia (CH) heat acclimation (HA), and before and after five days of HRA. Each HST consisted of 35 min of cycling at 1.5W·kg^-1 body mass (33°C and 65% relative humidity), followed by a graded exercise test. For HRA, participants were re-exposed to either CH (CH-CH, n = 6), hot water immersion (water temperature ~40°C for 40 min; CH-HWI, n = 5) or control (CH-CON, n = 4). LSR, sweat sodium, chloride, lactate and potassium concentrations were determined on the arm and back. LSR increased following HA (arm +18%; back +41%, P ≤  0.03) and HRA (CH-CH: arm +31%; back +45%; CH-HWI: arm +65%; back +49%; CH-CON arm +11%; back +11%, P ≤ 0.021). Sweat sodium, chloride and lactate decreased following HA (arm 25-34; back 21-27%, P < 0.001) and HRA (CH-CH: arm 26-54%; back 20-43%; CH-HWI: arm 9-49%; back 13-29%; CH-CON: arm 1-3%, back 2-5%, P < 0.001). LSR increases on both skin sites were larger in CH-CH and CH-HWI than CH-CON (P ≤ 0.010), but CH-CH and CH-HWI were not different (P ≥ 0.148). Sweat sodium and chloride conservation was larger in CH-CH than CH-HWI and CH-CON on the arm and back, whilst CH-HWI and CH-CON were not different (P ≥ 0.265). These results suggest that active HRA leads to similar increases in LSR, but more conservation of sweat sodium and chloride than passive HRA. Abbreviations: ANOVA: Analysis of variance; ATP: Adenosine triphosphate; BSA (m²): Body surface area; CH: Controlled hyperthermia; CH-CH: Heat re-acclimation by controlled hyperthermia; CH-CON: Control group (no heat re-acclimation); CH-HWI: Heat re-acclimation by hot water immersion; CV (%): Coefficient of variation; dt (min): Duration of a stimulus; F: Female; GEE: Generalized estimating equations; HA: Heat acclimation; HRA : Heat re-acclimation; HST: Heat stress test; LSR (mg·cm^-2·min^-1) : Local sweat rate; LOD (mmol·L^-1): Limit of detection; M: Male; $$ (mg): Mass of x; RH (%): Relative humidity; RT: Recreationally trained; SA (cm²): Surface area; t (min): Time; T: Trained; T_sk (°C): Skin temperature; T_re (°C): Rectal temperature; USG : Urine specific gravity; VO_2peak (mL·kg^-1·min^-1): Peak oxygen uptake; WBSL (L): Whole-body sweat loss; WBSR (L·h^-1): Whole-body sweat rate.