Does the mode of heat acclimation affect the kinetics of adaptation?

Background: Heat acclimation can improve exercise performance in the heat, cardiovascular function and longevity in the general population. However, the most effective method of acclimation is unknown. Each method (i.e., passive vs. active; water vs. land-based) will induce different profiles of strain, and thus potentially a different extent and nature of outcomes. The aim of this study was therefore to investigate adaptive responses to exercise in the heat (ExH), hot water immersion (HWI), Sauna, and a thermoneutral water immersion control (TWI), focussing on hypervolaemic responses. It was hypothesised that ExH would be most effective at increasing plasma volume (PV). Methods: In a crossover repeated measures design, 13 physically-active participants (5 female) undertook four, 5-day heat exposures (60 min/day) in randomised order, wherein rectal temperature (Tc) was clamped at neutrality (36˚C water; TWI) or raised by 1.5˚C in HWI (40˚C), Sauna (55˚C; 55% RH), or ExH (40˚C; 50% RH). Exercise strain (cardiovascular and thermal) was measured and compared to outcome measures of PV change (Dill & Costill, 1974), blood pressure (ambulatory), and fluid regulatory hormones/proteins (ELISA/RIA). Data were analysed using linear mixed model analysis. Results: In ExH, PV was elevated (7.2 ± 4.7%), but at day 6 this was not significantly larger than HWI, TWI or Sauna (p = 0.077). Approximately two thirds of the six-day PV expansion occurred within the initial 24 h, regardless of condition (r = 0.790). The stress-induced change in MABP at 45 and 120 min after exposure was only weakly correlated to PV change at 24 h (r = 0.290). Peak HR and thermal strain (Tc; area under curve) were also weakly associated to day 6 PV (r = 0.241, 0.258). The [aldosterone], as a potential mediator of PV expansion, was significantly elevated 45 min after exposure to ExH (147%) and Sauna (63%; p ≤ 0.050). However, [aldosterone] did not differ reliably between ExH, Sauna or HWI (p ≥ 0.406). Albumin content, as another mediator of PV expansion, was significantly increased by day 2, and remained so at day 5, and was comparable between ExH, HWI and Sauna (p > 0.559). By day 6 of HWI, the resting Tc was reduced (0.30˚C ± 0.25˚C; n=12), which was similar to ExH and Sauna (p ≥ 0.153) but larger than TWI, by 0.31˚C ([0.05, 0.57]; p = 0.013). By day 6, SBP was significantly lower than baseline by 4 mm Hg ([1, 8]; p = 0.001), and this reduction was not significantly different between conditions (p = 0.494). Conclusion: Short-term (5 * 1 h) heat acclimation effected the typically observed changes in resting PV, Tc, and SBP, and these did not differ significantly between the three modes of acclimation (ExH, HWI or Sauna). The control immersion condition (TWI) also led to changes in PV and SBP. The mode of short-term heat acclimation used should depend on the individual and accessibility to equipment