Adapting to heat in a process called heat acclimation/acclimatisation (HA) can improve heat tolerance and cardiovascular health outcomes. The primary purpose of this thesis was to explore how to efficaciously apply and dose HA, and whether heat acclimation affects cold defence.

Study One compared the more commonly studied clamped-hyperthermia method of HA (CH) with self-regulated HA (SELF), using a repeated measures cross-over design in eight young, aerobically- trained volunteers. Acclimation involved 1 h/d of exercise in humid heat (40 °C, 27 g/m3 [52% relative humidity]), for 9 d, with rectal temperature (Tcore) clamped at +1.5 °C in CH, or with only internal cues of intensity in SELF. Both regimens successfully elicited classic HA adaptations; increased resting plasma volume, decreased resting Tcore, decreased heat-stressed heart rate. SELF was at least as effective as CH, and seven of the eight participants preferred it. During SELF, daily heart rate and △Tcore were higher than in CH, and those with higher daily thermal or cardiovascular strain did not necessarily adapt more.

Study Two compared the effect of heat strain intensity during HA on adaptations. In a randomised cross-over design, eleven young, recreationally-active participants undertook 1 h/d of spa bathing in 39-40 °C water for 9 d, raising Tcore by +0.9 or +1.4 °C, versus bathing in 36.6 °C water for 5 d without raising Tcore. Increased daily thermal strain increased sweat rate, thermal sensation and discomfort, and worsened feeling state during acclimation sessions, but did not affect sudomotor or cardiovascular adaptations. Interestingly, even thermoneutral spa bathing may have elicited plasma volume expansion (+3.6 ± 6.1%; 90% CI: ± 2.4%) and a reduction in resting mean arterial pressure (-9 ± 6 mm Hg; 90% CI: ± 4 mm Hg). Modest thermal strain intensities may be more accessible for those with limited thermal tolerance but seeking cardiovascular adaptation.

Study Three compared the effectiveness of exercise-based HA with spa-based HA, using data from seven participants who completed both studies One and Two. Resting blood pressure and skin blood flow adaptations were evident only after spa-based HA, which was also the better tolerated regimen (lower daily heart rate and Thermal Discomfort). Thus, for people seeking cardiovascular adaptation, spa bathing may be the mode of choice, yet both regimens require resources that people may not have access to.

Study Four investigated how heat acclimation affects cold defence, by comparing thermoregulatory responses to 30 min of cold-water immersion (20.4 °C water) before and after spa-based HA (n = 8). Cold-stressed vasoconstriction was blunted (attenuated drop in skin temperature), as was the increase in metabolism, yet participants felt worse. Therefore, HA may facilitate an earlier onset of hypothermia but preserve behavioural (discomfort) protection.

In summary, the evidence in this thesis preliminarily supports the use of self-regulated HA for athletes, lower thermal intensities for cardiovascular adaptation via spa bathing, and possibly spa- over exercise-HA for cardiovascular adaptation, while the effects of HA on cold tolerance are more nuanced. Future research could further explore self-regulated HA, test whether thermoneutral spa bathing provides reliable cardiovascular adaptations, and if so, what physiological processes are responsible.