Seven days of mixed-method heat acclimation improved markers of cardiovascular and fluid-regulatory strain during exercise-heat stress.

A mixed-method heat acclimation (HA) protocol may optimise performance by supporting the training taper while promoting thermal adaptation; however, the impact on cardiovascular and fluid-regulatory adjustments to protect health is unknown. Therefore, we examined the effects of a mixed-method heat protocol on physiological responses, including cardiovascular and fluid-regulatory strain with exercise-heat stress, and self-paced performance in the heat. Twenty (15 males, five females) triathletes were randomised to 8 days of HA (HOT), or exercise in thermoneutral conditions (TEMP). A heat stress test (HST) comprising 45 min of cycling in a climatic chamber (32°C, 70% relative humidity) was performed on days 1, 5 (HOT only) and 8. Before and after the intervention, a cycling time trial was conducted in the same climatic conditions (days 0 and 10). Venous blood samples were analysed at rest and post-HST (days 1 and 8 only) for the catecholamine product normetanephrine and the vasopressin surrogate copeptin. Following 7 days of HA (days 1 vs. 8) resting rectal temperature was significantly lower in the HOT compared to the TEMP group (-0.32 ± 0.36°C, P = 0.002). Normetanephrine was 24.3% lower after 7 days of HA (P = 0.012), and copeptin was 53.4% lower at the post-HST time point (HOT vs. TEMP, P = 0.012). However, HA had no effect (0.3%, P = 0.984) on self-paced performance in the heat. Mixed-method HA elicited a progressive reduction in cardiovascular strain and a net reduction in fluid-regulatory strain without improving self-paced performance in the heat.