Effectiveness of an Intermittent Heat Exposure Protocol to Maintain Heat Acclimation

Background: Heat acclimation (HA) adaptations are temporary and must be sustained for the continued safety of those periodically exerting themselves in hot conditions. Purpose: To assess whether an intermittent exercise-heat exposure protocol can mitigate HA decay 25 days after initial acclimation. Methods: Sixteen males (VO2max=54.98±5.5 ml·kg-1·min-1) were pair-matched using physical training duration, VO2max, and body surface area then randomly allocated to a no heat (NHE; n=7) or intermittent exercise-heat exposure (IHE; n=9) group. All participants heat acclimated by completing 10-11 days of low-to-moderate intensity exercise (90-240 min) in hot conditions (40°C, 40%RH). Both groups completed a Pre HA and Post HA heat stress test (HST) consisting of two hours of exercise at 45% VO2max in hot conditions to assess HA. After Post HA, participants completed the HST in either a hot (IHE; 40°C, 37%RH) or thermoneutral environment (NHE; 24°C, 21%RH) every fifth day for 25 days with both groups exercising in the hot condition at day 25 (+25d). Thermoregulatory, cardiovascular, and circulating biomarkers of stress were evaluated. Self-led out-of-lab physical activity duration and intensity (heart rate [HR]) were recorded for 25 days after HA. Results: Both groups heat acclimated as post-exercise HR and rectal temperature (Tre) were lower and sweat rate higher at Post HA versus Pre HA (all p≤0.05). At +25d, post-exercise HR was attenuated in IHE versus NHE (mean difference [NHE-IHE]=28 bpm (95%CI [8, 48], effect size [ES]=1.41, p=0.01) but sweat rate (0.13 L·hr-1, 95%CI [-0.21, 0.46], ES=0.36, p=0.44), skin temperature (0.65°C (95%CI [-0.17, 1.47], ES=0.85, p=0.11) and Tre (0.47°C, 95%CI [-0.24, 1.19], ES=0.68, p=0.18) were similar. Post-exercise cortisol and epinephrine concentrations were higher in NHE versus IHE at +25d (p≤0.046). At +25d, heat adaptation decay was greater in NHE than IHE for Tre (87% versus 2.7%), skin temperature (44% versus 18%), and HR (163% versus 53%). Out-of-lab exercise intensity and +25d post-exercise HR were inversely related in IHE (r=-0.89, p=0.017). Conclusions: Periodic exercise-heat exposure every five days mitigated rectal temperature decay and cardiovascular strain 25 days after initial HA efforts. Intense exercise in thermoneutral environments in addition to exercise-heat stress after HA aids in minimizing adaptation decay