Background Heat acclimation (HA) reduces the risk of exertional heat illnesses (EHI). However, HA adaptations are temporary and must be sustained to safely perform physical activity in hot environments.

Objective To determine if intermittent exercise-heat exposures every fifth day mitigates HA decay, 25 days after initial HA (+25 d).

Design Randomized pair-matched design.

Setting Laboratory.

Participants Sixteen non-heat acclimatized recreationally active males (age=22.7±3.4 y; weight=72.9±7.8 kg; VO_2max=55.0±5.5 ml·kg⁻¹·min⁻¹).

Interventions All participants heat acclimated by completing 10–11 days of low-to-moderate intensity exercise (90–240 min; 40°C, 40%RH). A heat stress test (HST; 120 min, 45% VO_2max) before (Pre) and after (Post) HA in similar hot conditions assessed HA status. Matched participants were randomly allocated into a control group receiving no heat exposure (CON; n=7) or treatment group receiving intermittent heat exposure (IHE; n=9). After the Post HA HST, participants completed an HST every fifth day for 25 days in either a hot (IHE; 40°C, 40%RH) or mild environment (CON; 24°C, 21%RH) with both groups exercising in the hot environment +25 d.

Main Outcome Measurements Rectal temperature (T_re), heart rate (HR), and sweat rate (SR).

Results Both groups accomplished HA after the 10–11 day initial protocol, as evidenced by lower post-exercise HR and T_re, with higher SR compared to Pre HA (p≤0.05). Moderate-to-large between-treatment mean differences (MD; CON-IHE) and effect sizes (ES) were observed for HR (MD=28 bpm, 95% CI [8, 48], ES=1.41) and T_re (MD=0.47°C, 95% CI [−0.24, 1.19], ES=0.68) but not SR (−0.13 L·hr⁻¹, 95% CI [−0.47, 0.21], ES=0.36). At +25 d, HA adaptation decay was greater in CON than IHE (T_re: 87% vs. 2.7%; HR: 163% vs. 53%; SR: 77% vs. 70%).

Conclusions Periodic exercise-heat exposures every five days after initial HA better sustained HR and T_re in the IHE, suggesting periodic exercise-heat exposures may help reduce EHI risk for up to one month after initial HA.