Individual variability in achievement of short-term heat acclimation during a fixed intensity protocol

Heat acclimation (HA) occurs with ∼8-10d of repeated heat exposure; however, adaptations can occur earlier (<7d; termed short-term heat acclimation; STHA).

To test the hypothesis that some, but not all, young healthy men would achieve STHA after 5d of a standard HA protocol.

We conducted a novel, post-hoc analysis of data from 13 healthy men (21 ± 3 yrs; 173 ± 8 cm; 75.1 ± 12.2 kg) who participated in an 8d HA protocol (120 min treadmill walking: 5 km h^-1, 2% grade; 40 °C, 40% relative humidity). Core temperature (T_c), heart rate (HR), and whole body sweating rate (SR) were compared across 1d, 5d, and 8d of HA. Criteria for HA were delta (Δ)T_c ≤ 0.9 °C and ΔHR≤33 beats·min^-1.

Group averages on 1d, 5d, and 8d of HA for end exercise T_c (38.1 ± 0.34; 37.9 ± 0.3; 37.8 ± 0.3 °C) and HR (134 ± 17; 122 ± 13; 121 ± 13 beats·min^-1) were decreased by 5d (p < 0.05) and did not decrease further by 8d. ΔT_c (1.1 ± 0.36; 0.95 ± 0.45; 0.95 ± 0.33 °C) and ΔHR (36 ± 18; 33 ± 17; 33 ± 15 beats·min^-1) showed similar patterns (p > 0.05). At 5d, 31% (4/13) of the volunteers achieved HA; the remaining 9 volunteers had further adaptations between 5d and 8d. SR was not different across 1d, 5d, and 8d (860 ± 148; 908 ± 210; 873 ± 203 mL h^-1 respectively; p > 0.05).

Our results suggest that during a fixed-intensity HA protocol in young men, using criteria of changes in T_c and HR, ∼1/3 achieve STHA. Future research should evaluate potential mechanisms, and whether similar inter-individual variability occurs in women.