Inter-individual variation in the adaptive response to heat acclimation.

Aim: To investigate inter-individual variance in adaptive responses to heat acclimation (HA).

Methods: 17 males (VO=58.8(8.4) mL·kg·min) undertook 10-days (exercise + heat-stress [40 °C, 50%RH]) HA. Adaptation was assessed by heat stress tests (HST; 60-minutes cycling, 35% peak power output) pre- and post-HA.

Results: Inter-individual variability was evident in adaptive responses e.g. mean(range) reduction in end-exercise T= -0.70(-0.20 to -1.32)°C, but, in the main, the variance in adaptation was unrelated across indices (thermal, sudomotor, cardiovascular, haematological), indicating independence between adaptation indices. Variance in adaptive responses was not correlated with aerobic capacity, history of previous HA, or the accrued thermal-dose. Some responses to the initial HST were related to the subsequent adaptations e.g. ∆T̅ during the initial HST and the reduction in the within HST ΔT after HA (r = -0.676), but responses to the initial HST may also have been influenced by HST design e.g. ΔT correlated with metabolic heat production (r = 0.609). Metabolic heat production also correlated with the reduction in the within HST ΔT after HA (r = -0.514).

Summary: HA indices are mainly independent; 'low', or 'high', responders on one index do not necessarily demonstrate similar response across other indices. Variance in HA responses was not related to aerobic capacity, previous HA, or thermal-dose. Thermo-physiological responses to a HST might identify individuals who will benefit from HA. However, some initial responses are influenced by HST design, which may also affect the scope for demonstrating adaption.

Conclusion: Variance in the HA response remains largely unaccounted for and future studies should identify factors contributing to this variance.