Inter-individual variation in the adaptive response to heat acclimation

Area of Science:

• Exercise physiology
• Environmental physiology
• Human adaptation

Background:

• Heat acclimation (HA) improves thermoregulation and exercise performance in hot environments.
• Individual responses to HA exhibit significant variability, impacting training efficacy.
• Understanding this variance is crucial for optimizing heat adaptation strategies.

Purpose of the Study:

• To investigate inter-individual differences in adaptive responses to a standardized 10-day heat acclimation protocol.
• To determine if adaptive responses across various physiological systems (thermal, sudomotor, cardiovascular, haematological) are independent.
• To identify factors, such as aerobic capacity or previous HA history, that may influence HA response variability.

Main Methods:

• 17 healthy males completed 10 days of combined exercise and heat-stress (40°C, 50% RH) HA.
• Heat stress tests (HST) were conducted pre- and post-HA, involving 60 minutes of cycling at 35% peak power output.
• Adaptation was assessed by measuring changes in core and skin temperature, sudomotor, cardiovascular, and haematological parameters.

Main Results:

• Significant inter-individual variability was observed in HA adaptive responses, such as reductions in end-exercise rectal temperature (T_re).
• Adaptation variance was largely independent across thermal, sudomotor, cardiovascular, and haematological indices.
• No significant correlations were found between adaptive variance and aerobic capacity (VO_2max), previous HA history, or accrued thermal dose.

Conclusions:

• Heat acclimation indices are predominantly independent; responders do not necessarily show similar adaptations across all measures.
• Aerobic capacity, previous HA experience, and thermal dose do not explain the variance in HA response.
• Initial thermo-physiological responses during a heat stress test may help identify potential HA responders, though HST design can influence results.