Physiological and hematologic responses to summer and winter dry-heat acclimation

Area of Science:

• Environmental Physiology
• Human Adaptation
• Thermoregulation

Background:

• Seasonal variations in ambient temperature influence human physiological responses.
• Understanding heat acclimation differences between seasons is crucial for public health and performance.
• Previous research indicates potential differences in heat adaptation based on the time of year.

Purpose of the Study:

• To investigate and compare the effects of heat acclimation at the end of winter versus the end of summer.
• To determine if seasonal differences persist despite controlled heat exposure.
• To analyze changes in core body temperature, heart rate, sweat rate, and blood volume.

Main Methods:

• Eight male volunteers underwent 10 days of heat exposure (40°C, 30% rh) twice, once in summer (S) and once in winter (W).
• Daily exposures included rest and walking periods.
• Measurements included rectal temperature (Tre), heart rate (HR), sweat rate (msw), plasma volume, and red cell volume.

Main Results:

• Winter (W) acclimation led to decreased Tre and HR, unchanged msw, and expanded plasma and red cell volume.
• Summer (S) acclimation resulted in decreased HR, unchanged Tre and msw, and increased plasma volume.
• Acclimated subjects maintained higher Tre and lower msw in W compared to S.
• W acclimation induced greater plasma and red cell volume expansion than S acclimation.

Conclusions:

• Heat acclimation does not fully eliminate seasonal differences in thermoregulatory set point and sweating sensitivity.
• Acclimation to severe heat did not lower the thermoregulatory set point achieved by milder natural acclimatization.
• Acclimation primarily affects cardiovascular adjustments and plasma volume expansion, with winter acclimation showing greater effects on blood volume.