Acclimatization (or acclimation) is the process by which the human body makes physiologic adaptations to reduce the stress of an environment. (31) Heat acclimatization refers to an organism’s ability to survive an otherwise lethal heat stress from a prior heat exposure sufficient to cause the cellular accumulation of heat shock proteins.(24) Studies have shown that a period of 9 to 10 days is generally sufficient to attain most of the physiologic benefits associated with acclimatization,(31) and that physical endurance for exercise in hot, dry environments appears to be limited by the attainment of a critical level of core temperature.(5) High core temperature– and not circulatory failure or metabolic depletion– is the critical factor in heat stress, both before and after acclimation (5) Heat acclimatization results from a series of elevations in core temperature, generated by performing work in the heat, (24) and results in a number of physiological changes including the following:
- Improved thermo-regulatory control: Thermoregulatory control is improved via activation of the sympathetic nervous system and the resulting increases in the flow of blood to the skin and the rate of sweating.(2) Acclimatization to work in the heat brings an earlier onset of sweating, increase in sweat rate and evaporative cooling that reduces heart rate in proportion to decreased core temperature.(2,45) After acclimation, sweating occurs at a lower core temperature and the sweat rate is maintained for a longer time period.(2)
- Reduced resting core temperature and greater heat-dissipating capacity: Heat exposure causes a cascade of cardiovascular adaptations to heat including higher heart rates.(33) Heat acclimation reduces resting core temperature and increases heat-dissipating capacity. (24) Heat acclimation has also been shown to increase stroke volume, plasma volume (by 13%) and sweat rates.(5,29,32)
- Greater ability to dissipate excessive body heat and maintain lower core temperature: ATE lengthens the time before the core temperature reaches 40 degrees C.(1) The resulting improvements in evaporative cooling enhances the dissipation of metabolic heat during exercise in heat.(29)
- Prolongs ability to continue exercising before exhaustion: Trained athletes generally reach the point of exhaustion when core temperatures reach 39 degrees Celsius.(34) Heat acclimatization allows the organism to tolerate a higher core temperature and therefore prolongs the ability to continue exercising before exhaustion. (24) A study using a climatic chamber to study exercise in dry heat found that acclimation increased average endurance before reaching exhaustion of the study subjects from 48 minutes to 80 minutes.(5)
- Reduced lactate accumulation: Studies have also shown that heat acclimation results in reduced lactate accumulation in blood and muscle.(3)
- Results in increased intracellular heat shock proteins (HPS): HSPs are involved in maintaining cellular protein conformation and homeostasis during stress.(23, 24). The increase in HSPs resulting from heat acclimation is illustrative of a cellular adaptation to repeated heat stress in humans.(23)