PHYS-29 · Principle
How long does it take to adapt to the heat?
Most of it comes in one to two weeks of exercising in the heat: heart rate and core temperature run lower at the same work, sweating rises. Without heat it fades over weeks; people differ.
How the body works, agreed across the reviews below; a study entry shows what one experiment measured.
Practical Application
How long does it take to adapt to the heat? Most of it comes in one to two weeks of exercising in the heat: heart rate and core temperature run lower at the same work, sweating rises. Without heat it fades over weeks; people differ.
What it does not mean: that early comfort means the limit has moved (in non-acclimatised adults five days did not raise an occupational heat-tolerance limit, nine did, REF-3489), or that a heat block reliably adds performance in cool races.
Mechanism
Exercise-heat acclimation improves sweating and skin blood flow, lowers body temperatures and cardiovascular strain, improves fluid balance and cellular protection; the dose depends on intensity, duration, frequency and number of exposures and on dry or humid heat (REF-561).
In 7 trained men, 10 days of heat training raised plasma volume 6% and sweat output 26%, cut sweat sodium 19%, and improved VO2max and a time trial at 38 °C, not at 18 °C (Keiser et al. 2015).
In 17 trained men the fall in end-exercise rectal temperature ranged from 0.20 to 1.32 °C, and one index did not predict another (Corbett et al. 2018); body size, not fitness, went with the sweat and heart-rate responses in 24 adults (Alkemade et al. 2021).
Common misreading
"I felt fine by day four, so I'm acclimatised"; "a heat block is a shortcut for every race".