Heat acclimatization

Heat acclimatization is the body’s adaptation to a hotter environment that improves exercise tolerance.^{1USA Triathlon, Complete Triathlon Guide, p. 133.} Heat raises core and muscle temperature and heart rate at a given workload, along with sweat rate and dehydration risk.^{1USA Triathlon, Complete Triathlon Guide, p. 133.} The first signs of acclimatization are lower heart rate and rating of perceived exertion (RPE), followed by a higher sweat rate that helps control core temperature.^{1USA Triathlon, Complete Triathlon Guide, p. 133.}

ACSM lists a 10% to 20% increase in plasma volume, earlier sweating, less sodium lost in sweat, lower rectal temperature, heart rate and RPE, a higher sweat rate, and longer exercise tolerance.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 511.} A structured program of moderate exercise in the heat over 10 to 14 days can produce these adaptations.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 511.}

In practice

• The CSCCa and NSCA recommend 7 to 14 days for acclimatization; fitter clients adapt faster, while those with a BMI of 30 or more generally take longer.^{3CSCCa and NSCA, Joint Consensus Guidelines for Transition Periods, p. 6.}
• Start in the cooler part of the day and build training volume in the heat before adding intensity.^{1USA Triathlon, Complete Triathlon Guide, p. 133.}
• Watch intensity early on: high-intensity exercise can push an unfit or unacclimatized client’s core temperature to dangerous levels in under 30 minutes.^{3CSCCa and NSCA, Joint Consensus Guidelines for Transition Periods, p. 6.}

How long does heat acclimatization last?

USA Triathlon reports that exercising in the heat every third day for 30 days produced the same acclimatization as daily exercise for 10 days.^{4USA Triathlon, Complete Triathlon Guide, p. 151.} It also reports differing rates of loss without continued exposure: some studies show significant losses in under a week, while others show adaptations holding fairly well for up to a month.^{5USA Triathlon, Complete Triathlon Guide, p. 152.}