If you are an avid outdoor exerciser, you know how challenging your workout can become once the summer heat settles in. You probably also know that it can take a while to get used to exercising in the heat – and sometimes it’s not easy. To avoid the risk of heat exhaustion, dehydration, heat stroke and other heat-related illnesses, it is extremely important for your body to acclimatize to the heat.

Heat acclimatization can be defined as an improved tolerance to exercise in hot conditions. When you body has acclimatized, the incidence or severity of heat illnesses is reduced, and cardiovascular, thermal and metabolic strain lessens.

You can get used to heat by gradually increasing the time you spend exercising outdoors. Heat acclimatization generally takes anywhere from a few days to approximately two weeks, with maximum results after about two months. The idea is to gradually increase your heat exposure as the air temperature increases outside.

William O. Roberts, M.D., medical director of the Twin Cities Marathon, recommends preparing your body with a few daily walks during the heat of the day (beginning the first hot days of late spring), and only running during cool morning or evening hours. As your body begins to adjust to the heat, you can increase the intensity and duration of your workout.

After approximately five to eight days of heat acclimatization, your heart rate will begin to decrease and you will begin to sweat more, earlier. This, in conjunction with the decrease in cardiovascular strain, helps to decrease central body temperature (which is critical for exercise in heat). In addition, undergoing heat acclimatization in humid conditions produces a greater sweat rate than in dry-heat environments.

Although it is extremely important to drink plenty of fluids to prevent dehydration and other heat-related illnesses, drinking an excess of water or sports drinks containing electrolytes will not speed up the process of heat acclimatization.

Once you have adapted to the heat, you should be able to go about your normal exercise routine; however, periods of inactivity (a few days to a few weeks) may cause the adaptations to disappear. Cardiovascular function (heart rate) is generally one of the first adaptations to disappear. The rate at which they disappear depends on the number of heat exposures per week, the duration and intensity of exercise, and the degree to which core body temperature is elevated.

Sources: Armstrong, L.E. Heat acclimatization. Encyclopedia of Sports Medicine and Science, T.D. Fahey (Editor), 1998; Runner’s World Magazine