Heat Acclimatization

(The information in this topic contains material from Marathoning, Start to Finish, by Patti and Warren Finke, 2004).


With warmer weather, runners need to allow time for their bodies to adjust to the higher temperatures and increased humidity. Knowing how the weather changes affect you physiologically can help you prepare and ensure that you train and race safely.

The body is better at handling cold than heat, and exercise raises internal temperatures in addition to the outside weather. Two problems with hot weather are increased body temperature, which immediately affects performance, and dehydration, which can cause a more gradual drag on your ability to function in hot conditions.

Performance starts to suffer at just three degrees over normal temperature, and especially in a race, runners are not inclined to back off to allow for the warmer weather. This is where acclimatization can help; a properly acclimatized body makes adaptations to allow for optimal performance when it recognizes the challenge of extreme heat.

Acclimatization is the process of adapting your body to be able to run more efficiently in hot conditions. When it is hot the blood goes to the skin to cool the body as well as to the working muscles. This increases the workload of the heart and the exercising heart rate. Intensity of exercise will need to be reduced when running in the heat and when acclimatizing for proper adaptation to the warmer weather.

The body makes several adjustments during the heat acclimatization process. The circulatory adaptations provide better transport of heat from the core to the skin. There is better distribution of the blood to regulate temperature. Sweating mechanisms undergo changes, too. A runner’s first exposure to running in warm, humid weather often results in salty residue on their skin. Acclimatization helps the sweat become more dilute since it contains less salt and is more evenly distributed over the skin. Some runners perspire a lot more than others in identical heat conditions even with matching body composition, weight and running speeds so, if you are a “heavy sweater”, acclimatization is increasingly important. Major changes occur during the first week of heat exposure and are mostly complete after 10 days.  

Heat acclimatization can also be lost in 10 days. If it is likely that your goal race will be in hot weather, wearing extra clothing during unusually cool spring/summer weather should help to maintain acclimatization. While preparing for a race in a warmer climate, wearing an extra layer of dry clothing (i.e. going from short to long sleeved shirt or from shorts to tights), is equivalent to 15 to 20 degrees in temperature. (To avoid heat related illnesses, wearing extra layers of clothing is not recommended while running on hot days).

Some ways to acclimatize are:

  • Begin early in the season when the temperature is cool or moderate and wear one more layer of clothing than usual on one or two runs per week. If you would normally wear a tshirt wear a long sleeved one or a jacket. This provides a hot, humid micro atmosphere and prevents evaporation. Don’t overdo it by wearing extra layers for every workout.
  • If the weather suddenly turns hot, reduce the training load, run slower and less distance. Slowly build back up to usual mileage and intensity. Work on heat acclimatization a day or two each week and make sure to replace lost fluids. Do not overdo it and end up suffering heat symptoms.
  • If you plan to race under hot conditions, remember that acclimatization takes about 10 days. Plan to be acclimatized a week in advance. During the week before the event, avoid extra heat stress that may dehydrate and fatigue you for the race.
  • Make sure you adequately hydrate yourself when you are heat acclimatizing. This will prevent injury and train your body to use fluids.