How Long Does It Take to Adjust to Heat?

There’s been lots of recent interest in heat acclimation, the physiological adjustments your body makes in response to repeated exercise in hot conditions, for a couple of reasons. One is that we’re learning more about the factors (like temporary dehydration or even saunas) that may accelerate the process; the other is the mounting evidence that heat acclimation can actually make you faster even in moderate (not just hot) conditions, like a version of altitude training.

A recent review of the topic, published by Julien Périard of Aspetar Hospital in Qatar and his colleagues in the Scandinavian Journal of Medicine and Science in Sports, has some interesting details.

One of the perennial questions about heat adaptation is how long it takes. Researchers have found that as few as two 2-hour runs are enough to stimulate significant increases in plasma volume, but full adaptation takes longer—and different adaptations take different amounts of time, as the following chart from the new review illustrates:

A few adaptations take place right away, like a slight decrease in core temperature. Many of the big changes, such as plasma volume, seem to max out after about a week. The full benefits keep stretching out to about two weeks, at which point exercise capacity plateaus. Of course, this graph is just a guide: the precise changes will depend on the details of your acclimation protocol (how hot it is, how much you exercise, and so on).

One interesting detail to point out. The plasma volume change, which researchers suspect is the most important for boosting performance in normal conditions, seems to peak after a week and then start declining. Is this a “real” effect, suggesting that too much heat acclimation is counterproductive?

The short answer is we don’t know, but probably not. The problem with many prolonged heat acclimation protocols, especially in a lab setting, is that they get easier once you start to adjust. Running for an hour at a certain pace and a certain temperature will initially be thermally stressful, so you’ll start to adapt. After a week, though, you’ll be partly adapted, so that same pace and temperature will no longer be a stress—which may be why plasma volume appears to start declining again.

The solution? One approach that some studies now use is to clamp internal body temperature. Instead of saying “Run for an hour at 7:00/mile in a room at 35 degrees,” they might say “Run for an hour in a room at 35 degrees  at whatever pace it takes to keep your core temperature 1.5 degrees higher than baseline.” That way, you’ll continue getting the same thermal stress throughout the adaptation period, and hopefully continue to improve plasma volume for longer.

In practice, few of us would be enthusiastic about doing many training sessions with a rectal thermometer inserted. The authors of the review suggest exercising at a fixed level of cardiovascular strain—in practice, this would mean maintaining a target heart rate rather than a target pace or power output. As you adjust, you’ll be going faster at the same heart rate.

The review also delves into lots of other aspects of heat acclimation—whether fitness matters, dry versus humid heat, sweat rates, and so on. The overall message is that we know that adaptation to heat is important, and we know many of the changes that happen in the body, but there’s still lots of work to be done to figure out the best way to get the benefits. The main thing to keep in mind is to give yourself time to adapt.

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