Adjusting to hot weather: what does it take?

Conventional wisdom says that we adapt to deal with heat after a week or two of high temperatures. But a study in the current European Journal of Applied Physiology suggests that it doesn’t happen automatically. These days, we spend a lot of time in air-conditioned homes, offices, cars and even gyms — so we may no longer get the stimulus we need to adjust to exercising in heat.

To test this proposition, researchers at the University of Ottawa tested a group of 8 volunteers in mid May and early September. They measured core temperature, skin temperature, skin blood flow, sweat rate, heart rate, and a few other variables during a 90-minute bike session at 60% VO2max — and found no significant differences even after a long, hot summer. The key: their subjects reported spending an average of just 18 minutes a day doing “moderate” or “intense” physical activity outdoors over the summer.

In comparison, chamber-based heat acclimation protocols known to elicit physiological adaptations require a minimum of 1 h of exercise at 50% of VO2max for ten successive days in order to elicit a physiological acclimatization.

There’s no doubt that heat acclimatization effects are real — enhanced sweat rate and greater blood flow to the skin, resulting in lower core temperatures. But you have to get out there and sweat to make it happen.