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July 10, 2000
Defeat the Heat
By Mary C. Weaver

illustration: Michael Alto

Two runners jog along a heat-baked asphalt path during their lunch break. One zooms past the other, eventually running a total of five miles with ease. The other runner makes it only one mile before suffering heat exhaustion. What happened?

The athletic jogger's body has adapted to exercising in the heat by retaining more fluids, sweating earlier and in higher quantity, and diluting the sweat. These changes all make it easier for her to dissipate body heat. The out-of-shape jogger's body hasn't yet made that adjustment.

Although individuals vary tremendously, anyone can improve heat tolerance through acclimatization. "Start gradually, and go slowly," advises Scott Flinn, M.D., sports medicine director and senior medical officer at Parris Island, South Carolina, the home of a United States Marine Corps base.

"It takes about 10 days for your body to acclimatize to the heat," says Dr. Flinn, who works at the hot and muggy site where 20,000 Marines undergo basic training each year. "If it's hot and humid outside and you're not used to exercising in it, do just a little activity until your body catches up. You could even exercise in the desert of Saudi Arabia if you had to, but you'd have to get used to it," he points out. Begin with brief exercise sessions of lower intensity, Dr. Flinn suggests, and gradually work your way up.

Keeping your cool

Acclimatization alone won't make you impervious to heat injury, but it's a good start. To further protect yourself when exercising outdoors, follow these guidelines:

  • Watch the weather. Work out early in the morning or late at night, when the temperature outside is lower. Pay attention to the "heat index," or "apparent temperature," devised by the National Weather Service to approximate how hot the air feels at various levels of humidity. At heat indexes of 90 to 105, sunstroke, heat cramps, and heat exhaustion are possible with prolonged exposure; from 105 to 130, those maladies are likely, and heatstroke is possible; at 130 or more, heatstroke is highly likely with continued exposure.
  • Dress properly. Wear light-colored, loose-fitting shorts and tops made from fabric with a tight weave. Dr. Flinn recommends high-tech fibers that help wick away moisture and speed evaporation. A lightweight, light-colored hat with a brim can help protect you too.
  • Stay hydrated. It's essential to drink plenty of water both before and after exercise: If you're dehydrated, your body won't be able to sweat as much as it needs to for optimal cooling. The standard recommendation for water intake is eight 8-ounce glasses a day -- in addition, you must replace any water your body loses during workouts. "Drink an extra quart to a quart and a half for every hour of activity," advises Dr. Flinn. If your workout exceeds 50 minutes, you should consider consuming a sports drink during exercise as well.

It's hard for most people to drink that much water, notes Patrick Bird, Ph.D., dean of the College of Health and Human Performance at the University of Florida. To encourage the habit, he suggests keeping a bottle of water with you throughout the day. "The thirst mechanism in human beings is not very good, so you have to drink [water] even when you're not thirsty," Bird explains.

And lastly, a note for all of you who enjoy alcohol: Alcoholic beverages (as well as caffeinated drinks) cause the body to lose water, so if you drink heavily one night, avoid exercising in the heat the next day.

Related links:

Rx.magazine feature story: Under the Sizzling Sun

Rx.magazine feature story: Making Exercise Work for You

Outside link: To request a copy of the American Running Association's brochure "Beat the Heat: How to Stay Cool and Fueled," go to their site