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Keep your cool

Fatigue, dizziness, cramps -- that's how overheating starts. Now scientists are learning what causes it and how to prevent it.

June 04, 2007|Anna Gosline and Jeannine Stein | Special to The Times

EXERCISING al fresco is one of the great pleasures of living in Southern California. The trees, the hills, the beach, the (sort of) fresh air can make a long run go by faster. But summer heat waves and vicious Santa Ana winds can turn refreshing outdoor exercise into a sweat-drenched experiment in heat exhaustion.

Overheating, the mild form, causes fatigue and dizziness. That's annoying enough. As internal temperatures rise above 100 degrees, athletes may experience cramps, headaches, nausea and vomiting. By the time core temperatures reach 104, the body rebels from hyperthermia. If the athlete keeps on pushing and internal temperatures pass 104, the athlete risks "organ failure and death from heat stroke," says Dr. Aurelia Nattiv, professor in UCLA's Department of Family Medicine, Division of Sports Medicine.

Scientists are learning more about the factors that influence overheating -- and ways to help the athlete avoid it. Just how hot and bothered you get on the inside depends on a number of factors: body size, fitness level, intensity of exercise, the heat and humidity of the environment, and how acclimatized you are to exercising in hot weather.

Some tips science offers are unsurprising: Lower the intensity of exercise! Wear the lightest, littlest clothing possible!

For The Record
Los Angeles Times Monday June 11, 2007 Home Edition Health Part F Page 5 Features Desk 2 inches; 84 words Type of Material: Correction
Overheating: A June 4 Health article on ways to prevent overheating during exercise said that a study found taking 3 or 6 milligrams of caffeine daily did not raise body temperature or affect heat tolerance. The correct numbers are 3 or 6 milligrams of caffeine per kilogram of body weight. The same article said that in another study, dehydrated cyclers lasted an average of 13.6 minutes, and hydrated cyclists 19.6 minutes, in a cycle-to-exhaustion test. The correct numbers are 13.9 minutes and 19.5 minutes.

Others are more nuanced, or evolving: Cool drinks are best during workouts, but afterward, warmer's better. (If, that is, you drink at all during workouts: Not all scientists agree that it's needed, or advisable.)

Immersion in an ice-cold bath before exertion is helpful. And caffeine, long thought to be a no-no because it contributes to overheating, may be fine to indulge in on race day.

Follow the advice on these pages and those canyon runs can still be a pleasant -- if unavoidably sweaty -- part of summer.

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Cooling systems

For 46-year-old Laura Garcia, a legal secretary and avid runner, the worst overheating experience of her life came during the 2004 L.A. Marathon. It was the second of seven that she's run. Temperatures were in the 90s. Scores of runners ended up in medical tents. "It was unbelievably difficult," she says. "I could feel my muscles start to seize."

She took advantage of spectators who were cooling people down with their garden hoses. It did make her feel cooler. But at mile 18 or 19, she says, "I was done. I could just feel that overwhelming heat, like I was going to fall over. I really scaled back. I walked, I sipped water consistently, and drank Gatorade.... Around mile 22 or 23 I thought, 'I don't think I'm going to make it.' " (She did.) "When you see people around you dropping like flies, it's scary."

Working out uses energy we derive ultimately from food that we eat. A mere 25% of that energy ever leverages muscle force. The rest goes to waste -- as heat.

Fortunately, the human body comes well-equipped with heat-loss mechanisms. As core temperatures rise, sweat glands pump water through the skin. It evaporates into the air, taking a thwack of body heat with it.

Sweating's not the only way we have to cool down. Higher body temperatures cause the heart to pump more blood to the skin. Skin blood vessels dilate, exporting more heat.

As anyone running in midday heat knows, these mechanisms can be severely impaired by weather. "Exercise in the heat poses a formidable challenge to the body's ability to control its internal environment," says Susan Shirreffs of Loughborough University in Britain. As the difference between body temperature (98.6 degrees) and ambient temperature shrinks, heat moves less readily to the air.

When the mercury passes 100, we actually begin to absorb heat from the environment -- that's on top of the heat we're absorbing directly from the sun.

Humidity (a problem occasionally in L.A. and routinely elsewhere in the U.S.) adds an extra whammy. If the surrounding air is heavy with water, sweat cannot evaporate off the skin.

Other factors determine how hot we get -- such as body size. In a 2000 study, Frank Marino of Charles Sturt University in Australia tested 16 trained runners whose body weights ranged from 121 to 198 pounds. The lighter runners produced and stored less heat at the same running speeds, probably because smaller bodies require less effort to move and have a greater ratio of surface area to volume to dissipate heat. Thus, lighter runners can run faster or farther before reaching exhausting core heats.

This doesn't mean larger-framed athletes must exercise in the confines of a humidity-controlled, air-conditioned gym. Merely being fit helps too. The stronger the cardiovascular system, the easier and more efficiently it pumps blood to the skin where it can dump excess heat, says Glen Kenny of the University of Ottawa.

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