"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 the Department of Family Medicine, Division of Sports Medicine at the University of California, Los Angeles. "

"Scientists are learning more about the factors that influence overheating and ways to help an athlete avoid it. Just how hot 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. "

"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 here and those outdoor runs can still be a pleasant - if unavoidably sweaty - part of summer. "

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

"As core temperatures rise, sweat glands pump water through the skin. Sweat evaporates, taking 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 adds an extra whammy. If the surrounding air is heavy with water, sweat cannot evaporate. "

"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. "

"So if you can't be small, be fit. And while you're at it, shed excess body fat, which strikes a double blow against heat tolerance. "

"It adds more weight to move and insulates - like any clothing that's bulky or doesn't breathe - making it harder for heat to escape. "

"Fit and lean people aren't just better at cooling down, they also seem able to withstand greater heat. A 2001 study gave 24 men and women of either high or low fitness and fatness an extreme
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