Is working out in the heat bad for you?

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Sad news for folks (like me) who, at the end of a sweltering day, opt for the bar instead of a run, dismissing exercise in hot weather as downright dangerous.

Assuming the mercury is south of 100 degrees, and that you are not obese and are somewhat fit, go ahead and get out there (or in there, if you're signed up for an elevated-thermostat boot camp)—albeit with a few caveats.

But first, let's get a few facts about working out in the heat straight.

First, most heart attacks happen in cold weather, according to the American Heart Association. That's because frigid temperatures constrict your veins and slow your blood flow; hot weather has the opposite effect.

Next, the handful of high-school football players who sadly die each year during summer practice are, well, not you. Generally the kids who keel over are extremely large (linemen, specifically) and overweight (fat traps heat) and may not be in good overall shape. They may also be wearing heavy equipment, not to mention being screamed at to go harder.

What about those über-fit marathoners and triathletes who succumb to heat-related illnesses during a race? Well, no one really knows why some are stricken and others aren't.

Which brings us to three important rules for working out in the heat:

1. If you suddenly feel way off, stop. This is one of the beauties of the human body: It generally tells you when to pull back. Listen to it.

2. Stay hydrated. Heat makes you sweat more, so replenish often. But, more important, high internal body temperatures and metabolic rates cause heat illness; downing cool H2O cools your engine.

3. Start off slow. More athletes get heat sickness at the start of hot-weather season, according to the British Journal of Sports Medicine. Put another way, your body needs to acclimate to steamy temps, so up your intensity and workout times gradually.

Heed these rules and get out there—but know that heated sweat sessions don't necessarily spell better workouts. Because heat saps energy, you often can't push as hard as you can in, say, an air-conditioned gym. So don't think one hot-weather run equals two days on a treadmill. If you do, you'll be fat by fall.

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