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Choosing Your Hydration Fluid

If you are not running a race, "the rule of thumb is to drink 8 ounces of a sport drink or water possibly every 20 minutes," says Maharam, who advises the ING New York Marathon. No more, no less. "If you are exercising less than 40 minutes, water is fine, but for anything over 40 minutes, you want a sports drink that has sugar or salt in it because this helps you increase the fluid that goes into the body. Most sports drinks contain the equivalent of an "active pump" that gets more water into the body faster than the unassisted process - simple diffusion of water - would have.

When choosing a sports drink, look for salt and sugar on the label and choose a flavor that you like. While shoppers may be bombarded with vitamin-infused beverages, Maharam says added vitamins are useful for recovery and post-event muscle soreness -- not for hydration the day of the event.

It's also important to replace the fluid you lose during exercise, he says. Weigh yourself right before and after workouts and for every pound lost, drink eight ounces of fluid.

Moreover, "step out of bed every morning and onto the scale, and if you're anywhere from 1% to 3% lighter than yesterday, rehydrate by drinking eight ounces of fluid for each pound lost before training again," he says. "If you are between 3% and 6% lighter, rehydrate and back off that day's training intensity. And if you lost over 7%, get to the doctor."

Dehydration is somewhat insidious, adds Cohen. You can't always tell when it's starting.

"Humans don't have a 'fuel gauge' like your car, so there is no way to tell if you're full or even approaching empty, and thirst is typically a poor guide," he says. Early signs of dehydration may include poor concentration, headache, and inability to think clearly.

"Most people are chronically dehydrated as it is," says Eric von Frohlich, a group exercise instructor at Equinox in New York City and the chief exercise officer of Roadfit, an outdoor training group fitness organization. "Drink before your thirst kicks in," he says, touting the benefits of prehydration. "Drink 16 ounces before an event or session so have some extra fluid. You don't want to suck down two glasses of water and bolt out the door for a run." So wait about two hours before engaging in your activity of choice.

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Ensure you're exercising hard enough to get a good workout, but not strain your heart.

While you are exercising, you should count between...

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