The Truth About Hydration

We all know that when we work out, we need plenty of liquids. But beyond that basic tenet, things get murky fast—how do we know what, when and how much to drink?

 Sports nutrition expert Monique Ryan, RD, author of Sports Nutrition for Endurance Athletes, separates the facts from the hype:

Hype: REPLACE EVERY LOST OUNCE The truth is, your body can't absorb fluids as fast as it loses them, and not every ounce of weight is lost through sweat anyway.

Truth: KEEP UP WITH SWEAT LOSS—MOSTLY Replace about 75 percent of lost sweat during your workout. "To do that, you need to know your sweat rate," says Ryan, who recently coached a heavy-sweating triathlete who routinely lost 40 ounces of fluid an hour. To determine your sweat rate, weigh yourself before and after a short workout.

Hype: OVERFLOW BEFOREHAND Guzzling gallons of fluids before you workout will do little more than send you searching for the restroom!

Truth: TOP OFF AS YOU GO Take in about six to eight ounces (two to three gulps) every 15 to 20 minutes while you workout.

Hype: CAFFEINE WILL DEHYDRATE YOU Caffeine has long been demonized as a diuretic. On paper, that means it should lead to dehydration and heat stress, especially when you consider that it also raises your heart rate and increases your metabolism.

Truth: CAFFEINE IMPROVES CARB BURNING A review of ongoing research recently revealed that caffeinated drinks don't make you pee that much more than equal amounts of beverages without the buzz. The stimulant also doesn't worsen the effects of summertime heat. In fact, caffeine makes you feel better. Numerous studies have shown that it lowers your rate of perceived exertion while improving your strength, endurance and mental performance. Even better, researchers from the University of Birmingham, in England, found that athletes who drank a caffeinated sports beverage burned the drink's carbs 26 percent faster than those who consumed a noncaffeinated sports drink, likely because caffeine speeds glucose absorption in the intestine.

Hype: HYDRATION DURING EXERCISE IS THE BE-ALL AND END-ALL Big beverage companies would have you grabbing your sports drink during every workout, no matter how long or short the effort, lest you suffer the ill effects of dehydration.

Truth: DRINKING EVERY DAY IS ESSENTIAL "Your first priority should be staying on top of your daily hydration," says Ryan. Research on gym-goers found that nearly half began their workouts in a dehydrated state. "Many people don't consume enough fluids during the day," Ryan says. "If you hydrate properly on a regular basis, you won't need to worry as much about getting dehydrated during a typical moderate workout." The old eight-glasses-a-day dictum is a good guidepost.