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Loss of More than 2 Percent of Your Body Weight During Exercise Degrades Performance

This debate, popular among exercise gurus and professional trainers, centers on how much water weight an athlete can lose without sacrificing performance. Lab tests have suggested that a body-weight loss of more than 2 percent impairs athletic performance. This information has become accepted. But a study of marathon runners in France published late last year found exactly the opposite. The fastest finishers were the most dehydrated, having lost 3.1 percent of their body weight, while the slowest finishers lost only 1.8 percent. It turns out that the body has hidden reservoirs that can generate several pints of water during exercise. For example, when your body burns fat or carbohydrates to fuel muscles, one of the byproducts is H2O. So drinking water when thirsty—and no more—is the best course of action.