The Scientist: Dee Anna Glaser, M.D., is a cosmetic dermatologist, professor and vice-chairman of the department of dermatology at Saint Louis University School of Medicine.

The Answer: Sweat is typically about 99 to 99.5 percent water. The other half percent or so is a combination of minerals and electrolytes. The curious cocktail consists largely of potassium and sodium. The latter gives sweat its saltiness. Also in the mix: calcium, chloride, magnesium, trace amounts of zinc, copper, iron, chromium and nickel, plus some urea and lactate, which is produced when muscles are working hard. (And, yes, urea is usually found in urine. But don’t worry—you’re not sweating out pee, we swear.)

If you’re really exerting yourself, say, competing in a triathlon, you can pump out a couple liters of sweat per hour. At times like that, perspiration is more like 99.9 percent water, because your body wants to cool you off without losing precious minerals. That is why it’s important to replenish both water and electrolytes to maintain a healthy balance when you’re pushing yourself to the max. For a regular workout, though, what you really need is just plain ol’ water—the body does a pretty good job of regulating minerals and salts on its own.

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