Sweat is good. When you're too hot, the sweat evaporating on your skin can cool your body off. Sweat also provides a good waste disposal system for your body, getting rid of toxins and other waste products as it cools you down. However, for top athletes who sweat a lot, too much of a good thing can spell trouble.

These athletes can actually start sweating minerals like calcium out of their bodies. Since calcium is the main ingredient of bones, these athletes can lose bone density, and develop a host of bone problems including stress fractures and premature osteoporosis.

Just how much sweat are we talking about here? Is this finding a good excuse to stop exercising and watch more TV? Not really. The researcher who discovered this, Robert Klesges of the University of Memphis, studied university basketball players. In the course of an intensive three-hour workout, these players would lose an astonishing six pounds of sweat or more. That's around three quarts of sweat!

Klesges collected the sweat by wringing out the athletes' tee-shirts. He discovered that they were losing a significant amount of calcium with each workout, enough for the average player to lose almost four percent of his bone material during three months of practices and games.

Fortunately, this bone loss can be made up by drinking calcium-rich sports drinks during periods of heavy exercise, or excessive sweating. For those of us who aren't top athletes, it should be sufficient to make sure we get our recommended daily allowance of calcium-around a thousand to thirteen-hundred milligrams.