The difference is water loss, and they need to replace it. One pound of weight loss equals two cups of water. Dr. Kimmel says 2 percent dehydration is very common among young athletes. That translates to six cups of water for a 150-pound person. Even if you're as little as 2 percent dehydrated, that starts affecting your performance and concentration.
Starvation is the best diet.
Research has shown that starvation isn't a good way to lose weight. The body can't exist on calories from fat alone, so it breaks down muscle tissue to provide calories from protein. Dr. Kimmel notes that ideal weight loss is best achieved by reducing daily intake by about 500 calories. He encourages athletes to eat small frequent meals rather than few large meals, and he urges them to eat breakfast. A healthy diet contains no fat.