If a dieter persists long enough with the self-imposed famine, the body begins to break down muscle tissue for fuel. When protein is broken down, it releases nitrogen. Your body will quickly wash away the nitrogen by releasing water from tissue cells, causing an immediate reduction in water weight and a noticeable drop on the scale. However, water and muscle loss is nothing to celebrate. The water weight will be quickly regained as soon as you have something to drink, and the missing muscle can wreak havoc on your metabolism for a good long time.
Muscle is a metabolically active tissue.
It requires a certain number of calories each day to maintain itself. Therefore, the more muscle you have, the more calories you burn even when you're just sitting around. As your muscle mass drops, so does your daily calorie requirement. Now suppose that each pound of muscle had been burning 50 calories a day just sitting there. Together, those 10 pounds of muscle had been burning 500 calories a day.