Not having enough body fat can be as harmful as having too much. When your body doesn't have enough fuel to burn, it goes into starvation mode. True, you may lose weight, but your metabolic functions will slow down, leaving you with no energy, little ability to concentrate, a weakened immune system, and anemia. You also lose muscle density in starvation mode. In addition, your skin becomes thin, pale, and furry, and your hair falls out. And, as your body loses nutrients like calcium your bones age prematurely. For more information follow the link.