A certain amount of body fat is essential to cushion the joints, protect the organs and help regulate body temperature, as well as to store vitamins and help the body sustain itself when food is not available. Therefore, it's possible to have a body fat percentage that's too low.
Numerous athletes, some of the healthiest people around, have five per cent fat and are extremely healthy. Some researchers feel that with elite athletes, strength and mental concentration deteriorate below seven per cent body fat. As well, loss of muscle can occur below five per cent, as the body will turn to muscle for fuel in the absence of fat.
Generally speaking, seven percent to ten percent body fat is found on elite athletes such as long-distance runners. Eleven per cent to 13 per cent is considered exceptionally fit; 14 per cent to 18 per cent is considered average. These numbers are for men in their 20s.