Average expected weights should not be considered ideal weights. The correct weight varies from 10 percent below to 10 percent above the average weight. Weight well above or below this is probably abnormal. A person may decide that he or she is underweight, but this may be because of racial or inherited characteristics. If, however, there has definitely been a weight loss, a physician should be consulted and the cause found.
Some people become dangerously underweight by starving themselves.
Anorexia nervosa is an extreme form of self-starvation common in adolescent girls and needs specialized drug and psychiatric treatment. Depression, anxiety, and general fatigue are other factors in weight loss. Nearly always because they consume more calories than they burn off. When people eat more than they need, the excess calories are stored as fat. Some people may be overweight because of a physical disorder, and some gain weight because of compulsive eating, often a sign of underlying anxiety or depression.