Anorexia nervosa is an eating disorder in which the affected individuals perceive themselves to be overweight even when they may be grossly underweight. The disorder is characterized by a deep desire to keep body weight low at all costs. It is a psycho-physiological condition in which the fear of obesity leads to an unwillingness to eat and the individual may exercise excessively. Physiological changes include alterations to the menstruation cycle or impotence. The disorder mainly affects young adult women or adolescents.