Individuals with anorexia nervosa are unable or unwilling to maintain a healthy body weight that is expected for their age and height. There is no definition of "normal" or "too low", so the medical community has generally agreed to use 85% of normal (healthy for age and height) weight as a guide. Individuals with anorexia nervosa have a strong fear of weight gain and a dread of becoming fat even though they are markedly underweight. Weight and shape concerns affect how the person evaluates him or herself. The seriousness of the weight loss and its health implications is usually minimized, if not denied, by the individual.

There are two subtypes of the anorexia nervosa: First, individuals with the restricting type maintain their low body weight purely by restricting food intake and, possibly, by exercise. Second, those with the binge-eating/purging type usually restrict their food intake as well, but also regularly engage in binge eating and/or purging behaviors such as self-induced vomiting or the misuse of laxatives, diuretics or enemas.