Those suffering from anorexia nervosa have an emotional disorder that disrupts normal eating habits. People who suffer from anorexia will tend not to eat. For information on how anorexia is diagnosed and treated go to this site: http://www.ask.com/wiki/Anorexia_nervosa.
Depression is an emotional disorder that can be manifested by severe weight loss or failure to gain weight. Depressed people often withdraw themselves from their old routines and eventually end up neglecting their physiologic needs which includes eating and drinking.