Anorexia nervosa. Illustration of anorexia nervosa which is a mental disorder characterised by intense fear of being fat, wilful avoidance of food and consequent severe weight loss. The fundamental cause of anorexia is unknown. It mostly affects girls soon after puberty and young women. It has been proposed that anorexia nervosa is a self-protective device employed by vulnerable young persons who fear the prospect of growing up, leaving home and the family and facing their new sexuality.