Eating disorders are not very stable. There is a lot of movement between the disorders. People with anorexia nervosa choose a thin figure as their ideal and overestimate their own body size, but are fairly accurate at reporting their weight. Maintaining thinness is strongly linked to self-esteem in anorexia nervosa.

There are several biological consequences of anorexia nervosa: falling of blood pressure (1), slowing of heart rate (2), decline of bone mass (3), kidney and gastrointestinal problems (4), drying of the skin (5), brittle nails (6), change in hormone levels (7), alteration of electrolytes (e.g., potassium) (8), tiredness (9) and lanugo (10).

Lanugo is a fine, soft hair that develops on the body in people with anorexia nervosa. 50%-70% of the people with anorexia nervosa recover, but it takes ±6-7 years to recover. Severity ratings in anorexia nervosa are based on BMI.

Bulimia nervosa and binge-eating disorder include binge eating. A binge is most likely to occur after a negative social interaction. Maintaining normal weight is strongly linked to self-esteem in bulimia nervosa. Most people with bulimia nervosa were slightly overweight before the onset of the disorder and binge eating started during an episode of dieting.

There are several biological consequences of bulimia nervosa: potassium depletion (1), diarrhoea (2), irregularities in heartbeat (3), menstrual problems (4), ragged teeth (5), swollen salivary glands (6) and tearing of tissue in the stomach and throat (7). Close to 75% of the people with bulimia nervosa recover. There are several biological consequences of binge eating disorder, with most of them being a consequence of obesity: sleep problems (1), anxiety (2), depression (3), irritable bowel syndrome (4) and early onset of menstruation (5).

Eating disorders are highly heritable. The hypothalamus regulates the level of cortisol in the body and cortisol levels differ in people with anorexia nervosa, but these hormonal differences are most likely to occur because of self-starvation. Self-starvation and excessive exercise may also increase endogenous opioids, substances produced by the body that reduce pain sensations, enhance mood and suppress appetite. Endogenous opioids may positively reinforce the behaviour that maintains the disorder.

Serotonin promotes satiety and severe food intake restrictions could interfere with the serotonin system. Dopamine is linked to the pleasurable aspects of food that compel an animal to go after food. Restrained eaters may be more sensitive to food cues. People with anorexia nervosa and bulimia nervosa had a greater expression of the dopamine transporter gene DAT. Expression of DAT influences the release of a protein that regulates the reuptake of dopamine back into the synapse.

Perfectionism and a sense of personal inadequacy may lead a person to become especially concerned with appearance. Western culture emphasizes and reinforces the desirability of being thin more for women than for men. The objectification of women’s bodies exaggerates this effect. Body shame has a negative influence on eating disorders, as well as criticism from peers and parents about being overweight. The intense fear of becoming fat reflects an ideal more widely espoused in western cultures. There is variation in the clinical picture of anorexia nervosa across cultures and bulimia nervosa is more common in industrialized societies.

Behaviours that achieve or maintain thinness are negatively reinforced by the reduction of anxiety about becoming fat. Cognitive behavioural theories of anorexia nervosa emphasize the fear of fatness and body-image disturbance as the motivating factors that reinforce weight loss.