Causes and contributory factors

Genetic factors
Family and twin studies have suggested that genetic and environmental factors account for 74% and 26% of the variance in anorexia nervosa, respectively. This evidence suggests that genes influencing both eating regulation, and personality and emotion, may be important contributing factors. In one study, variations in the norepinephrine transporter gene promoter were associated with restrictive anorexia nervosa, but not binge-purge anorexia (though the latter may have been due to small sample size).

Neurobiological factors
Anorexia may be linked to a disturbed serotonin system, particularly to high levels at areas in the brain with the 5HT1A receptor - a system particularly linked to anxiety, mood and impulse control. Starvation has been hypothesised to be a response to these effects, as it is known to lower tryptophan and steroid hormone metabolism, which might reduce serotonin levels at these critical sites and ward off anxiety. Other studies of the 5HT2A serotonin receptor (linked to regulation of feeding, mood, and anxiety), suggest that serotonin activity is decreased at these sites. There is evidence that both personality characteristics and disturbances to the serotonin system are still apparent after patients have recovered from anorexia.

Changes in brain structure and function are early signs often to be associated with starvation, and is partially reversed when normal weight is regained. Anorexia is also linked to reduced blood flow in the temporal lobes. It is possible that it is a risk trait rather than an effect of starvation.

Anorexia may be linked to an autoimmune response to melanocortin peptides which influence appetite and stress responses.

Nutritional factors
Zinc deficiency may play a role in Anorexia. It is not thought responsible for causation of the initial illness but there is evidence that it may be an accelerating factor that deepens the pathology of the anorexia. A 1994 randomized, double-blind, placebo-controlled trial showed that zinc (14 mg per day) doubled the rate of body mass increase compared to patients receiving the placebo.

Psychological factors
Anorexic eating behavior is thought to originate from an obsessive fear of gaining weight due to a distorted self image and is maintained by various cognitive biases that alter how the affected individual evaluates and thinks about their body, food and eating. This is not a perceptual problem, but one of how the perceptual information is evaluated by the affected person. People with anorexia nervosa seem to more accurately judge their own body image while lacking a self-esteem boosting bias.

People with anorexia nervosa also have other psychological difficulties and mental illness. Clinical depression, obsessive compulsive disorder, substance abuse and one or more personality disorders may be the most likely conditions to be comorbid with anorexia. High-levels of anxiety and depression are likely to be present regardless of whether they fulfill diagnostic criteria for a specific syndrome.

Research into the neuropsychology of anorexia has indicated that many of the findings are inconsistent across studies and that it is hard to differentiate the effects of starvation on the brain from any long-standing characteristics. One finding is that those with anorexia have poor cognitive flexibility.

Other studies have suggested that there are some attention and memory biases that may maintain anorexia.

Social and environmental factors
Sociocultural studies have highlighted the role of cultural factors, such as the promotion of thinness as the ideal female form in Western industrialised nations, particularly through the media. A recent epidemiological study of 989,871 Swedish residents indicated that gender, ethnicity and socio-economic status were large influences on the chance of developing anorexia, with those with non-European parents among the least likely to be diagnosed with the condition, and those in wealthy, white families being most at risk. People in professions where there is a particular social pressure to be thin (such as models and dancers) were much more likely to develop anorexia during the course of their career, and further research has suggested that those with anorexia have much higher contact with cultural sources that promote weight-loss

There is a high rate of reported child sexual abuse experiences in clinical groups of who have been diagnosed with anorexia. Although prior sexual abuse is not thought to be a specific risk factor for anorexia, those who have experienced such abuse are more likely to have more serious and chronic symptoms.