Environmental. Historical and cross-cultural variability in the prevalence of anorexia nervosa supports its association with cultures and settings in which thinness is valued. Oc- cupations and avocations that encourage thinness, such as modeling and elite athletics, are also associated with increased risk.
Genetic and physiological. There is an increased risk of anorexia nervosa and bulimia nervosa among first-degree biological relatives of individuals with the disorder. An in— creased risk of bipolar and depressive disorders has also been found among first-degree relatives of individuals with anorexia nervosa, particularly relatives of individuals with the binge—eating/purging type. Concordance rates for anorexia nervosa in monozygotic twins are significantly higher than those for dizygotic twins. A range of brain abnormali- tional magnetic resonance imaging, positron emission tomography). The degree to which these findings reﬂect changes associated with malnutrition versus primary abnormalities associated with the disorder is unclear.
Anorexia nervosa occurs across culturally and socially diverse populations, although available evidence suggests cross-cultural variation in its occurrence and presentation. Anorexia ner- vosa is probably most prevalent in post-industrialized, high-income countries such as in the
United States, many European countries, Australia, New Zealand, and Japan, but its incidence in most low- and middle-income countries is uncertain. Whereas the prevalence of anorexia nervosa appears comparatively low among Latinos, African Americans, and Asians in the
United States, clinicians should be aware that mental health service utilization among individ- uals with an eating disorder is significantly lower in these ethnic groups and that the low rates may reﬂect an ascertainment bias. The presentation of weight concerns among individuals with eating and feeding disorders varies substantially across cultural contexts. The absence of an expressed intense fear of weight gain, sometimes referred to as ”fat phobia,” appears to be relatively more common in populations in Asia, where the rationale for dietary restriction is commonly related to a more culturally sanctioned complaint such as gastrointestinal discom- fort. Within the United States, presentations without a stated intense fear of weight gain may be comparatively more common among Latino groups.
The following laboratory abnormalities may be observed in anorexia nervosa; their pres- ence may serve to increase diagnostic confidence.
Hematology. Leukopenia is common, with the loss of all cell types but usually with ap- parent lymphocytosis. Mild anemia can occur, as well as thrombocytopenia and, rarely, bleeding problems.
Serum chemistry. Dehydration may be reﬂected by an elevated blood urea nitrogen level. Hypercholesterolemia is common. Hepatic enzyme levels may be elevated. Hypo- magnesemia, hypozincemia, hypophosphatemia, and hyperamylasemia are occasionally observed. Self-induced vomiting may lead to metabolic alkalosis (elevated serum bicarbon- ate), hypochloremia, and hypokalernia; laxative abuse may cause a mild metabolic acidosis.
Endocrine. Serum thyroxine (T4) levels are usually in the low-normal range; triiodothy- ronine (T3) levels are decreased, while reverse T3 levels are elevated. Females have low se- rum estrogen levels, whereas males have low levels of serum testosterone.
Electrocardiography. Sinus bradycardia is common, and, rarely, arrhythmias are noted.
Significant prolongation of the QTc interval is observed in some individuals.
Bone mass. Low bone mineral density, with specific areas of osteopenia or osteoporo- sis, is often seen. The risk of fracture is significantly elevated.
Electroencephalography. Diffuse abnormalities, reﬂecting a metabolic encephalopa- thy, may result from significant ﬂuid and electrolyte disturbances.
Resting energy expenditure. There is often a significant reduction in resting energy ex— penditure.