Genetic Influences on Eating Disorders

For decades, anorexia nervosa, bulimia nervosa, and binge-eating disorder were considered to be disorders influenced primarily by family and sociocultural factors. However, over the past decade, it has become clear that these disorders run in families, that the observed familiality is due primarily to genetic and unique environmental factors, and specific areas of the genome and specific genes have been implicated in etiology. Eating disorders are relevant to obesity insofar as they represent forms of dysregulation of appetite and weight, and genetic mechanisms that are operative in eating disorders may be of direct relevance to understanding genetic factors that influence risk for obesity.

Anorexia nervosa is a serious psychiatric illness marked by an inability to maintain a healthy body weight, often dropping below 85 percent of expected weight. Despite increasing emaciation, individuals with anorexia nervosa continue to obsess about body weight and shape, remain dissatisfied with the perceived size and shape of their bodies, and engage in unhealthy behaviors to perpetuate weight loss (e.g., purging, dieting, excessive exercise, and fasting). Bulimia nervosa occurs in individuals of all body weights and is marked by binge eating (i.e., eating an unusually large amount of food and feeling out of control) coupled with inappropriate compensatory behaviors (e.g., self-induced vomiting, laxative abuse, diuretic abuse, excessive exercise, and fasting). Binge-eating disorder, often associated with obesity, is characterized by recurrent episodes of binge eating and associated distress, and the absence of regular inappropriate compensatory behaviors.

Family Studies

Family studies have revealed consistent evidence for familial aggregation of eating disorders. Relatives of individuals with anorexia and bulimia nervosa are approximately 10 times more likely to have an eating disorder than relatives of unaffected individuals. Family members of an individual with binge-eating disorder are more than twice as likely to have the disorder themselves compared to those with an overweight or obese relative without binge-eating disorder. Intriguingly, [Page 317]anorexia and bulimia nervosa do not “breed true,” meaning that risk is increased in relatives for an array of eating disorders, not just the disorder of the index case. Family studies are unable to determine the extent to which the familial pattern is due to genetic or environmental factors.

Twin Studies

Although adoption studies are a valuable tool for unpacking the extent to which genes and environment contribute to risk for a disorder, no adoption studies of eating disorders exist. Therefore, attempts at determining the relative contributions of genes and environment to eating disorders have focused exclusively on twin studies. Twin studies utilize the fact that monozygotic (identical) twins share 100 percent of their genome, whereas dizygotic (fraternal) twins share only 50 percent. Thus, if both members of monozygotic twin pairs are more frequently affected (i.e., concordant) than members of dizygotic twin pairs, then a genetic contribution is suggested. Additional analyses enable more detailed decomposition of liability into genetic, shared environmental, and unique environmental influences.

The heritability of anorexia nervosa has been estimated to be between 33 and 84 percent, and the heritability of bulimia nervosa between 28 and 83 percent, with the remaining variance in both disorders attributable to individual specific environmental factors, and with negligible impact of shared environmental factors. The studies on which these estimates are based are primarily twin populations of European ancestry, and given the relative rarity of the traits under study, confidence intervals tend to be wide. One twin study has been conducted for a broad definition of binge-eating disorder, and yielded a heritability estimate of 41 percent. Individual environmental factors accounted for the remaining 59 percent of the variance. In summary, findings from twin studies have been replicated in several countries throughout the world—all indicating a substantial contribution of genetic factors to eating disorders. Additional analyses on more diverse populations will enable a more complete understanding of the role of genes and environment in risk for these disorders across racial and ethnic groups.

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