The magnitude of inherited influences is unclear. Several studies have found an increased rate of anorexia nervosa and bulimia nervosa among the relatives of probands with bulimia nervosa,(25) whereas other studies have found either no increase or only an increase in atypical eating disorders. (30) The findings of the first twin series of note suggested that the inherited contribution was minimal. ^ More recent studies from the Virginia Twin Registry have yielded inconsistent findings with estimates of heritability varying from negligible (48> to 55 per cento to 83 per cent.(iE> A number of factors are likely to have contributed to the variability in the findings, including the small sample sizes (which render twin models unstable) and the difficulty making lifetime diagnoses of bulimia nervosa. It is also of note that, in contrast with other psychiatric disorders, the findings suggest that there may be violations of the equal environments assumption that underpins twin studies. (4 ,49)
The nature of any inherited vulnerability is a matter of speculation. The liability appears not to be shared with that for other psychiatric disorders, (25> although there is evidence suggestive of shared familial transmission between anorexia nervosa and obsessive-compulsive personality disorder, (30) a construct which overlaps with that of perfectionism. It is possible that there is an inherited abnormality in the regulation of weight and eating habits in view of the raised rates of parental and childhood obesity. This may include susceptibility to adverse consequences of dieting. Dieting is known to affect 5-hydroxytryptamine neurotransmission particularly among women,(50) and there is evidence of abnormalities in brain 5-hydroxytryptamine function in women who have recovered from bulimia nervosa. For example, one study found that acutely lowering brain 5-hydroxytryptamine function resulted in the temporary reappearance of bulimic symptoms (51) and another found abnormal levels of cerebrospinal fluid 5-hydroxyindoleacetic acid. (52> The molecular genetic studies of eating disorders are focusing on the genes involved in
5-hydroxytryptamine neurotransmission but no consistent findings have emerged.(25)
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