Males with twin sisters are more likely to develop anorexia nervosa than other males, including males with a twin brother. Does exposure to female sex hormones in the womb increase the risk of getting it? If so, is anorexia a sex steroid hormone issue rather than strictly a psychological one?

“Anorexia nervosa is approximately 10 times more common in females than in males,” write study authors in the Archives of General Psychiatry. “The reasons for this difference are not known, and it is likely that their unraveling will represent an important step forward in the understanding of the etiopathogenetic factors involved in the development of eating disorders.”

Marco Procopio, M.D., M.R.C.Psych., of the University of Sussex, Brighton, England, and Paul Marriott, Ph.D., of the University of Waterloo, Ontario, Canada, analyzed data from a study of Swedish twins born between 1935 and 1958. Two sets of diagnostic criteria, one broader and one more narrow, were used to determine which twins had anorexia nervosa.

Overall, female twins were more likely than male twins to develop anorexia nervosa. The one exception was among males who had a dizygotic (fraternal) twin sister. “In fact, their risk is at a level that is not statistically significantly different from that of females from such a pair,” the authors write. Among 4,478 dizygotic opposite-sex twins, 20 females and 16 males had anorexia nervosa using narrow criteria and 32 females and 27 males qualified under the broad criteria. Risk for these female twins was not significantly different from than that of other female twins.

“A plausible explanation for this phenomenon is that in pregnancies bearing a female fetus, a substance is produced, probably hormonal, that increases the risk of having anorexia nervosa in adulthood,” the authors write. “Because the male half of an opposite-sex twin pair would also be exposed to this substance, it could account for the observed elevated risk in males with female twins. The most likely candidates are sex steroid hormones.”

“The results of our study are compatible with the hypothesis that intrauterine exposure to sex hormones might influence neurodevelopment, affecting the risk of developing anorexia nervosa in adult life,” they conclude. “This might be a factor contributing to the higher risk of developing anorexia nervosa in females.”

Arch Gen Psychiatry. 2007;64(12):1402-1408.

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