A male twin who shares the womb with a female is nearly as likely as girls from female twin pairs to develop anorexia, researchers have found. It suggests that prenatal conditions influence the likelihood of developing the eating disorder.
"It is a step forward ... in the understanding of the origins of eating disorders," Dr. Marco Procopio of the University of Sussex in Brighton, England, one of the study's authors, told Reuters Health. "Something is happening in the womb."
Anorexia nervosa is ten times as common in females as in males, Procopio and Dr. Paul Marriott of the University of Waterloo in Ontario, Canada note in their report in the Archives of General Psychiatry, but the reasons for this sharp gender difference in prevalence remain unclear.
The researchers looked a Swedish study of thousands of twin pairs born between 1935 and 1958 to determine whether prenatal exposure to sex hormones might be a factor. They hypothesized that males with a female twin would be more likely to develop anorexia because they would have been exposed to female sex hormones in the womb.
Their analysis bore out this hypothesis: men with a female twin were much more likely to develop anorexia than men from same-sex twin pairs. In fact, the anorexia risk for these male twins was not significantly different from that of their female twin.
On the other hand, females with a male twin were no less prone to anorexia than females from same-sex twin pairs, suggesting that male hormone exposure is not protective against the eating disorder.
"The one thing we are certain of is that there is a genetic disposition to anorexia, but at the same time this is not enough," said Procopio, pointing out that if the eating disorder was due to genes alone, an identical twin of an anorexia patient would also have the disease, but this isn't the case.
Some scientists have suggested that upbringing may be a factor in the gender difference in rates of occurrence of the disorder, he added, but studies haven't borne this out.
The latest findings could eventually lead to ways to treat and even prevent anorexia, said Procopio.
More attention should be paid to the condition, which exacts a lifelong toll on the psychological and physical health of its victims, he added. "People think that it's very uncommon, but it isn't. It has become one of the more common psychiatric problems."
