A boy/man who has a twin sister has a higher chance of developing anorexia nervosa, an eating disorder, compared to other males, including those who have a twin brother, according to an article in Archives of General Psychiatry (JAMA/Archives), December issue. The authors suggest that this latest finding adds evidence to the hypothesis that exposure to female sex hormones in the womb is perhaps linked to a higher risk of developing anorexia nervosa.

The researchers explain “Anorexia nervosa is approximately 10 times more common in females than in males. 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., University of Sussex, Brighton, England, and Paul Marriott, Ph.D., University of Waterloo, Ontario, Canada, examined data on Swedish twins born during 1935-1958. They used two sets of diagnostic criteria to determine which twins had developed anorexia nervosa – a broad set and a narrower one.

They found that female twins had a higher chance of developing the eating disorder compared to male twins, except for male twins who had a fraternal (dizygotic*) twin sister. “In fact, their risk is at a level that is not statistically significantly different from that of females from such a pair,” they wrote.

Out of 4,478 dizygotic opposite-sex twins they analyzed, 16 males and 20 females had developed anorexia using the narrow criteria, while 27 males and 32 females had done so under the broad criteria – showing that there was not a significantly different risk.

The researchers explained “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 authors concluded “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. This might be a factor contributing to the higher risk of developing anorexia nervosa in females.”

* (dizygotic =The product of fertilization of 2 separate eggs by 2 separate sperm; nonidentical twin pair.)

“Intrauterine Hormonal Environment and Risk of Developing Anorexia Nervosa”
Marco Procopio, MD, MRCPsych; Paul Marriott, PhD
Arch Gen Psychiatry. 2007;64(12):1402-1407.
Click here to view abstract online

Written by – Christian Nordqvist