Male Twins at Risk for Anorexia
An Observed Phenomenon
While women become anorexic ten times more often than males, a study from April 2007 shows that males with a twin sister have a greater tendency than other males of developing anorexia nervosa, an eating disorder characterized by self-imposed starvation. This finding mirrors the hypothesis that one factor related to the risk for anorexia nervosa is exposure to female sex hormones in the womb.
Speaking to the preponderance of anorexia in female populations as opposed to the rate of this disorder in men, the authors of the report comment, "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 [cause and development of] factors involved in the development of eating disorders."
For purposes of this study, data was analyzed from a previous study of Swedish twins born between the years 1935-1958. 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, in Ontario, Canada, used two sets of diagnostic criteria, one broad and one narrow, to determine which twins had developed anorexia nervosa.
In general, female twins developed anorexia at a rate higher than male twins. The only exception was among males having a fraternal twin sister, of whom it was found that anorexia occurred at similar rates to those of the female halves of such pairs. Of 4,478 opposite sex fraternal twins, 20 females and 16 males were defined as having anorexia nervosa according to the narrow criteria, whereas 32 females and 27 males were anorexic when broad criteria were applied.
In Utero Exposure?
Procopio and Marriot believe that the phenomenon may be explained by the fact that pregnancies bearing a female fetus cause a substance to be produced that is probably hormonal in nature and which increases the apparent risk of contracting anorexia nervosa in adulthood. In a fraternal pregnancy containing both a female and a male twin, the male half of the pair would also be exposed to such a substance, which would account for the observed phenomenon of elevated risks for the eating disorder in males with female twins. The authors posit that the substance could well be a sex steroid hormone.
The authors of the study believe that such in utero exposure might be a factor contributing to the higher risk of females in developing anorexia nervosa.