Sex hormones and anorexia

Anorexia could be determined by exposure to sex hormones in the womb, reported The Times on New Year’s Eve. The female sex hormone oestrogen “may be overproduced by some mothers, affecting the baby’s brain and making it susceptible to the eating disorder”, the newspaper said.

The newspaper reports are based on research into twins that has found that, although anorexia occurs more often in females than males, when they looked at twin pairs of different sexes, males who had shared the womb with a female were ten times more likely to develop anorexia in later life than if they had been in the womb with another male. However, anorexia is a complex psychological condition, and this research cannot prove that the higher rate of anorexia amongst girls, and boys of mixed twin pregnancies, is caused by higher exposure to sex hormones in the womb, and not by a number of other genetic, environmental or social factors.

Where did the story come from?

Dr Marco Procopio of the University of Sussex, Brighton and Paul Marriott of the University of Waterloo, Ontario, Canada carried out this research. No sources of funding were reported by the study. The study was published in the (peer-reviewed) medical journal: Archives of General Psychiatry .

What kind of scientific study was this?

In this cohort study of more than 12,000 pairs of Swedish twins, the researchers examined the rates of anorexia in same-sex and opposite-sex twins to investigate whether exposure to sex hormones in the womb affects brain development and causes a predisposition to anorexia nervosa in later life. The researchers suggested that when a male and female share the womb, the mix of hormones during development means that there is feminisation of the male and masculinisation of the female, and that the presence of a male in the uterus may be expected to protect against anorexia for the female or increase the risk in the male.

The researchers examined twins that had been identified through other research into the causes of anorexia, which included all members of the Swedish Twin Registry born between January 1935 and December 1958 who fulfilled the diagnostic criteria for anorexia nervosa. They looked at the number of twins with anorexia from female-female and male-male pairs (identical and non-identical) and in male-female pairs. They used both strict diagnostic criteria for anorexia (according to the Diagnostic and Statistical Manual of Mental Disorders), and a broader definition looking at weight and dietary history. 

What were the results of the study?

The researchers found there was a much higher rate of anorexia in females, when compared with all other males in the study. This finding was significant regardless of whether they used the strict or less strict ways of diagnosing anorexia. The rate of anorexia in males of male-female twin pairs was not significantly different from the female co-twins in the pairs, although it was significantly greater than males of male-male twin pairs (about 10 times greater).

This contrasted with females of male-female twin pairs, whose rate of anorexia was no different from females of female-female twin pairs. However, the rate of anorexia amongst any of the twins overall was low – 1.12% in non-identical female twin pairs using the loosest criteria.

What interpretations did the researchers draw from these results?

The researchers conclude that their results are “compatible with the hypothesis that intrauterine exposure to sex hormones might influence neurodevelopment, affecting the risk of developing anorexia nervosa in later life”. The authors present an extensive discussion of the possible explanations for their findings, which have not been covered in depth here.

What does the NHS Knowledge Service make of this study?

This study has looked at a large number of twins to measure the prevalence of anorexia. The authors acknowledge that their findings do not identify a proven cause of anorexia and there are several important things to consider when interpreting the findings:

• This research has been carried out to explore whether exposure to sex hormones in the uterus may affect neurological development and affect the predisposition to anorexia in later life. However, it has looked at the prevalence of anorexia amongst same sex and opposite sex twins only; it cannot prove that the higher prevalence of anorexia amongst girls, and boys of mixed twin pregnancies is caused by higher exposure to sex hormones and not to other genetic, environmental or social factors.
• This study does nor record any of the circumstances of the twins’ family history, upbringing, lifestyle or exposures. It is widely recognised that eating disorders are more common in females than in males; it is possible that the higher prevalence of anorexia seen amongst boys with a twin sister compared to those with a twin brother may be due to a number of complex reasons, such as shared social exposure with a close sibling, rather than because of influences in the womb. 
• The study has only examined twins born in a particular birth cohort (1935 to 1958), and those born in Sweden only; therefore it may not apply to other populations, or to those born in later generations.

Anorexia is a complex disorder, and the fact that not all girls develop an eating disorder in adolescence suggests that oestrogen exposure in the womb is not the main cause of anorexia and that there are many other factors at play.

Sir Muir Gray adds...

There is little scope for prevention or treatment, even if the association was found to be one of cause and effect.