Prenatal exposure to female hormones increases the risk of anorexia nervosa, according to new research released Monday that bolsters the theory that the disorder has a biological basis in addition to a social and cultural one.
In an analysis of 4,478 pairs of opposite-sex twins, researchers found that the males had the same chance of becoming anorexic as women in general, an indication that hormones circulating in their mother's womb increased their susceptibility to the disorder.
The study, published in the Archives of General Psychiatry, shed new light on the causes of the eating disorder, which is often linked to a cultural obsession with thinness and is 10 times as common in women than men.
"Any information that points to a biological origin is helpful, at least for future development of medication and other therapies," said Dr. Thomas Weigel, a psychiatrist who treats eating disorders at McLean Hospital in Belmont, Mass.
"If there is medical origin, that suggests a medical solution," said Weigel, who was not connected to the study.
An often intractable disorder, anorexia affects about 1% of females in the United States and leads to death in 10% of the cases. People with anorexia are obsessed with body weight and diet to the point of starving themselves. About half the people with the disorder binge on food and then purge it (inducing vomiting, taking laxatives and diuretics, or both). The other half restricts food intake and exercises excessively.
Standard treatment for the disorder is behavioral therapy, and in severe cases, patients are hospitalized to restore their weight. The relapse rate in the worst cases is high, and half are readmitted within a year.
The disorder has long been seen as a consequence of a culture that equates beauty with thinness. People involved in activities that emphasize weight or body image, such as modeling, ballet, gymnastics or figure skating, have a higher risk of becoming anorexic. Male jockeys and wrestlers who must stringently watch their weight are also at increased risk.
More recently, researchers have found evidence that a proclivity to anorexia may be genetic. A large study of female twins published in Archives of General Psychiatry last year found that identical twins were more likely to share the disorder. The study concluded that 60% of the risk of developing anorexia was linked to heredity and the remaining risk was related to social factors.
The latest study set out to explain why anorexia was more prevalent among women.
Researchers analyzed data on more than 18,000 pairs of twins born in Sweden between 1935 and 1958. Among same-sex twins, the study found, the prevalence of anorexia among females was 0.6%, which the researchers said was the same as the baseline rate they used for adult females in the general population. The researchers found that females were about 12 times as likely as males to develop anorexia.
In opposite-sex twins, the females had the same risk of developing anorexia as females generally -- but the females' twin brothers also had approximately the same risk.
Coauthor Dr. Marco Procopio, a psychiatrist at the University of Sussex in Britain, said the findings implicated prenatal exposures to the female sex hormone estrogen, which probably had an effect on the developing brain. He said it was likely that hormones produced by female fetuses and their mothers had a role.
"This study shows the importance of the intrauterine environment in the development of anorexia," he said. "Possibly there is a need for genetic predisposition and a certain hormonal environment in utero to develop the illness."
Procopio said he suspected that prenatal conditions had a role in other mental disorders. He is beginning studies to look for the influence of intrauterine hormonal exposures on other conditions such as autism and attention deficit hyperactivity disorder, both of which are more common in males.