CHAPEL HILL, N.C., March 6 - Anorexia nervosa -- presumed to be a pathologic response to images of slim fashion models -- is actually largely controlled by genetic factors, according to data from the Swedish Twin Registry.
The calculation that the disorder has a strong genetic basis comes from a study of 31,406 participants in the Swedish Twin Registry, according to Cynthia Bulik, Ph.D., director of the University of North Carolina's eating disorders program and a professor of psychiatry.
Dr. Bulik said "this is the best estimate so far" of the heritability of anorexia nervosa, mainly because the sample population was so large.
Her study, reported in the March issue of Archives of General Psychiatry, also found that the only significant predictor of the development of anorexia was neuroticism, which Dr. Bulik defined as an "innate tendency to be depressed, anxious and emotionally reactive."
Other possible predictors -- including body mass index, excessive exercise, gastric problems early in life, and perceived life stress -- were not significantly associated with the risk of anorexia, Dr. Bulik and colleagues found.
The findings -- especially the genetic link -- are "good news for patients and good news for their parents," Dr. Bulik said in a telephone press conference.
"We have gone through far too much time when parents are blamed for causing this disorder," she said. Patients with anorexia -- mainly young and adolescent females -- are "fighting their biology," she said.
Dr. Bulik and colleagues studied two cohorts of Swedish twins, those born between 1938 and 1944 and those born between 1944 and 1958. The cohorts were chosen because their members would have been teenagers or young adults in 1972 and 1973, when a prospective study -- the Screening Across the Lifetime of Twins (SALT) study -- was begun.
Between 1998 and 2002, the researchers conducted interviews with 31,406 members of the twin registry who had taken part in the SALT study; twins who already had anorexia in 1972-1973 were excluded, to allow investigators to study prospective predictors of the disorder.
The study found:
--The overall prevalence of anorexia was 1.29% in females and 0.29% in males, roughly comparable with earlier research.
--In females, the prevalence increased, from 0.65% among the older twins to 1.56% in those born between 1945 and 1958. There was no change for men.
--The heritability of the disorder, assessed by comparing concordance between monozygotic and dizygotic twins, was 0.56, although the 95% confidence interval was wide -- from 0 to 0.87.
--The contributions of shared environment and unique environment were 0.05 and 0.38, respectively.
--Neuroticism in the early 1970s - about three decades before the 1998-2002 interviews - was significantly associated with the later development of anorexia. The odds ratio was 1.62, with a 95% confidence interval of 1.27 to 2.05.
The heritability finding places anorexia "well within the more heritable psychiatric disorders," Dr. Bulik said, and shows "clearly that genes play a substantial role in liability to this illness."
The increase in prevalence among females, she and colleagues noted, might be an artifact, because many of the participants in the older cohort were born before there was widespread awareness of eating disorders.
That could cut both ways -- some people might have had the disorder but never been labeled or detected, while more awareness later might have led to increased detection, the researchers said.
The researchers also noted that the point estimate for heritability is similar to those found in earlier, smaller studies, but the wide confidence intervals mean the "estimates remain imprecise.
Dr. Bulik said the finding should lead to more study of the genetic basis for anorexia, with the goal of developing medications. The finding that anorexia runs in families should also help parents be more aware of the issue.