Researchers studying anorexia in twins conclude that more than half a person's risk for developing the sometimes fatal eating disorder is determined by genes.

Most experts already believe there is a strong genetic component to the disorder, which mostly affects girls and women. The new study "hammers home the fact that these are biologically based disorders," said Cynthia Bulik, lead author of the study who is a psychiatrist at the School of Public Health at the University of North Carolina-Chapel Hill.

"We need to stop viewing them as a choice. ... The patients feel guilty, the providers tell them things like they should just eat, parents are blamed, the insurance companies won't fund treatment because they think it's a choice. It's held us back for decades."

People with anorexia have a distorted body image and refuse to maintain a minimally acceptable body weight. Bulik said anorexics are about 10 times more likely to die in a given period of time than peers the same age.

Anorexia's rarity — slightly more than 1 percent of females and well under 1 percent for males — has made it hard for scientists to gather large groups of patients for study.

The study by researchers at UNC and Sweden's Karolinska Institute looked at a Swedish registry of 31,406 twins — both identical and fraternal — born between 1935 and 1958. Identical twins are genetic clones, while fraternal twins are no more similar genetically than a brother and sister born in separate pregnancies.

Anorexia was more prevalent between identicals, and statistical analysis led to the scientists' conclusion that 56 percent of the liability for developing anorexia is due to genetics, with environmental factors determining the rest, Bulik said.

That means not everyone with a genetic predisposition to anorexia develops it.

"A person may have genetic liability for anorexia nervosa, but they also may have — from a different parent, for example — genes that buffer them from expression of the disorder," she said. The person's environment might also provoke anorexia or prevent it.

Michael Strober, a clinical psychologist at the University of California at Los Angeles and editor of the International Journal of Eating Disorders, said conventional wisdom is that genetic factors do play a role in susceptibility.

This latest study, published in the March issue of the Archives of General Psychiatry, further confirms previous research, Strober said.

The study also found a link between anorexia and childhood "neuroticism," which Bulik describes as "a tendency to be depressed or anxious, and also to be emotionally reactive."

"For some kids, insults come right off them like water off a duck's back," she said. "These kids are more like emotional Velcro. Things stuck to them, get under their skin, and it influences them."

For Strober, the new study also lends support to the belief that personality traits, including neuroticism, are important in the development of anorexia. He believes that nearly all anorexia sufferers exhibit neurotic behavior in childhood.

Bulik and Strober are both involved in a large, federally funded multiyear study of anorexia. Headed by Dr. Walter Kaye, a psychiatry professor at the University of Pittsburgh Medical Center, that study is seeking families with two or more members with anorexia.

"This is a disorder where we haven't seen great treatments," Kaye said. "At least some of us have thought there's a very powerful biology at work here. ... The next step, of course, will be to determine what the biology is, what genes are involved and what difference they make as far as how the brain works."

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