Bulik noted that there is a female preponderance in bulimia, but the diagnostic criteria are "basically an accepted set of female norms."
"We need to look at the ways men experience body dissatisfaction and to focus on the methods men use to change their bodies," she said. "They [men] usually focus on increasing their lean mass or becoming more muscular."
Bulik continued with a brief description of genetic epidemiology, which looks at how genes and environment influence the risk for specific disorders. The first question asked is whether eating disorders run in families. If the answer to the family studies is that the disorder does run in families, the next step is twin and adoption studies to help determine the extent to which the disorder is due to genes or the environment. If genes are determined to be important, the researchers move on to conducting linkage and association studies to identify where the genes are and what they do.
Both AN and BN are strongly familial and are, in fact, as familial as schizophrenia and bipolar disorder, Bulik said.
"They don't breed true, in that a patient with anorexia doesn't only have family members with anorexia. You will see a mixture of eating disorders among the family members--bulimia, eating disorders [not otherwise specified] and threshold eating disorder behaviors. But relatives of individuals with eating disorders are at seven to 12 times higher risk than relatives of individuals without eating disorders. The familiality of anorexia nervosa is the highest; they tend to have the densest family history," she said.
While there are no published adoption studies for AN or BN, Bulik said, twin studies do exist. In twin studies, researchers compare concordance rates (the frequency with which both members of the twin pair have the disorder or the trait of interest). They compare those concordance rates in monozygotic and dizygotic twins.
Bulik explained that eating disorders are complex, and investigators know they are looking for multiple genes and environmental factors. To do that, the investigators are going beyond concordance rates in twins and looking at three different pockets of variance: the additive effect of genes; shared environmental effects, such as family religion, parental rearing style or socioeconomic status in which the twin pair was raised; and unique environmental effects, those things that happen in the environment that will influence one of the twins but not the other. For example, one of the twins joins a gymnastic club where an incompetent coach pressures her to diet to an unrealistic weight, while the other twin joins a soccer team where weight is not a major issue.
"When we started to do twin studies, we were all surprised, given that we had all grown up in that sociocultural tradition of eating disorders. When we look at those three pockets of variance, we actually find that the heritability of bulimia nervosa, for example, is somewhere between 59% and 83%. So what we are saying is the liability for developing bulimia nervosa is predominately affected by genetic factors. Shared environment wasn't terribly important. The remainder of risk comes out in those unique environmental factors--those slings and arrows of outrageous fortune," Bulik said. "Similar results came for anorexia nervosa--with the heritability of 58% to 76%. å Again, shared environment wasn't terribly important, and the unique environment seemed to influence risk for anorexia nervosa."