Uncovering Links between AN and BN

A moderate overlap of genetic and environmental factors was noted.

Although anorexia nervosa (AN) and bulimia nervosa (BN) are often dealt with as if they were distinct disorders, for some time researchers have sought possible overlaps between the two disorders.

Clinically, pure forms of AN (e.g., such as restricting subtype AN) and BN (e.g., no history of AN) exist. However, most patients with AN develop some features of BN during the course of their illness. Likewise, up to 27% of individuals with BN report a history of AN, and from 20% to 50% develop AN after meeting full criteria for BN. For example, while AN is marked by low weight, provisions are made for the presence of bulimic symptoms in the binge-purge subtype of AN. No provisions are made in BN criteria for a past history of AN. And, importantly, the current classification system fails to capture the considerable flux in symptoms during the course of both disorders: from 8% to 54% of patients have frequent crossover of symptoms.

To determine if the partial overlap in symptoms is due to shared genetic or environmental factors, Dr. Cynthia M. Bulik, University of North Carolina at Chapel Hill, and colleagues from the University of North Carolina, the Karolinska institute, Stockholm, and the University of Southern California conducted a bivariate twin analysis of AN and BN (Biol Psychiatry 210; 67:71). The researchers used data from the Swedish Twins Study of Adults: Genes and Environment (STAGE) study, a population-based prospective sample of Swedish twins born between 1959 and 1985 (ages 20-47 years). More than 25,000 individuals from a total sample of 43,000 participated in the study (10,510 males and 13,295 females). The final sample for modeling included 7,000 females from identical (i.e., monozygotic [MZ]) and fraternal (i.e., dizygotic [DZ]) same-sex twin pairs.

Zygosity was determined with a series of questions concerning the physical simi8larity of co-twins. For example, twins who responded ‘alike as two peas in a pod’ after being asked about their physical similarities were classified as MZ. If both twins responded that they were clearly ‘not alike,’ they were classified as DZ.

Genetic and environmental factors were identified

Dr. Bulik and colleagues found a moderate overlap of both genetic and unique environmental factors that influenced AN and BN. Their results suggest that about half of the genetic factors contribute to liability to both disorders, while the remaining genetic factors contribute independently to AN and BN. The authors’ heritability estimates are broadly similar to those reported from other twin studies; using the full models, they found an additive genetic effect for narrow AN of .57 and .29 for broad AN. Heritability estimates are derived from bivariate structural equation modeling applied to a broad definition of AN and BN.

The study did have several limitations, including the fact that diagnoses of AN and BN were based on computer-generated self-reports. Also, although the study reported a 56% response rate, this relatively low rate may have introduced bias. Third, because the age range of the participants was from 20 to 47 years, not all of the individuals might have reached their ultimate diagnostic profile and crossover could still occur at a later time. Also, the results apply only to females. Strengths included a large patient population with enough persons with AN and BN threshold DSM-IV diagnostic criteria, in addition to broader case definitions.

The authors note that their findings may be helpful in the study of diagnostic crossover and may also inform future revisions of eating disorders diagnosis classifications. Currently, the only way to account for the presence of bulimic features in a patients with AN is if the features currently meet diagnostic criteria for AN and warrant inclusion of the binge/purge subtype. If they currently meet criteria for AN restricting subtype, there is no way to report a history of BN or binge/purge AN—this transition is rare but does occur. An optimal diagnostic method for eating disorders could account for both concurrent symptoms expression of AN- and BN-related traits as well as the historical presentation or one or the other eating disorder.