Most of my readers know by now that I don't dispute the fact that eating disorders are biologically-based mental illnesses. They clearly have a biological component, although this isn't entirely without controversy. Research has yet to tease out exactly what these components are and how they interact with both each other and with the world at large. But the latest science points to a strong biological basis for eating disorders.

I recently read a fantastic review article by none other than my "homeboy" Walter Kaye* called The Neurobiology of Anorexia and Bulimia Nervosa, the full text of which is now freely available, understandable, and well worth the read. A few lines that lodged in my head:

Because AN and BN present most often during adolescence in women, they are often theorized to be caused by cultural pressures for thinness since dieting and the pursuit of thinness are common in industrialized countries. Still, AN and BN affect only an estimated 0.3% to 0.7% and 1.5% to 2.5%, respectively, of females in the general population. This disparity between the high prevalence of pressures for thinness and the low prevalence of eating disorders (EDs), combined with clear evidence of AN occurring at least several centuries ago, the stereotypic presentation, substantial heritability, and developmentally specific age-of-onset distribution, underscores the possibility of contributing biological vulnerabilities...Twin studies of AN and BN suggest there is approximately a 50 to 80% genetic contribution to liability accounted for by additive genetic factors. These heritability estimates are similar to those found in schizophrenia and bipolar disorder, suggesting that AN and BN may be as genetically-influenced as disorders traditionally viewed as biological in nature.

Additive genetic factors are pretty much what they sound like: several independent genes that together influence the likelihood of developing an eating disorder. The more factors, the higher the risk. The math is far more subtle and complicated than that, but that's essentially how it works.

In a recent article in Time magazine (featuring none other than our lovely Laura Collins) about the relationship between anorexia and autism (I've blogged about the subject here), Dr. Kaye said these wonderful words:

"[Anorexia is] highly heritable, it runs in families, and it's clear now that it's affected by a cluster of [early life] vulnerabilities like anxiety and perfectionism. If you don't have those vulnerabilities, you are very unlikely to develop anorexia," says Dr. Walter Kaye, director of the eating-disorders program at the University of California, San Diego..."I think that anorexia is as biological as autism. It's just 20 years behind in terms of research."

A 2005 article titled Anorexia could be caused by brain dysfunction quoted Bryan Lask who explained the biological predisposition as follows:

"The biological cause has been under-emphasised and the socio-cultural pressures have been over-emphasised. There has to be a biological contribution otherwise everyone would be anorexic given the socio-cultural pressures in our society," he said. "I am not saying people are born anorexic any more than they are born asthmatic or diabetic. But I am saying some people are born with a genetic pre-disposition to anorexia which makes it more likely to develop. If you live in a society which promotes thinness as the ideal then you are more likely to develop anorexia if you have the biological predisposition."

Biology isn't simple, though, and no disease--no characteristic, no phenotype--is entirely biological. Environment always plays a role, like Lask said. I have a longer post on this tomorrow that will be full of geeky goodness.

*I got to tell him that I called him my "homeboy" at last year's NEDA conference, which was a combination of funny and embarrassing- for both of us!