Wednesday, July 9, 2008

Unearthing anorexia's genetic roots

While eating disorders remain complex, multifaceted brain diseases, research is uncovering the importance of genetics in causing these diseases. Some families have long known that eating disorders, like other illnesses such as depression, bipolar disorder, and anxiety disorders, run in families.

The question that has remained is how: how are risks for these illnesses inherited? And what are these risks, anyway? Despite all of the talk, there will almost certainly be no "anorexia gene" or "bulimia gene" uncovered. Issues likely involve differences in appetite regulation (especially the hormones leptin and ghrelin), and altered levels of the neurotransmitters serotonin and dopamine, which regulate mood.

Abnormal levels of serotonin have also been found in those with obsessive-compulsive disorder, an anxiety disorder that is several times more common in those with anorexia nervosa.* The close links between OCD and AN have prompted some researchers and clinicians to question whether anorexia should be classified as an obsessive-compulsive spectrum disorder. This spectrum includes Tourette's syndrome, tic disorders, compulsive skin picking, trichotillomania (compulsive hair pulling), and body dysmorphic disorder. All of which are, not surprisingly, more common in people with anorexia than in the general population.** This confirms and strengthens the contribution of serotonin to anorexia.

Now, researchers have made links between anorexia and what appears to be the illness' mirror image: 'bigorexia,' or muscle dysmorphia. Those who suffer from muscle dysmorphia, almost exclusively men, believe themselves to be small and unfit. Their response (again, the eerie mirror image of anorexia) is to bulk up through weight lifting, special diets, and occasionally injectible steroids.

The initial commonality between anorexia and bigorexia are the obvious body image distortions. But the similarities may go deeper than that. Finnish researchers studied five pairs of twins, which included at least one male with anorexia nervosa in each twin pair. A story on Reuter's Health revealed that the researchers found "a 'striking familial liability' for traits related to the eating disorder, including major depression, muscle dysmorphic disorder (which is sometimes called 'bigorexia), and obsessive compulsive disorder. The findings suggest that all of the symptoms have similar genetic roots, the researchers point out."

Just as telling, eight out of the ten twins had suffered from a mood disorder during their lifetime. And body dysmorphic disorder (especially muscle dysmophia) was common in the twins without AN.

The abstract of the study says this: In males, overweight commonly predated AN, and symptoms of body dysmorphic disorder, particularly of muscle dysmorphia, were common among the anorexia-discordant co-twins. Affective and anxiety disorders were present in both the probands and their co-twins. CONCLUSION: We found a strong familial clustering of AN, affective and anxiety disorders, and symptoms of muscle dysmorphia among men in the general population. In men, muscle dysmorphia may represent an alternative phenotype of AN.

While a study involving five pairs of twins is by no means conclusive, the link to muscle dysmorphia and mood disorders implicates a serotonin dysfunction as a contributing factor to anorexia and the body image disturbances that so often accompany it.

*And the reverse is also true: those with OCD are more likely to have AN or BN. Although this study out of Spain provides evidence that eating disorders and OCD might not be related.
**Sadly, research on bulimia, EDNOS, and binge eating disorder is limited at best.

(Cross-posted on ED Bites)


Ai Lu said...

Though it's fascinating to consider the genetic underpinnings of eating disorders, I question the generalizability of a study that only uses five pairs of twins to draw conclusions about genetics and eating disorders. With such a small number of participants, it's likely that the results are not typical of the general population.

Also, genetic factors should not be confused with environmental influences that twins often shares, such as the family that they were raised in.

It's very appealing to look to genetics to explain eating disorders -- and it also behooves this community, as insurance companies are more likely to look favorably upon claims for biologically-grounded mental illnesses than mental illnesses that are perceived as a choice (I hope that this myth gets dispelled before long), and more research is likely to be directed our way. However, I would be cautious to jump too quickly to the conclusion that eating disorders are genetically based; as with most complex human behaviors, there are multiple influences, both genetic and environmental, that probably intermingle in particular ways to form eating disorders. I would like to see more research on these interactions.

Carrie Arnold said...

ANY disease has both genetic and environmental components, and eating disorders are no exception.

But this study is hardly an exception to showing a strong genetic component to eating disorders. It's estimated that about 75% of your risk for developing an ED is genetic.

I firmly believe that genetics is a necessary but not sufficient cause of an eating disorder. What that means is without the genetics, no eating disorder. But there also needs to be outside input, too. And environment can be things like living in a diet-crazy culture, going through puberty, etc. Matt Ridley has a very interesting book called "The Agile Gene" where he looks at these factors in depth. It's a fascinating read.

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Anonymous said...

Bigorexia is hard to eradicate because our society promote this kind of lifestyles.

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I think that all those matters are related to DNA. Eventually, human brain is the way it is just because of DNA, so in order to fix a mental problem, I think anorexia can be genetically treated in order to heal the ill person.