Today I thought I’d take the time to do an overview of what researchers know about the genetics of eating disorders and try to clear up some common misconceptions. The bulk of the content in this blog post comes from a very nice review paper published in 2011 by Drs. Stephanie Zerwas and Cynthia Bulik on the genetics and epigenetics of eating disorders. In an effort to keep blog posts short, this will be a multi-part mini-series.
When it comes to the genetics of eating disorders, there are two main questions that research ask: What is the relative contribution of genetic factors to eating disorder behaviours? And what are those genetic factors? I’ll talk about research attempting to answer the first question in this post and the second question in my next post.
In order to understand the role that genetics plays in influencing eating disorder behaviours, researchers use family, twin, and to a lesser extent, adoption studies.
In family studies, researchers are typically asking What is the probability that a relative of someone with an eating disorder will also develop an eating disorder compared to the probability of a relative of someone without an eating disorder?
These studies can tell us if eating disorder aggregate (run) in families, but they can’t tell us if that aggregation is due to genetic factors or due to a shared environment. We simply can’t figure that out using this study design. It could be either one or a mix of both.
What do we know about eating disorders from family studies?
We know that the likelihood of a first-degree relative of someone with anorexia or bulimia is much more likely to have also experienced an eating disorder during their lifetime than first-degree relatives of healthy controls.
Here’s an important thing to remember though: “much more likely” is a probability statement.For example, first-degree relatives of AN patients are 6-10x more likely to report lifetime prevalence of AN than first-degree relatives of health controls. That means that if the lifetime prevalence of AN in first-degree relatives of healthy controls was 0.6%, we’d expect somewhere between 3.6-6% of first-degree relatives of AN patients to also report suffering from anorexia. That leaves 94-96.4% of relatives unaffected (at least with AN).
Twin studies on the other hand are able to give us an idea about the relative contribution of genetic factors to a particular behaviour.
How? Well, identical twins share roughly 100% of their genes and fraternal twin share roughly 50% of theirs. Say a behaviour was 100% genetic and it was governed by one gene. In that case, we’d expect that if one identical twin showed that behaviour, the other one would, too (and vice-versa), but for fraternal twins, if one twin showed the behaviour, there would only be a 50/50 chance that the other one did, too. Conversely, if a behaviour wasn’t genetic at all, then we wouldn’t expect to see this pattern and there would be no difference between the likelihood of identical and fraternal twins exhibiting the same behaviour, because the genes they’d have wouldn’t matter.
In reality, things are way more complex, but we can take a large sample of both types of twins and compare the rates of concordant identical and fraternal twins for eating disorders and see how those numbers compare with our ideal scenarios (like the ones I described above).
Twin studies enable us to estimate the heritability of a particular trait, in our case, eating disorders.
Heritability estimates describe the proportion of the variation between people that is due to genetic variation for a specific population at a specific point in time. Shared environment estimates describe environmental factors experienced by both twins that lead twins to be more similar. Unique environment estimates describe environmental factors that lead twins to be more dissimilar (eg, events experienced by only one twin, or differential reactions to the same experienced event).
Heritability calculations involve lots of assumptions and so, like most things, they are not perfect (and the values depend on the sample size, too). But they do give us an idea of the extent to which genetic factors contribute to eating disorders.
What do we know about eating disorders from twin studies?
Studies have reported the heritability for anorexia nervosa and bulimia nervosa somewhere between 33-84% and 28-83%, respectively. Interestingly, the rest of the variability that we seen in the behaviour seems to be due to unique–and not shared–environmental factors.
Heritability is a concept that is commonly misunderstood, and it is easy to understand why: it is complicated and confusing. Here are some things you should remember, though:
- It does not apply to individuals. You can’t say your eating disorder is caused 60% by your genes and 40% by the environment because you read a study that showed a heritability of 60%. Heritability is a population measure. It has nothing to say about individuals. Nada.
- It is not static. Heritability for a particular behaviour (like, say, purging) isn’t necessarily static. It can vary with time within a population and between populations, too. This is because it depends on the genetic factors that are occurring/present in the population and on the environment.
Think about it like this: in a stable and homogeneous environment, any variation we see in a trait (like height) will be due to genetic factors, but in a heterogeneous environment where access to food is variable (for example), heritability will decrease because now environmental factors will play a role in explaining why some people are tall and some are short. The really important this to remember is that this is true even if the genetic factors that control height are completely unchanged, even if it is the same number of genes acting in the same way.
- High or low heritability doesn’t tell us anything about the number of genes or the kind of genes that are involved. More importantly, high heritability doesn’t mean the disorder is incurable or the behaviour is unchangeable because the value doesn’t take into account how extreme some treatment interventions can be.
Here’s a nice example to illustrate how confusing the concept might be (source):
Heritability sounds simple, but it can act in unexpected ways. For example, if everyone in a population has the same allele [think a version of a gene, or just gene] for a trait and shows little variation (differences) on that trait, then the heritability for that trait is zero. It is zero because that trait has no genetic variation. One example is hair color among Eskimos (N. Carlson & W Buskirt, 1997). The whole Eskimo population appears to have the same alleles for hair color, so for Eskimos, the heritability for hair color is 0.00, even though the color is under strong genetic control! It’s 0.00 because there is no genetic variation for hair color.
Adoption studies are good for understanding the role of the environment, particularly shared environment, on behaviour. Researchers compare how similar biological and adopted children are to their parents. If biological parents are more similar than adoptive parents, we can conclude that genetic factors contribute to the similarity. On the other hand, if adoptive parents are more similar than biological parents, then we know that shared environmental factors likely contribute to that similarity.
To date, one adoption study has been done in the field of eating disorders (Klump et al., 2009). In that study, researchers looked at disordered eating symptoms and not eating disorders because of the low prevalence of eating disorders in the population.
What do we know about eating disorders from adoption studies?
The findings from the adoption study confirm what researchers have found in twin studies. Namely, heritability ranged between 59-82% and shared environmental factors didn’t contribute significantly to the variability observed in disordered eating behaviours.
In summary, evidence to date suggests that 50-80% of the variation we seen in eating disorder behaviour (at least in the populations we’ve studied, predominantly those of European ancestry) is due to genetic factors and about 20-50% is due to unique environmental factors (and a tiny bit is due to shared environmental influences).
Next up, we’ll look at what specific genes have been implicated in eating disorders. (Warning: we know a lot less about that!)
Zerwas, S., & Bulik, C. (2011). Genetics and Epigenetics of Eating Disorders Psychiatric Annals, 41 (11), 532-538 DOI: 10.3928/00485713-20111017-06