By JESSICA BAKER
Published: November 1, 2013

Individuals with eating disorders are at higher risk for substance use disorders compared with the general population. For instance, one study reported that 32% of women with bulimia nervosa (BN) had a substance use disorder whereas only 6% of women in the general population did.⁽¹⁾ The association holds both ways, as individuals with substance use disorders are also more likely to have an eating disorder. Technically, this is called comorbidity (or, having more than one disorder) A study of comorbidity of eating disorders and substance use disorders was conducted in 1994 and showed that approximately 20% of females with a substance disorder have a current or past history of BN while 17% of females with BN report a current or past history of a substance use disorder.⁽²⁾ This pattern is not unique to BN, individuals with anorexia nervosa (AN) are also more likely to have had a substance use disorder than women in the general population. For example, one study reported that 17-22% of women with AN had a lifetime history of a substance use disorder,⁽³⁾ but the association is strongest in individuals with the bing/purge subtype of AN.

We know the comorbidity occurs, but we know less about why it occurs. A review paper published in 2000⁽⁴⁾ introduced several possible explanations, including a “shared etiology (cause)” hypothesis. Some possibilities are:

(1) Addictive personality hypothesis. An addictive personality style may predispose an individual to both eating disorders and substance use disorders. Certain personality traits could make them vulnerable to addictive line behaviors (be that alcohol, drugs, eating, gambling etc.)

(2) Shared family risk hypothesis. Family studies have shown increased rates of substance use disorders in relatives of women with BN.⁽²⁾ But family rates are highest in those individuals who have both BN and substance use suggesting that it is the substance use that is really tracking through families These studies typically conclude that eating disorders and substance use disorders do not share familial risk factors.

Twin studies, however, provide more convincing evidence. This design tease apart genetic and environmental effects (which family studies cannot do), and suggest that eating disorders and substance use disorders do in fact show shared family risk—or the same genetic factors can contribute to each. Twin studies allow us to calculate a “genetic correlation,” which represents the correlation between the genetic factors that influence risk for an eating disorder and the genetic factors that influence risk for a substance use disorder. In this statistical analysis, if the correlation is estimated at 1.0, this would indicate perfect overlap in the genetic factors responsible for eating disorders and substance use disorders.

To date, twin studies have focused on exploring the genetic correlation between BN, BN-symptoms, and substance use disorders. Two studies examining the genetic correlation between BN and an alcohol use disorder found genetic correlations of 0.53⁽³⁾ and 0.23,⁽⁵⁾ respectively. Additionally, a genetic correlation between BN and an illicit drug use disorder has been estimated at 0.39,⁽⁶⁾which is similar to the correlations estimated for an alcohol use disorder. The take home message of these correlations is similar to other types of statistical correlations—the strength of the association between two things. These correlations were not 1.0, so the genetic factors responsible for BN and substance use disorders are not 100% the same. However, the correlations were also not 0, so there is some (a moderate amount) overlap in genetic factors. Finally, providing even more evidence of a shared genetic link between eating disorders and substance use disorders, shared genetic risk has also been observed among binge eating, purging, and alcohol use.⁽⁷⁾

(3) The final shared etiology hypothesis suggests that individuals, girls specifically, are more vulnerable to societal or cultural pressures such that they are more susceptible to cultural pressures towards thinness and experimenting with drugs. This vulnerability to cultural pressures may then result in an increased risk for both eating disorders and substance use during adolescence. However, this hypothesis “falls short in explaining why most female adolescences can engage in dieting behavior and recreational drug use without encountering serious problems or developing a disorder” (Wolfe and Maisto, 2000, p. 622).⁽⁴⁾

What Do These Results Mean?

The increased risk of having a substance use disorder if one has an eating disorder and vice versa has led some to question whether eating disorders should be classified as an addictive disorder, such as substance use disorders. Classifying eating disorders, specifically BN, as an addiction is based on various arguments including:

• Relatives of BN patients have high rates of substance abuse;
•  Treatment options for BN that are inspired by current treatments for addictions, such as Overeaters Anonymous and have shown some benefits;⁽⁹⁾
• Individuals with BN and a substance use disorder sometimes show similarities in their symptoms such as reporting “cravings”, a sense of loss of control, and a preoccupation with the “substance.”

This is still a hotly debated topic in the literature and some references are included below for further reading.

Despite an extensive literature demonstrating a high co-occurrence of eating and substance use disorders, the reasons for this frequent co-occurence remain unclear. Twin studies suggest there may be a shared genetic link between the two disorders. However, much more work is necessary in this area so that we can crack the code as to why these two disorders frequently co-occur. Soon, we will be able to conduct cross-disorder genome-wide association studies between anorexia nervosa and alcohol and substance use disorders. Unlike twin studies that are based on statistical models, this approach will allow us to explore concretely which genes influence risk for both disorders. These studies are likely to help us determine which of the hypothesis outlined above best account for comorbidity between eating disorders and substance use disorders.

References

1.         Bushnell JA, Wells E, McKenzie JM, Hornblow AR, Oakley-Browne MA, Joyce PR. Bulimia comorbidity in the general population and in the clinic. Psychol Med. 1994;24:605-11.

2.         Holderness C, Brooks-Gunn J, Warren M. Co-morbidity of eating disorders and substance abuse. Review of the literature. Int J Eat Disord. 1994;16:1-35.

3.         Baker JH, Mitchell KS, Neale MC, Kendler KS. Eating disorder symptomatology and substance use disorders: prevalence and shared risk in a population based twin sample. Int J Eat Disord. 2010;43(7):648-58.

4.         Wolfe WL, Maisto SA. The relationship between eating disorders and substance use: moving beyond co-prevalence research. Clin Psychol Rev. 2000;20(5):617-31.

5.         Trace SE, Thornton LM, Baker JH, Root TL, Janson LE, Lichtenstein P, et al. A behavioral-genetic investigation of bulimia nervosa and its relationship with alcohol use disorder. Psychiatry research. in press.

6.         Baker JH, Mazzeo SE, Kendler KS. Association between broadly defined bulimia nervosa and drug use disorders: common genetic and environmental influences. The International journal of eating disorders. 2007;40(8):673-8.

7.         Slane JD, Burt SA, Klump KL. Bulimic behaviors and alcohol use: shared genetic influences. Behavior genetics. 2012;42(4):603-13.

Further reading:

Davis C, Claridge G. The eating disorders as addiction: a psychobiological perspective. Addict Beh. 1998;23:463-75.

Wilson G. The addiction model of eating disorders: a critical analysis. Advances in Behavioral Research and Therapy. 1991;13:27-72.

photo credit: D.H. Parks via Creative Commons