Should You Choose Your Friends More Carefully?

The “buzz” today among medical journalists is about an article published today (available to subscribers and news organizations yesterday) in the New England Journal of Medicine. The article was written by Nicholas Christakis and James Fowler and entitled “The Spread of Obesity in a Large Social Network over 32 Years.” The citation is New Engl J Med 2007;357:370-9. There was an article about the study today in the New York Times ( “Find Yourself Packing It On? Blame Friends,” written by Gina Kolata- Thursday July 26, 2007, page A1). There was also a piece on National Public Radio. I’m sure this study will be widely reported on.
The scientists studied a network of people who were part of the famous Framingham Heart Study which was initiated in 1948. The authors actually started with the “offspring cohort” from the original Framinham subjects (n= 5124) and created a network data set which included both original Framingham study subjects and offspring cohort subjects. Linkage between the offspring cohort subjects (called “egos”) and other Framingham study subjects (called “alters”) was based on marriage, sibship, and friendships. A total of 12,067 people ended up as either egos or alters.

So, what’s with these egos and alters?

The authors took their carefully constructed social network and studied if obesity was related to the linkage, much like one might study epidemiology of an infectious disease. Obesity was defined as a BMI 30 or greater (if you forgot what a BMI is check out one of my earlier entries). The study included only people older than 21 years of age. The average age of the study subjects at the start of the study was 38 years and 53% of the subjects were women. The data collection was from 1971-2003. The 5124 egos from the offspring cohort had 38,611 observed family and social ties (e.g., spouse, neighbor friend).

The authors found that obesity in this study population was in clusters related to the social contacts. Thus, if a person (the ego) had a friend (an alter) who became obese during the period of observation, the ego was 57% more likely to also become obese. If the friend was a “close friend,” the risk was increased by 171%. Risks were increased with sibs and spouses but not with friends of the opposite sex or with neighbors. So, what this all means is that having a friend or a spouse or a sibling who is or who becomes obese, means that you are much more likely to also become obese. The converse is also true; if one’s relations and friends are lean, he is more likely to remain lean.

Ok. Now what?

I find the data interesting but not so surprising. Many previous studies have demonstrated, at least within families, the effects of parental obesity on their childrens’ weights independent of genetic influences. The authors of the study speculate that the social influence on the spread of obesity can perhaps be “harnessed” to to slow the epidemic. The authors are a bit fuzzy as to how this actually might be accomplished. It would appear from their data that the best approach would be to have people at risk for obesity (most Americans) or those who are already obese hang out only with thin people. Of course, some of those thin people might be smokers and the person hoping to avoid the bad influence of being around someone who will become or who is already obese might be trading one “evil” for another.

When and if all of this ever gets sorted out, I think it is likely we will find that there are many reasons for the observed clusering of obese people. For some it will be shared acceptance of the obesity resulting in less anxiety about the problem. For others, it will be similar lifestyles, eating, and exercising habits that promote the obesity. I doubt many friendships are based on the idea that “opposites attract.” People generally want to be around people who are like them. I view the data as just one more interesting piece of information about the environmental causes side of the obesity epidemic (the other side is genetics). Whether we can take these data and use them to help people trim down other than by abandoning their overweight friends and relations remains to be seen. I for one intend to screen my new prospective friends by having them submit their BMIs to me for review.

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