Can Obesity Be Literally Contagious?

The first paper reporting that obesity could spread like a virus was published in 2007 in The New England Journal of Medicine.

In it Dr. Nicholas Christakis of Harvard University and Dr. James Fowler of the University of California used data gathered from 12,067 subjects in along running American government study, the Framingham Heart Study.

The data included 32 years of medical records, including such routine data as body weight and smoking habits. But the Framingham researchers also happened to know who among the subjects were friends.

In analyzing the Framingham data, Dr. Christakis and Dr. Fowler found that friends, and friends of friends, had similar levels of obesity, but neighbors did not. The researchers proposed a possible explanations.

One was homophily, the tendency to choose friends like oneself. A second explanation was that people are affected in the same ways by the environments they share with their friends. The third explanation, and the one that garnered so much attention, was contagion.

Does obesity spread like a virus through networks of friends and friends of friends? Do smoking, loneliness, happiness, depression and drug use proliferate through social networks?

In recent years, a series of highly publicized studies of two researchers have concluded that these behaviors can be literally contagious, passed from person to person.

And there was an important public health corollary, the researchers said: It should be possible to curb a behavior like obesity by focusing on small groups of people who influence their networks.

But now those surprising conclusions have drawn criticism from other scientists who claim that the studies methodology was flawed and the original date were inadequate to estimate the role that contagion might play in the spread of these behaviors.

The social scientists who published the original studies, Drs. Christakis and Fowler, say they are aware of the limitations of their analyses but mantain that their conclusions are robust.

But critics are not convinced that it is possible to separate homophily from contagion with observational data. “It is very hard to be sure you have properly accounted for all the confounding variables in any observational study,” said Hans Noel, a social scientist at Georgetown University in Washington D.C., who has criticized Dr. Christakis’s and Dr. Fowler’s work.

The explanations have different implications for public health. If behaviors cluster together because of homophily or shared environments, there is no need to regard other people as potentially harmful.

But if contagion is real, it might follow that people who are fat should stay away from fat people to control their weight.

Dr. Christakis and Dr. Fowler focused on this as a cause of obesity, saying that they could estimate its effect and it was large. They theorized that a person’s idea of an acceptable weight , or an acceptable portion size, changes when he sees how big his friends are or how much they eat.