Study: Obesity "Contagious" among Friends, Social Networks

The weight of your friends can have a direct influence on your own weight, according to a new study by the University of California-San Diego and Harvard University.

The study, which appeared in a recent issue of the New England Journal of Medicine, discovered that obesity seems to be "socially contagious" among friends within a given social network. Researchers studied data collected on 12,067 adults for 32 years as part of the Framingham Heart Study, which is a project of the National Heart, Lung and Blood Institute. Using the information, they were able to create a detailed map of the subjects' social networks, including both family members and friends. Body-mass index was one of the factors recorded.

Upon studying the data, researchers found that the entire network of subjects grew to weigh more over time, keeping in line with the nation's "obesity epidemic." The information also revealed clusters of thin and heavy people, which the scientists could not attribute to social factors related to weight.

"It's not that obese or non-obese people simply find other similar people to hang out with," said Nicholas Christakis, one of the study's co-authors, in a press release. "Rather, there is a direct, causal relationship."

The researchers also found that friends have more impact on a person's weight than family members, and that the friends don't need to live in close proximity to make an impact. People of the same sex also tended to have more of an impact on each other.

The impact can be striking. According to figures from the study, if an individual's friend becomes obese, that person's chances of also becoming obese go up by 57 percent. An obese sibling can make you 40 percent more likely to become obese, and obese spouses increase the chance by 37 percent.

Obesity is defined as (in adults) having a body-mass index of 30 or higher, according to the Centers for Disease Control. An estimated 32 percent of adults in the United States meet this definition of obesity, according to CDC statistics.

The findings of this study mean that social factors could have an influence on which weight is considered "normal" in a circle of friends, Christakis said.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads," Christakis said in the press release.

The US National Institutes of Health/National Institute on Aging sponsored the study.

UCSD Press Release. "Obesity is Socially Contagious, Study Finds." http://ucsdnews.ucsd.edu/newsrel/soc/07-07ObesityIK-.asp

Centers for Disease Control. "Prevalence of Overweight and Obesity Among Adults." Available at: http://www.cdc.gov/nchs/products/pubs/pubd/hestats/overweight/overwght_adult_03.htm

Centers for Disease Control. "Defining Overweight and Obesity." http://www.cdc.gov/nccdphp/dnpa/obesity/defining.htm