Is Obesity Contagious?

A groundbreaking new study says obesity is contagious, spreading through social networks. Friends, more than family or neighbors, are the ones propagating the epidemic.

Obesity spreads through social networks, according to the study, so if your friends put on weight, you're more likely to put on the pounds, too. Your family members or spouse can also influence you; as they get heavier, you're more likely to gain along with them. But, your friends even if they don't live anywhere near youhave the most sway. A close friend's weight gain can even be downright dangerous.

"If your close friend becomes obese in a given time interval, there's triple the risk that you will follow suit," says Nicholas Christakis, a coauthor of the study and a professor of medical sociology at Harvard Medical School. "Before you know it you have an obesity epidemic, where we're all kind of gaining weight together, like a fashion spreading through society, rising in lockstep."

The researchwhich Richard Suzman, director of the National Institute on Aging's Behavioral and Social Research Program, calls "one of the most exciting studies in medical sociology that I have seen in decades" focuses on 12,067 participants in the Framingham Heart Study, a multidecade government health-research project. Each participant was asked to name a list of friends and family members when he or she joined the program in 1971. Then the participants and their friends and family were tracked over the years.

When one person in the study became obese, his siblings' risk of also becoming obese jumped by 40 percent, while his spouse's risk jumped by 37 percent. More strikingly, if that person had been named as a "friend' by another participant, the second participant's risk of becoming obese shot up by 57 percent. If the friends were of the same gender, the risk was even higher, at 71 percent. (The study found a man's weight gain would have no significant effect on his female friend's weight, and vice versa, but the study did not have many male-female friendships to examine.) If the friends were particularly close, judged in the study by the fact that they both named each other on their lists of loved ones, the risk that one's weight would follow the others' increased by a whopping 171 percent.

Even people who’d never met each other were affecting each other in a six-degrees-of-separation way. If your friend’s friend’s friend, or your friend’s sibling’s friend, gains weight, “that will have a subtle effect on you over the course of two to four years,” says James Fowler, an associate professor of political science at the University of California, San Diego, and the other coauthor of the study. “When we change our own lifestyle and become heavier or thinner, that has a ripple effect through the whole population.”

The study’s effects don’t just come down to the idea that thin people seek out other thin people as friends, while heavy people seek out other heavy people. In fact, what’s going on is much more interesting, according to the researchers: heavy and thin people are causing their friends to become more like them. The reason people have such a powerful effect on each other’s weight is hinted at by one of the study’s most intriguing findings, says Fowler: “Friends who are hundreds of miles away from you have as much of an effect as friends who are [geographically] close.”

The study suggests a new explanation for the obesity epidemic, says Matthew Gillman, director of the Obesity Prevention Program at Harvard Medical School. “Genes can certainly affect whether one individual is obese rather than the other, but they can’t really explain the obesity epidemic, because they haven’t really changed in the last 30 years,” he says. True, plenty of changes in American society have contributed to the epidemic: most obviously, an increase in fatty, carb-heavy processed foods and a decrease in built-in daily exercise. But social networks have changed, too. Compared to the years before the epidemic started, Americans also now have more ways to keep in contact with their loved ones, such as e-mail, instant-messaging and videoconferencing. The study suggests that the obesity norms could indeed be transmitted via those technologies; a friend 1,000 miles away can still send an e-mail bemoaning his recent weight gain.

There’s still a lot left to figure out about these new dynamics of obesity. One question the research brings up, but fails to completely answer, is where neighbors fit into the picture. They appear to have no influence: if your neighbor becomes obese, your risk of doing likewise doesn’t change. It’s unclear why neighbors aren’t playing a larger role, although Christakis notes that if you don’t particularly admire or even know your neighbors, you're not likely to base your ideas about body size on theirs.

The study also brings up several other questions: Why are same-sex friendships and relationships so much more influential over weight than male-female friendships are? Where does the ripple effect stop? Does the same dynamic apply to other behavior-related health problems, such as drinking, smoking and risky sexual behavior? It may be some time before researchers fully know the answers.

It’s not too early, however, for public-health officials to start thinking about the study’s implications. Over the last 25 years, obesity in the United States has doubled; 66 percent of Americans are overweight and 32 percent are on the next level, classified as obese, according to the National Center for Health Statistics. And measures to combat the problem aren’t bringing those numbers down. “We see no evidence that the obesity epidemic has peaked,” says Christakis. And it’s possible the epidemic won’t peak until weight-loss groups and health advocates start taking social ties into account. But in a way, that's good news, says Fowler: “The flip side of this is that thinness is contagious, too." If you really want to lose weight, he adds, maybe you should encourage some of your buddies to trim down as well.

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