Obesity is contagious among friends, study suggests

Your own close friends can can certainly influence your weight even more compared to genes or even your family members, based on a new research appearing in the July 26 issue of the New England Journal of Medicine. The study's authors suggest thatobesity isn't just spreading; rather, it may be contagious between people, like a common cold.

Researchers from Harvard and the University of California, San Diego, reviewed a database of 12,067 densely interconnected people — that is, a group that included many families and friends — who had all participated in a major American heart study between 1971 and 2003. The participants met with heart researchers every two to four years. To facilitate study follow-up, the researchers asked participants to name family members and at least one friend who could be called on if the participant changed addresses. It was that information the NEJM authors mined to explore obesity in the context of a social network.

According to their analysis, when a study participant's friend became obese, that first participant had a 57% greater chance of becoming obese himself. In pairs of people in which each identified the other as a close friend, when one person became obese the other had a 171% greater chance of following suit. "You are what you eat isn't the end of the story," says study co-author James Fowler, a political scientist at UC San Diego. "You are what you and your friends eat."

It's not just that people who share similar lifestyles become friends, Fowler says. He and co-author Nicholas Christakis of Harvard Medical School considered the possibility — and were surprised. For one thing, geographic distance between friends in the study seemed to have no impact: friends who lived a 5-hour drive apart and saw each other infrequently were just as influenced by each other's weight gains as those who lived close enough to share weekly take-out meals or pick-up basketball games. The best proof that friendship caused the weight gain, says Fowler, is that people were much more likely to pattern their own behavior on the actions of people they considered friends — but the relationship didn't work in the other direction. If you had named another person as a friend, and your friend became obese, than you were more than 50% more likely to get fat too. But if your friend had not named you as a mutual friend, and you became obese, it would have no significant impact on your friend's weight.