August 13, 2007

Friends and Family May Play a Role in Obesity

An overweight couple on the sofa eating popcorn.

Best buddies and family share life's ups and downs. They may also share a tendency to gain excessive weight. A new study reports that a person is more likely to become obese if a close friend or family member has put on some pounds, even if the friend or relation lives many miles away. The research provides the first detailed picture of how social ties may contribute to obesity. It offers clues for developing both clinical and public health interventions to slow the sharply rising rates of obesity in this country.

Obesity affects nearly 1 in 3 American adults, putting them at risk for serious health problems, including type 2 diabetes, heart disease and stroke. Genetics are known to affect a person's weight, but other factors, including a sedentary lifestyle, diet and the influence of friends and family, play a part as well.

With funding from NIH's National Institute on Aging (NIA), Dr. Nicholas Christakis of Harvard Medical School and Dr. James Fowler of the University of California, San Diego, examined how social networks affect obesity. They analyzed weight, height and other data, collected over a 32-year period from a large, socially intertwined network of 12,067 adults who took part in the Framingham Heart Study, an ongoing research project funded by NIH's National Heart, Lung and Blood Institute (NHLBI).

To uncover changing social ties among participants, Christakis and Fowler used previously untapped data from handwritten "tracking sheets" completed during regular medical checkups for the Framingham Study. The forms contained valuable information about more than 5,100 key participants and their parents, spouses, siblings, children and close friends who also participated in the study. In addition to identifying kinships, the researchers could distinguish mutual friendships versus "one way" friendships, in which a friendship was reported by one person but not the other.

As described in the July 26, 2007, issue of The New England Journal of Medicine, the researchers found that friendships can have a crucial influence on a person's weight. The likelihood of becoming obese increased by nearly 57% if a close friend had become obese. In same-sex friendships, a close friend becoming obese increased a person's chance of becoming obese by 71%. The effect was strongest among mutual friends, with the risk of obesity rising by 171% if a close mutual friend had became obese.

Among pairs of siblings, one becoming obese increased the other's likelihood of obesity by 40%. In married couples, one spouse becoming obese increased the likelihood of obesity in the other by 37%. There was no effect among neighbors unless they were also friends.

"We didn't find that people who were overweight simply flocked together," says Christakis. Rather, people who became obese seemed to raise the likelihood that those close to them to would become obese.

Exactly how social ties influence obesity is still unknown. One possibility is that norms may shift within a social network when one person gains weight, so that close friends and family find it more acceptable to put on extra pounds. Additional research may help to illuminate the influence of social networks on weight and suggest new strategies for preventing obesity.

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