Are Your Friends Making You Fat? A new study suggests that your best friend's weight may be very influential in determining whether you'll gain or lose weight over the years.

Are Your Friends Making You Fat?

Are Your Friends Making You Fat?

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The office doughnut isn't as influential in weight gain as the ideas of a person's social network, a new study bears out. Hein van den Heuvel/Zefa/Corbis hide caption

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Hein van den Heuvel/Zefa/Corbis

Harvard health researcher Dr. Nicholas Christakis stands in front of diagrams of social networks. Harvard Medical School hide caption

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Harvard Medical School

A new study suggests that your best friend's weight may be very influential in determining whether you'll gain or lose weight over the years. The research documents the spread of obesity from person to person in a study of more than 12,000 people.

The Office Doughnuts

Helen Hancock, a nurse in Washington, D.C., is trying to take off the weight she has gained over the years. She says it's hard to resist the Krispy Kreme doughnuts that one of her friends brings into the office every Friday.

"They smell wonderful," says Hancock. "It's not just one doughnut you eat. It's generally two doughnuts!"

Many of us face office temptations. But researchers think there's more to the spread of weight gain than simply copying a friend's food choices.

"Sharing a doughnut is obviously important," says James Fowler, Ph.D., who studies social networks at the University of California, San Diego. "But what's even more important are the ideas that we share with one another."

Our ideas about what's a normal amount of food or exercise, and what counts as a normal body size, all seem to be influenced by the people to whom we're connected. And researchers believe these "norms" ultimately shape our weight.

Fowler teamed up with Harvard Medical School's Dr. Nicholas Christakis to study this phenomenon. Together, they documented the spread of obesity in one town in Massachusetts.

Recreating a Social Network

Christakis and Fowler used data collected by the long-term Framingham Heart study to recreate a social network. This study included pairs of spouses, groups of siblings and next-door neighbors. Researchers even had old paper records where people had written down the names of close friends.

"What we were able to do is computerize these archived, handwritten records," Christakis says, "and reconstruct this densely inter-connected social network."

The people in the Framingham study gained weight at about the same rate as the rest of America during the 30 years they were studied.

And once Christakis and his colleagues assembled a bird's-eye view, they discerned patterns.

But Christakis says they found the strongest patterns among friends.

Researchers found that a person's chance of becoming obese increased by 57 percent if that person's close friend became obese. If it was a sibling or a spouse, the person's risk went up by more than one-third. Next-door neighbors, however, had no influence at all.

The results appear in the current issue of the New England Journal of Medicine.

Weight Gain and Weight Loss: Same Phenomenon

If the scope of this social influence is impressive, Christakis says the beauty is that it seems to work in both directions.

"The same kinds of phenomenon that contribute to an obesity epidemic could potentially be harnessed to the reversal of it," he says.

That's because researchers also saw that when one person loses weight, it contributes to other people around them losing weight, too.

Complexity of Obesity

Experts say this study shouldn't be used to blame fat people for making other people fat.

"There's enough stigma already," says Kelly Brownell, Ph.D., a researcher at Yale University.

A lot of factors influence obesity. Genes have a strong influence, as does income. Brownell says he would like to see more focus on prevention.

Since there's a domino effect to weight gain and weight loss, researcher James Fowler says people can use this to their advantage.

The take-home message here, says Fowler, is that if you're interested in losing weight, "you really need to get your friends and family involved in the process."