STEVE INSKEEP, host:

And let's have a look at Your Health this morning.

Researchers have new evidence to suggest that obesity is contagious. It's spread from person to person through shared ideas and behaviors. The study documents how friends and family influenced whether we gain or lose weight over the years.

NPR's Allison Aubrey reports.

ALLISON AUBREY: Helen Hancock is a nurse in her late 50s. Over the years, she's gained a lot of weight, and she says her office is partly to blame.

Ms. HELEN HANCOCK (Nurse, Washington, D.C.): I have a friend who always brings four dozen Krispy Kreme fresh doughnuts in, and they smell wonderful every singles Friday morning. And generally, it's not one donut you eat. It's generally two doughnuts.

AUBREY: Helen's office ritual is likely only part of the reason she's overweight. Many of us face these temptations, but researchers think there's more to it. James Fowler studies social networks at UC San Diego.

Professor JAMES FOWLER (University of California-San Diego): Sharing a doughnut is obviously important, but what's perhaps even more important is these ideas that we share with one another about, you know, how many donuts to eat and how much we should exercise the next day after we've eaten those donuts.

AUBREY: Fowler says our ideas about what's normal when it comes to food and exercise and even what counts as a normal body size all seem to be influenced by the people to whom we're connected. And this ultimately shapes our own weight. In order to study this phenomenon, Fowler teamed up with a health researcher at Harvard named Nicholas Christakis. Together they documented the spread of obesity through one town in Massachusetts.

Dr. NICHOLAS CHRISTAKIS (Harvard Medical School): I think our findings reinforced the idea that because people are interconnected, their health is interconnected.

AUBREY: Finding these connections in the town of Framingham was possible because decades ago researchers began tracking the weight of thousands of people there as part of a long-term heart study. There are husbands and wives in the study, groups of siblings, next-door neighbors. Researchers even had old paper records where people had written down the names and numbers of close friends, and Christakis says this really helped.

Dr. CHRISTAKIS: What we were able to do then is computerize these archived, handwritten paper records and reconstruct this densely interconnected social network, the individuals in which have been repeatedly measured across time for 30 years.

AUBREY: During those years, the folks of Framingham gained weight at about the same rate as the rest of America. Interestingly, once Christakis assembled a bird's eye view of exactly who became obese, a pattern emerged.

Dr. CHRISTAKIS: What we found was - is that the strongest effects were seen between friends. So a friend gaining weight had a very big effect on an individual gaining weight.

AUBREY: They found that if a person's close friend became obese, that person's chances of becoming obese within the next few years increased by 57 percent. For siblings, the risk increased 40 percent. And for spouses, the correlation held up too. Next-door neighbors, however, had no influence at all. The results appear in the current New England Journal of Medicine. If the scope of this social influence is impressive, Nicholas Christakis says, the beauty of it is that it seems to work in both directions.

Dr. CHRISTAKIS: The same kind of phenomenon that contribute to an obesity epidemic could potentially - potentially be harnessed to contribute to the reversal of the epidemic, because it's equally the case that we see that one person losing weight contributes to other people around them losing weight.

AUBREY: Kelly Brownell is an obesity researcher at Yale University. He says this is a terrific study but it should not be used to blame fat people for making other people fat.

Dr. KELLY BROWNELL (Yale University): There's enough stigma already that makes lives for overweight people pretty terrible.

AUBREY: And he says you can't forget about other factors. Genes have a strong influence, and income matters too.

Dr. BROWNELL: So for example, the relatively high cost of healthy foods compared to a relatively lower cost of unhealthy foods.

AUBREY: Brownell says he'd like to see more focus on prevention. And in fact this study suggests that working together in groups may help. That's what Sue Hidell is trying. She has joined Weight Watchers with a friend, and she's just had her weekly weigh-in.

Ms. SUE HIDELL (Member, Weight Watchers): And I'm going to be very motivated to be even more careful about what I eat this week, even though I was careful.

AUBREY: Sue says she did lose a little weight this week, but her friend lost more.

Ms. HIDELL: So I don't want that to happen again.

AUBREY: It's a little competition, I guess.

Ms. HIDELL: Friendly competition, and unofficial.

AUBREY: And all good, say the social network experts. That's because the friendship is more complementary than competitive. When one loses weight, the other is more likely to lose too.

Allison Aubrey, NPR News.

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