Study: Obesity is Socially Contagious
By Jeanna Bryner, LiveScience Staff Writer

posted: 25 July 2007 02:31 pm ET

People who notice a friend packing on pounds might want to steer clear if they value a sleek physique.

A new study finds that when the scale reads "obese" for one individual, the odds that their friends will become obese increase by more than 50 percent.

The study, published in the July 26 issue of the New England Journal of Medicine, suggests that obesity is "socially contagious," as it can spread among individuals in close social circles. The likely explanation: A person's idea of what is an appropriate body size is affected by the size of his or her friends.

Conversely, the researchers found that thinness is also contagious.

"Social effects, I think, are much stronger than people before realized," said co-author James Fowler, a social-networks expert at the University of California-San Diego. "There's been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity, and what our paper suggests is that you really should spend time looking at the social side of life as well."

An outside expert on social networks called the new research impressive, particularly in showing a causal link between obesity and friends. However, he cautioned that the evidence for the effect extending out to friends' friends, and so on, is weaker.

"The suggestion in their paper is that obesity sort of spreads through the network as if it were some kind of epidemic, some kind of contagious disease," said Duncan Watts, who studies social networks at Columbia University. While this is plausible, he noted, the current research doesn't provide direct evidence for this phenomena.

Social networks

Research has shown that peers influence each other's health behaviors. One past study showed that teens associating with friends who smoke and drink were more likely to take up the behaviors. However, no past research has looked at how the impact extends to friends' friends and beyond.

In the new study, Fowler and Nicholas Christakis of Harvard Medical School analyzed health data collected between 1971 and 2003 from more than 12,000 adults who participated in the Framingham Heart Study, an ongoing cardiovascular study. Participants provided contact information for close friends, many of whom were also study participants, resulting in a total of 38,611 social and family ties.

The researchers found that if a participant's friend became obese over the course of the study, the chances that the participant also became obese increased by 57 percent. Among mutual friends (both individuals indicate the other is a "friend"), the chances nearly tripled.

Among siblings, if one becomes obese the likelihood of their sister or brother becoming obese increases by 40 percent. Among spouses there is a 37 percent increased risk.

Gender also affected the degree of "obesity contagion." In same-sex friendships, individuals had a 71 percent increased risk of obesity if a friend became obese. If a guy's brother is obese, he's 44 percent more likely to also become obese. Among sisters, the risk was 67 percent.

Fat factors

Other studies have suggested that obesity might be physically contagious, possibly passing from one person to another by virus. But that idea has not been firmly supported. The new study doesn't address this possibility but instead looked at mindsets and attitudes as the controlling factors.

Fat-fueling factors were taken into consideration. For instance, the researchers made sure the effect wasn't a case of "birds of a feather flocking together." Body measurements were taken throughout the study period, showing when individuals became obese and whether they began the study with obese readings.

"It's not that obese or non-obese people simply find other similar people to hang out with," Christakis said. "Rather, there is a direct, causal relationship."

They also ruled out the idea that an outside factor, and not the friendship, caused the fatness. If an environmental factor were affecting both individuals in a friendship, then it shouldn't matter whether individuals are mutual friends or just one individual labels the other as a friend.

The study, however, found that it does matter which way the friend arrow points: If subjects named an obese person as a friend, they tended to be affected by that person's obesity.

But when the person on the receiving end did not label the first person as a friend, there was no "obesity contagion" effect in the other direction. The distinct variable here is who calls whom a "friend."

"The fact that it only has an effect when I think you're my friend is very strongly suggestive to me," Watts said. "That's about as good as you can do in terms of identifying a causal relationship."

Perhaps friends just spend a lot of time together and so would eat similar foods and engage in the same physical activities. But they found the results held no matter the geographic proximity of friends.

"So friends that are thousands of miles away have just as large an impact on you as friends who are right next door," Fowler told LiveScience.

The scientists suggest the findings can be explained if friends are influencing one another's norms for body weight.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size," Christakis said. "People come to think that it is OK to be bigger since those around them are bigger, and this sensibility spreads."

Bulging waistlines

In the past 25 years, obesity among U.S. adults has shot from 15 to 32 percent. The new study reveals friends could be feeding the fat epidemic, along with our large-serving, high-calorie, fast-food lifestyles.

"We show that one person's behavior ripples through the network to have an impact beyond those first-order friendships," Fowler said. "So we're talking about dozens of people that are affected by one person's health outcomes and health behaviors."

He added, "And that needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier."

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http://www.livescience.com/health/070725_fat_friends.html

Achoo! Look out, that obesity is CONTAGIOUS! Puh-leez!

Culture is contagious. Fads are proof of that. I believe our culture does promote a way of eating that leads to obesity. Also, I am amazed at the changes taking place at the elementary school level. I was always embarassed to be a fat child and I never heard my mom say I would grow out of it or that I was "normal" but I see and hear these things all the time at my kids' school. One little boy in my sons K class must have weighted over 200 pounds. This child could not sit on the floor and build a puzzle without wheezing. At nearly 300 pounds I could do that. My heart broke for this little boy, but his parents adamently maintain that "his real father was a big guy, his older brother is 6'5" and a big guy, [he] is just going to be a big guy too. While this is an extreme story, the same theme runs thru alot of conversations at the school. Parents seem to be aware that kids are "bigger" these days, but unwilling to see it for what it is and compare them to other heavy kids. We need to stop this sultural contagion somehow. I am starting with myself and providing a better model for my kids.

Great article discussing how factual this study is

Yep. Low-Carb "immunization"

Oh, I agree that the "study" was so flawed as to be laughable, but I do believe that even as our society still penalizes fat people, more people close their eyes to just how much they and other people really do weigh. I have always been the fattest person in a room until recently. I was trully amazed to go to a social gathering a few months ago and realize how heavy so many people there were. I still have 90 pounds to lose and at least 6 women out of a group of about 20 were heavier than I am now (75 pounds down from where I would not have attended the event). I find it mind boggling that our society is so tough on fat people but so many people have found it acceptable to become so heavy. Many of these people were people who were not heavy as children or teenagers. They were the ones who picked kids like me. What a contradiction in our society.

I think there may be something to it... if you picture a woman and her group of friends all piling into a cafe and all ordering a Giant Fluffy Beverage.

/knows the 'look' you get when you're the only one ordering a cappucino sans sugar

I think there is more of a pernicious undercurrent of fat hatred in a lot of these studies and media stories.

Not overt, so much....but it's like they have to find a scientific justification to pick on people struggling with obesity.

Biggest load of crud.