Fat Friends Make You Fat?

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According to a new article in the New England Journal ofMedicine, obesity is socially contagious, spreading from personto person within a social network. If your friend becomes obese,your odds of becoming obese go up 57 percent. And if your portlyfriend considers you a friend as well, your odds of becoming equallyplump rise 171 percent.

This social network effect extends (with progressively lessimpact) to three degrees of separation – to your friends' friends'friends. Not only that, but a faraway friend has as much impact onyour obesity as a friend close at hand. A stranger, however, evenone who lives next door, has no effect on your odds of obesity. Inother words, increasing social distance decreases the effect, butincreasing geographic distance does not.

To arrive at these conclusions, researchers analyzed the socialnetworks of 12,067 people who were followed for 32 years as part ofthe Framingham Heart Study. After painstakingly mapping 38,611social interconnections among all the subjects, they discovereddistinct clusters of thin and heavy people.

Statistical analysis revealed that the clusters were not due toany tendency for people to congregate with others like them.Furthermore, people's influence on their friends' weight could notbe attributed to eating the same foods as their friends or toengaging in similar activities.

The analysis revealed that the infectious effect is much greateramong friends of the same sex. You have a 71 percent increased riskof obesity if your same-sex friend gains a lot of weight. If you area woman and your sister becomes obese, your risk rises by 67percent. If a man's brother becomes obese, his risk rises by 44percent. And if you become obese, your spouse has a 37 percentincreased risk of excess adiposity.

How could this possibly work? Given the dramatic effect of distantfriends on your odds of obesity, common exposure to a particularenvironment cannot be the cause. The researchers hypothesize thatyour esteem for your friends may alter your conception of whatconstitutes normal and proper body size. They propose that thiscontagious social influence should be exploited to spread positivehealth behaviors from one person to that person's entire socialnetwork.

Source: New England Journal of Medicine, July 2007;EurekAlert

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