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updated 5/9/2011 5:30:18 PM ET 2011-05-09T21:30:18

Obesity is known to spread among friends, but how does this transmission happen?

Obesity spreads socially not because friends have shared ideas about acceptable body size, but rather because they share environments and carry out activities together that may contribute to weight gain, a new study suggests.

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In other words, shared social behaviors, such as eating out at restaurants, and shared surroundings, likely play a bigger role in the obesity "friend effect" than do shared social norms.

The study suggests ways public health professionals can best combat the obesity epidemic in America. For instance, it suggests interventions that try to change people's ideas about how fat or thin they should be won't be very effective.

"What people think about others' bodies is not that important to protecting them against weight gain," said study researcher Alexandra Brewis, director of the Center for Global Health at Arizona State University. "There's a lot of assumption that you can shame people into losing weight through social pressure," Brewis told MyHealthNewsDaily. "That strategy is probably not going to work very well," she said.

Instead, efforts should focus on promoting healthy environments, for instance, making people's neighborhoods more walkable and increasing access to healthy foods, the researchers say.

"We need to focus on what people do together, rather than what people think," Brewis said.

Obesity spread
Brewis and colleagues interviewed 101 women in Phoenix, Ariz., about their views on body size. Then, they tracked 812 of the participants' friends and family members (both men and women) and asked them the same questions. The researchers also calculated the body mass index (BMI) for everyone in the study. BMI is a ratio of height and weight that is considered an indicator of body fatness.

The results confirmed earlier findings that obesity spreads in social networks. The initial women interviewed in the study were 2.4 times more likely to be obese if their friends were obese. And they were 3.6 times more likely to be obese if their close friends were obese.

Participants were asked to choose their ideal body size from nine line drawings of people of different sizes. They were also asked how much they agreed with stigmatizing statements about obesity, such as "People are overweight because they are lazy." And they were asked whether they would rather be obese or have one of 12 other stigmatizing conditions, including herpes or alcoholism.

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The researchers found very little support for the hypothesis that friends' shared views about acceptable body size cause obesity. Although friends tended to have similar BMIs, their views about body size did not account for this effect.

Women in the study tended to prefer not to be obese. In fact, many women said they would rather have another socially stigmatizing condition than be obese. One-quarter said they would rather be depressed, 14.5 percent said they would rather be totally blind and nearly half said they would rather lose five years of their life.

Future work
More studies need to be done to find out what accounts for the spread of obesity among friends, the researchers say. For instance, some studies have found that simply seeing overweight individuals causes people to eat more junk food.

In addition, research should assess how shared activities, such as eating and exercising, might affect obesity spread.

The study was published online today (May 5) in the American Journal of Public Health.

Pass it on: Shared social norms do not account for the spread of obesity among friends, a study suggests.

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