The Social Side of Obesity: You Are Who You Eat With

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Sending your kids back to lunch-lady land this fall? Careful, your child's dining mates may be upping his chances of packing on the pounds. A study published in the August issue of the American Journal of Clinical Nutrition finds that how much tweens and teens eat can be influenced by how much their friends weigh.

In the study, 130 kids ages 9 to 15 were allowed to snack as much as they wanted while hanging out with a friend or with a peer they did not know. All the kids ate more when they were with a friend than with a stranger. But the overweight children ate the most when paired with an overweight friend — an average of 300 more calories than when they spent time with leaner friends. The research also found that friendship itself makes the appetite grow stronger: when overweight kids ate with similar-weight kids who were already their pals, they threw back an extra 250 calories than when they ate with chubby kids they had just met.

Lead researcher and clinical psychologist Sarah-Jeanne Salvy says her research demonstrates an eye-opening social theory: obesity can be contagious.

Really, an "obesity bug"? In 2007, Harvard researcher Nicholas Christakis and his colleagues analyzed 32 years' worth of data from an interconnected social network of 12,000 adults and found that a person's chances of becoming obese increased 37% if a spouse had become obese, 40% if a sibling had and 57% if a friend had.

Socializing with overweight people can change what we perceive as the norm; it raises our tolerance for obesity both in others and in ourselves. It's also about letting your hair down. Past research has shown that adults tend to eat more around friends and family than they do with strangers. They shed their inhibitions about how it looks when they go back for thirds or order the alfredo sauce instead of the marinara.

Finally, there's the idea that we like to hang with people who are like ourselves. Cornell food sociologist Jeffrey Sobal explains that "especially among two overweight people, there's a sort of permission-giving going on. We're encouraging each other to eat more."

Salvy, who is an assistant professor of pediatrics at the State University of New York at Buffalo, refers to this phenomenon as a sort of feedback effect. Conversely, she suggests, overweight diners are more likely to tone down how much they eat in front of skinny people to avoid the stigma of overeating.

When it comes to holding back, though, there's nothing like the opposite sex to curb our appetites, at least when we're single. In a study to be published in the October issue of Appetite, researchers at Montreal's McGill University secretly observed 460 college students eating in the campus cafeterias. They found that when a woman was with a man, she ate about 100 calories less than when she was with a woman. The more men present in larger eating groups, the fewer calories a woman had on her tray. Women ate roughly 100 fewer calories for each man at the table. But there was no such effect on men. And women who only ate with other women tended to slightly increase their calorie quotient.

The study's author Meredith Young, a cognitive psychologist, says the social comfort of a same-sex lunch partner probably makes a difference — but evolutionary instincts are also at play. The women are using food as a signal of attractiveness. "In past studies, when you compare the exact same woman either eating a meatball sub or a dainty salad, people find the salad eater more alluring and more desirable as a friend," she says. Young thinks that men, on the other hand, are probably focused on spending more money on the food instead of eating it, because evolutionary biology says that part of male sex appeal lies in the financial wealth they bring to the table.

For parents who are less worried about subconscious dating rituals and more worried about overweight kids influencing their own, the answer isn't to encourage ditching fat friends. Shaming kids over their bodies could drive them to social isolation, a much worse place to be, according to Salvy. Her previous research found that overweight teens eat an average of 400 calories more when they're alone, compared with when they're in the company of friends of any weight group.

Dr. Marc Jacobson, a member of the American Academy of Pediatrics' Task Force on Obesity, says the best thing to do is to model healthy eating and exercise habits yourself; research shows it's more effective than just talking about losing weight. So pack a healthy, satisfying lunch, but "focus more on making your home as healthy as you can," he says.