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Obesity is a social disease that seems to spread through friends and families a bit like flu. Researchers recently applied the relatively new discipline of network analysis to the men and women in the Framingham heart study, a cohort that has been tracked for more than 30 years. They examined the effect of friendships, family ties, and neighbourhood connections on the spread of obesity through the cohort, using named contacts provided by participants to help with follow-upfollow-up.
Obesity developed in clusters. As one adult became obese, the chances of their close friends becoming obese increased by 57% (95% CI 6% to 123%). Siblings had similar influences over each others body weight, particularly if they were the same sex, as did husbands and wives. When one spouse became obese, the other spouse's chance of obesity increased by 37% (7% to 73%). In this simulation, obesity did not spread between neighbours. So there is more to the clusters than shared environment, say the authors.
If obesity spreads from person to person through social networks, the authors hope that dieting and exercise might spread in the same way. It's certainly possible. Weight loss programmes that provide peer support are generally more successful than those that don't.