Fat friends: You're more likely to gain weight if your friends are heavier than you

Obesity treatments for teenagers should take social influence from their peers into account, say researchers By CLAIRE BATES PUBLISHED: 12:30 GMT, 10 July 2012 | UPDATED: 12:53 GMT, 10 July 2012 If you want to get slim, you may want to shed some of your larger friends, a new study suggests. Researchers from Loyola University found students were more likely to gain weight if they had friends who were heavier than they were. However, they were more likely to either slim down, or at least gain weight at a slower pace, if their friends were leaner than they were. A little larger? A study suggests a person's body mass index is influenced by their friends (posed) Dr David Shoham said the results could help them develop better interventions to treat obesity in teenagers. 'We should not be treating adolescents in isolation,' he said. The study was designed to determine the reason why obesity and related behaviours appear to cluster in social networks. Is it because friends influence one another's behaviour or is it that teenagers befriend people who look similar to themselves? The researchers examined data from two large high schools - one school, referred to as 'Jefferson High,' was in a rural area and has mostly white students. The second school, 'Sunshine High,' was an ethnically diverse urban school. Students were surveyed during the 1994-95 school year and surveyed again the following school year. Researchers examined data from 624 students at Jefferson High and 1,151 students at Sunshine High. Their body mass index was calculated from their height and weight. A BMI over 25 is considered overweight and a BMI over 30 is considered obese. Researchers found that even after controlling for this friend-selecting process, there still was a significant link between obesity and a student's circle of friends. If a borderline overweight student at Jefferson High School had lean friends there was a 40 per cent chance the student's BMI would drop in the future. However, if they had obese friends their was just a 15 per cent chance they would slim down. The authors said the findings show that social influence 'tends to operate more in detrimental directions, especially for BMI.' 'Effective interventions will be necessary to overcome these barriers, requiring that social networks be considered rather than ignored.' Dr Shoham noted the study relied on self-reported data and was collected more than a decade ago - before Facebook and at a time when childhood obesity rates were much lower. Nevertheless, he said the results raised important questions about peer influence. 'Our results support the operation of both homophily and influence,' he said. 'Of course, no one study should ever be taken as conclusive and our future work will attempt to address many of these limitations.' The study was published in the journal PLoS ONE.