Can Adults and Teens Catch Obesity?

Ketchup and Mustard

In 2007, researchers utilized data from the Framingham Heart Study to learn about the spread of obesity in both voluntary and involuntary social networks. The available information covered more than 12,000 people and was collected over a time span of more than 30 years. The researchers wanted to know whether a person’s weight gain was associated with weight gain on the part of others with whom the person had either selective social ties (friends and spouses) or nonselective ties (siblings and neighbors). The study found that:

A person’s chances of becoming obese increased by 57% if he or she had a friend who became obese in a given interval. Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40%. If one spouse became obese, the likelihood that the other spouse would become obese increased by 37%.

Siblings share genetic material and familial indoctrination. Those unavoidable influences are much stronger and of a quite different nature than relationships created and shared with voluntary contacts like friends and spouses, so it should come as no surprise that siblings resemble each other. Conversely, the other type of nonselective tie, consisting of only the geographical coincidence of being neighbors, seemed to have no effect one way or the other. The researchers also found that women’s obesity patterns are more influenced by female associates, and the proclivities of men are more influenced by other males.

Danger: Restaurant Ahead

The book A Big Fat Crisis by Deborah Cohen, M.D., contains fascinating information about why people overeat even when they know better. For instance, if someone is offered a second helping of same-shaped pasta with sauce, they might give it a pass. But if the second serving, identical in amount and with the same sauce, is made up of differently shaped pasta, the person is likely to eat it.

The most insidious thing is the multiplier effect, a hazard when people eat away from home. According to Cohen, the presence of one extra person at a shared restaurant meal will cause everyone at the table to eat 33% more, and when a group gets up to seven or more convivial diners, everybody eats nearly twice as much as they otherwise would have. Contagious overeating is right next door to contagious obesity.

The young

Now, what about the obesity-related mutual influence exerted by teenagers? PsychCentral senior news editor Rich Nauert wrote up an informal interpretation of a Loyola University study that scrutinized friendship circles among high school students.

Thanks to the National Longitudinal Study of Adolescent Health, data were collected in 1994 and 1995 from two large high schools. David Shoham, Ph.D., lead author of the study, issued several caveats. Any research that depends on self-reporting is susceptible to bias. Also, studies of social networks are observational, quite different from lab experiments where animals are fed calibrated calories followed by blood tests to assess results.

Researchers interested in cause and effect are working with approximations, not hard facts. Perhaps the most limiting factor in this particular study was that data collection took place before the childhood obesity epidemic really hit, and before Facebook and other online social networks were phenomena to be reckoned with. The Loyola team found that social influence “tends to operate more in detrimental directions,” and this is especially true relative to body mass index:

Students were more likely to gain weight if they had friends who were heavier than they were. Conversely, students were more likely to get trimmer — or gain weight at a slower pace — if their friends were leaner than they were.

If a borderline overweight kid — one who could go either way — pals around with leaner friends, there would be a 40% chance of losing weight, and only a 27% chance of gaining. If a borderline teenager hangs out with obese friends, there would only be a 15% chance of losing weight and a whopping 56% chance of gaining. The researchers arrived at the useful conclusion that treating an obese adolescent is likely to be more successful in a group setting rather than in isolation.

Your responses and feedback are welcome!

Source: “The Spread of Obesity in a Large Social Network over 32 Years,”, 07/26/07
Source: “Why gov’t should regulate food like tobacco & alcohol,”, 12/28/13
Source: “Social Networks Influence Obesity,”, 07/12/12
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