Researchers say obesity is not only caused by genes, but by overweight people being part of social networks of obese individuals. Investigators say the finding helps explain an epidemic of obesity over the past 30 years. VOA's Jessica Berman reports.
Since the mid-1970s, the number of overweight and obese individuals has skyrocketed. The World Health Organization estimates there are now 1 billion overweight people, at least 300 million of whom are obese.
Experts define obesity as weighing in excess of 30 percent of ideal body weight, a condition that puts obese individuals at extreme risk for heart attack, stroke and diabetes.
Medical researchers know that obesity runs in families because of genes. But, it turns out, that is not the whole story. Investigators have found that social networks play a significant role in determining body type.
Matthew Gilman is head of the obesity prevention program at Harvard University in Cambridge, Massachusetts.
Gilman says while genes can explain obesity, they cannot explain the so-called obesity epidemic.
"Genes haven't changed in the last 30 years, but our environment really has," he said. "And this study is adding a new perspective to that environment, that of social networks."
Sociologists at Harvard University and the University of California at San Diego studied the intricate social networks of more than 12,000 people between 1971 and 2003.
After an exhaustive study of the individuals' relationships - including friends, family and neighbors - the investigators found that a person's chances of becoming obese increased 40 percent if he or she had a sibling who became obese and 37 percent if they had a spouse who became obese.
Most surprising though, was the risk of obesity among unrelated friends. The study showed that if the friendship was casual, a person's risk of becoming obese increased by 57 percent if the friend became obese. If the relationship was strong and the two people claimed each other as friends, then if one became obese, the friend's risk of obesity jumped by 170 percent.
The findings are published in the New England Journal of Medicine.
James Fowler is a sociologist at the University of California and one of the study's authors.
"We were stunned to find that people who are hundreds of miles away have just as much impact on a person's weight status as friends who are right next door," he said. "And so what this suggests is that it's not the case that this causal relationship is due to people eating together or exercising together. Rather, it has to do with them sharing ideas about what healthy behavior is like."
Co-author Nicholas Christakis of Harvard University says the findings have implications for the development of effective weight loss programs.
"We think our results suggest that treating people in groups my be more effective than treating them individually," he noted. "So, collective or group dieting or efforts to participate in sports or other activities that are collective may be more effective than individual attempts alone."
Observers say this is the first time it appears that groups of people have infected one another with a non-infectious disease, and they predict the study may lead to a new field called network medicine.