Add friends to the possible causes of the nation's obesity epidemic.
A study being published today suggests that obesity spreads through social connections, particularly close friendships. This doesn't replace the effects of genetics, failing to exercise, or supersizing food portions. But researchers writing in The New England Journal of Medicine say it provides a possible explanation for the rapid increase in obesity over the last few decades. National surveys show that nearly one-third of US adults are now obese.
The condition appears to spread though what researchers are calling "social contagion," a tendency of people who become obese to influence the behavior of others and to convey -- perhaps subliminally -- the message that being overweight is OK.
"Obesity is not just an individual problem, but a collective problem," said Dr. Nicholas Christakis, a professor of medical sociology at Harvard Medical School and coauthor of the report. To slow the epidemic, he said, "treating people in groups may be more effective than treating them individually."
Studying more than 12,000 people linked to a long-running study of heart disease based in Framingham, the researchers found that an individual's chances of becoming obese increased by 57 percent if a friend grew obese. Within the closest friendships, the increased risk was 171 percent. The study found smaller influences between siblings and spouses; neighbors who aren't friends had no effect.
Surprisingly, obesity seemed to spread even if friends were geographically distant.
"We were stunned to find that friends who are hundreds of miles away have just as much impact on a person's weight as those who are geographically close," said James Fowler, the paper's other author, who is an associate professor of political science at the University of California at San Diego. This led researchers to suggest that the effect wasn't only due to sharing behaviors -- such as eating together -- but to sharing ideas about what constitutes an appropriate weight.
Because the study looked at relationships over time, the researchers were able to exclude cases in which obese people chose overweight friends, and therefore were able to make a stronger case for a causal effect.
An official at the National Institute on Aging, which funded the study, called it "one of the most exciting studies in medical sociology . . . in decades" and suggested it would spur research into how networks of interactions affect health.
"It takes what was seen as a noninfectious or noncontagious disease and shows that it's clearly got communicable factors," said Richard Suzman, director of the behavioral and social research program at the institute.
The major findings of the study ring true for Maureen Russell, who was obese for decades before recently losing weight. The Reading woman has four siblings, and she said they all began to struggle with their weight after she did. "We actively encouraged each other to order dessert," said Russell, a nurse case manager at Boston Medical Center.
But Dr. Caroline Apovian, director of the Center for Nutrition and Weight Management at Boston Medical, said she thought the study gives "a false perception" of what's occurring nationally.
"There are strong forces in this country that are moving us toward obesity, but I don't think social contagion is one of them," she said. Rather, she pointed to the abundance of fast-food options and people so busy with jobs and family life that they don't exercise.
Apovian also disagreed with the suggestion that obese people are changing the norms.
"We idolize thinness," she said. "People are not running around being happy and obese."
The study used records from the Framingham Heart Study, which contained addresses, heights, and weights of participants, as well as names of their close friends. The researchers traced the connections over a 32-year period and then examined individuals' body-mass index, a measure of height and weight, for signs of obesity. They found obesity seemed to spread quickly -- in as few as two to four years.
Among friends and siblings, obesity seemed to spread only between those of the same gender. It was strongest between male friends -- if one became obese, the risk for the other doubled -- and between sisters. In a counterintuitive finding, the spread was less among heterosexual married couples. Although couples may share many activities, Fowler suggested that people tend to look to others of their own gender when deciding what weight is appropriate.
The study offered a bit of encouragement for people trying to lose weight: The effect also seems to work in reverse. A person with a friend able to bring his or her weight down also seemed more likely to lose weight, Fowler said.
"We are not suggesting that people should sever ties with their overweight friends," said Christakis. "But forming ties with underweight or normal weight friends may be beneficial to you."
Alice Dembner can be reached at email@example.com.