Obesity spreads to friends, study concludes
By Gina Kolata
Published: July 25, 2007
Obesity can spread from person to person, much like a virus, according to researchers. When one person gains weight, close friends tend to gain weight too.
Their study, published Thursday in the New England Journal of Medicine, involved a detailed analysis of a large social network of 12,067 people who had been closely followed for 32 years, from 1971 until 2003. The investigators knew who was friends with whom, as well as who was a spouse or sibling or neighbor, and they knew how much each person weighed at various times over three decades.
That let them watch what happened over the years as people became obese. Did their friends also become obese? Did family members? Or neighbors?
The answer, the researchers report, was that people were most likely to become obese when a friend became obese. That increased one's chances of becoming obese by 57 percent.
There was no effect when a neighbor gained or lost weight, however, and family members had less of an influence than friends. It did not even matter if the friend was hundreds of miles away - the influence remained. And the greatest influence of all was between mutual close friends. There, if one became obese, the other had a 171 percent increased chance of becoming obese too.
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The same effect seemed to occur for weight loss, the investigators say, but since most people were gaining, not losing, over the 32 years, the result was an obesity epidemic.
Dr. Nicholas Christakis, a physician and professor of medical sociology at Harvard Medical School and a principal investigator in the new study, says one explanation is that friends affect each others' perception of fatness. When a close friend becomes obese, obesity may not look so bad.
"You change your idea of what is an acceptable body type by looking at the people around you," Christakis said.
The investigators say their findings can help explain why Americans became fatter in recent years: Persons who became obese were likely to drag some friends with them.
Their analysis was unique, Christakis said, because it moved beyond a simple analysis of one person and his or her social contacts, and instead examined an entire social network at once, looking at how a friend's friends' friends, or a spouse's siblings' friends, could have an influence on a person's weight. The effects, Christakis said, "highlight the importance of a spreading process, a kind of social contagion, that spreads through the network."
Of course, the investigators say, social networks are not the only factors that affect body weight. There is a strong genetic component at work too.
Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds, or 13.5 kilograms, for each person. But that leaves a large role for the environment in determining whether a person's weight is near the top of his or her range or near the bottom. As people have gotten fatter, it appears that many are edging toward the top of their ranges. The question has been why.
If the new research is correct, it might mean that something in the environment seeded what many call an obesity epidemic, making a few people gain weight. Then social networks let the obesity spread rapidly.
It also might mean that the way to avoid becoming fat is to avoid having fat friends.
That is not the message they meant to convey, say the study investigators, Christakis and his colleague James Fowler, an associate professor of political science at the University of California in San Diego. You don't want to lose a friend who becomes obese, Christakis said. Friends are good for your overall health, he explains.
So why not make friends with a thin person, he suggests, and let the thin person's behavior influence you and your obese friend?
That answer does not satisfy obesity researchers like Kelly Brownell, director of the Rudd Center for Food Policy and Obesity at Yale University.
"I think there's a great risk here in blaming obese people even more for things that are caused by a terrible environment," Brownell said.
On average, the investigators said, their rough calculations show that a person who became obese gained 17 pounds, and the newly obese person's friend gained 5 pounds. But some gained less or did not gain at all, while others gained much more.
Those extra pounds were added onto the natural increases in weight that occur when people get older. What usually happened was that peoples' weights got high enough to push them over the boundary, a body mass index of 30, that divides overweight and obese. (For example, a man 6 feet, or 1.8 meters, tall who went from 220 pounds to 225 would go from being overweight to........
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