By Gina Kolata

Obesity can spread from person to person, much like a virus, researchers are reported Wednesday. When one person gained weight, their close friends tended to gain weight, too.

Their study, published 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 examine what happened over the years as some individuals 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 a person'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 influence than friends.

Proximity did not seem to matter: The influence of the friend remained even if the friend was hundreds of miles away. And the greatest influence of all was between mutual close friends. There, if one became obese, the odds of the other becoming obese were nearly tripled.

The same effect seemed to occur for weight loss, the investigators say. But since most people were gaining, not losing, over the 32 years of the study, 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 have become fatter in recent years -- each person who became obese was 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 person's friend's friend's friends, or spouse's sibling's 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 as well.

Science has shown that individuals have genetically determined ranges of weights, spanning perhaps 30 or so pounds 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 may mean that something in the environment seeded what many call an obesity epidemic, leading a few people to gain weight. Then social networks let the obesity spread rapidly.

It also may 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.

Their research has taken obesity specialists and social scientists aback. But many say the finding is pathbreaking, and can shed new light on how and why people have gotten so fat so fast.

"It is an extraordinarily subtle and sophisticated way of getting a handle on aspects of the environment that are not normally considered," said Dr. Rudolph Leibel, an obesity researcher at Columbia University.

Dr. Richard Suzman, who directs the office of behavioral and social research programs at the National Institute on Aging, called it "one of the most exciting studies to come out of medical sociology in decades." The institute financed the study.

But Dr. Stephen O'Rahilly, an obesity researcher at the University of Cambridge, said the uniqueness of the Framingham data will make it hard to replicate the new findings. No other study that he knows of includes the same kinds of long-term and detailed data on social interactions.

"I don't want to look like an old curmudgeon, but when you come upon things that inherently look a bit implausible, you raise the bar for standards of proof," O'Rahilly says. "Good science is all about replication, but it is hard to see how science will ever replicate this."

"Boy, is the Framingham Study unique," O"Rahilly said. (NYT)