Obesity can spread from person to person, much like a virus, researchers have reported on 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 neighbour, 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 neighbours?

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 per cent. There was no effect when a neighbour gained or lost weight, however, and family members had less 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 close mutual friends. There, if one became obese, the other had a 171 per cent increased chance of becoming obese, too. 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.

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. The investigators say, social networks are not the only factors that affect body weight. There is a strong genetic component at work as well.

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.

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 behaviour influence you and your obese friend?