WASHINGTON - Obesity can spread from one person to another like a fad or the flu, researchers report today in a first-of-its-kind study that helps explain - and could help fight - one of the most intractable public-health problems.
The study, involving more than 12,000 people tracked over 32 years, found that "social networks" play a surprisingly powerful role in determining an individual's chances of gaining weight, transmitting an increased risk of becoming obese from wives to husbands, from brothers to brothers, and from friends to friends.
The researchers found that when one spouse became obese, the other was 37 percent more likely to do so in the next two to four years, compared with other couples. If a man became obese, his brother's risk rose by 40 percent.
The risk rose even more sharply among friends - between 57 and 171 percent, depending on whether they considered each other mutual friends. Moreover, friends affected friends' risk even when they lived far apart."It's almost a cliche to speak of the obesity epidemic as being an epidemic. But we wanted to see if it really did spread from person to person like a fashion or a germ," said Nicholas Christakis of Harvard Medical School, who led the study in today's New England Journal of Medicine.
"And the answer is, 'Yes, it does.' We are finding evidence for a kind of social contagion."
Christakis stressed that the researchers were not saying that obesity was literally caused by a virus or some other pathogen, or that factors such as poor diet, lack of exercise, or a biological propensity were unimportant. Rather, the findings suggest that once people become obese for whatever reason, it may make it more socially acceptable for those close to them to gain weight, and that new social norms can proliferate quickly.
"What spreads is an idea," he said. "As people around you gain weight, your attitudes about what constitutes an acceptable body size changes, and you might follow suit and emulate that body size. It may cross some kind of threshold, and you can see an epidemic take off. Once it starts, it's hard to stop it. It can spread like wildfire."
'Most exciting'Other researchers used words like brilliant and groundbreaking to describe the work, and said it was likely to lead to a flurry of research.
"This is one of the most exciting studies in medical sociology that I've seen in decades," said Richard Suzman, director of the behavioral and social research program at the National Institute on Aging, which funded the study. "These results are going to shift the way we think about some of these supposedly noncommunicable diseases."
In addition to offering novel insights into the obesity epidemic, the discovery could suggest new tactics for stemming the seemingly inexorable trend. The findings lend support to treating people in groups or even whole communities, for example. The researchers noted that their study also showed that people who were close to someone who lost weight were more likely to get thinner.
Turning it around"If these close social environments can promote a disease, they can also promote solutions," said William Dietz of the Centers for Disease Control and Prevention. "These same social networks might be used to turn a disease like obesity around."
More than two-thirds of adults are overweight, and one-third of those are obese. Obesity boosts the risk for a host of health problems, including diabetes, heart disease and cancer.
The new study is the first to explore the influence of social networks - people connected through family, friendships, neighborhoods or other relationships - for any chronic condition.
Christakis and James H. Fowler of the University of California, San Diego, took advantage of detailed records collected between 1971 and 2003 on 12,067 adults who participated in the well-known Framingham Heart Study. They were able to construct intricate maps of the social connections among the participants, identifying spouses, siblings, neighbors, and both casual and close friends.
Statistical analyses revealed distinct groupings of thin and heavy individuals and showed that the clusters could not be explained simply as a matter of similar-sized individuals gravitating toward one another because they already shared lifestyles.
"This is not birds of a feather flock together," Christakis said. "It's not that obese or non-obese people simply find other similar people. There is a direct causal relationship."
Some experts questioned whether the study had fully accounted for other factors.
"People pick friends because they are similar in the way they eat or the way they move," said Barry Popkin, who studies obesity at the University of North Carolina at Chapel Hill. "It's a nice piece of work but still stops short of being able to deal with causality. "
Christakis said the data showed siblings and spouses had less influence than friends, for example, supporting the idea that the findings could not be the result of people originally eating the same foods, engaging in the same activities, or sharing genes.
And while environmental factors such as living in neighborhoods with a lot of fast-food restaurants and few grocery stores or sidewalks probably play a role, the researchers found no effect among neighbors unless they were friends, and being friends had an effect regardless of whether they lived nearby. That ruled out common surroundings as explanations.
"We were stunned to find that people who were hundreds of miles away had just as much impact on a person's weight status as friends who are next door," Fowler said.
People of the same sex influenced each other the most. In same-sex friendships, an individual was 71 percent more likely to become obese if a friend did. But friends and siblings of opposite genders had no increased risk.
"People are more likely to copy the actions of people they resemble," Christakis said. "What we think is going on here is emulation."
The researchers cautioned that people should not sever relationships with friends who gained weight or stigmatize obese people, noting that close friendships have many positive health effects. But the results do support forming relationships with people who have healthy lifestyles.
Others said the findings should not be seen as relieving people of responsibility for watching their weight.
"This is very interesting and potentially helpful," said Gary Foster, president-elect of the Obesity Society, "but at the end of the day, an individual still controls what they eat and how they move."