By Rob Stein
Washington Post Staff Writer
Thursday, July 26, 2007
Obesity appears to spread from one person to another like a virus or a fad, researchers reported yesterday in a first-of-its-kind study that helps explain -- and could help fight -- one of the nation's biggest 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 climbed 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, and the influence cascaded through three degrees of separation before petering out, the researchers found.
"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 A. Christakis of Harvard Medical School, who led the study, being published tomorrow in the 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 are not saying that obesity is literally caused by a virus or some other pathogen, or that factors such as a poor diet, a lack of exercise or a biological propensity are unimportant. Rather, the findings suggest that once a person becomes obese for whatever reason, it may make it more socially acceptable for people close to him or her to gain weight, and that new social norms can proliferate quickly.
"What spreads is an idea. 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," Christakis said. "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."
Other researchers used words such as "brilliant" and "groundbreaking" to describe the work and said it is 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. "I think these results are going to shift the way we think about some of these supposedly noncommunicable diseases."
Some researchers, however, 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 M. 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."
But Christakis said the analysis showed that the results could not be explained simply as a matter of similar-size individuals gravitating toward one another because they share lifestyles.
"This is not 'birds of a feather flock together.' It's not that obese or non-obese people simply find other similar people to hang out with," Christakis said. "Rather, there is a direct causal relationship."
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 said their study also showed that people who were close to someone who lost weight were more likely to get thinner.
"If these close social environments can promote a disease, they can also promote solutions to disease," said William H. Dietz of the federal Centers for Disease Control and Prevention. "These same social networks might be used to turn a disease like obesity around."
The proportion of obese Americans has been rising steadily for decades; more than two-thirds of U.S. adults are overweight, and one-third are obese. Obesity boosts the risk for a host of health problems, including diabetes, heart disease and cancer.
The 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 at 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. The researchers were able to construct intricate maps of the social connections among the participants, identifying spouses, siblings, neighbors, and both casual and close friends.
Sophisticated statistical analyses revealed distinct groupings of thin and heavy individuals, and found that siblings and spouses had less influence than friends, supporting the idea that the study's findings were not the result of people eating the same food, engaging in the same activities or sharing genes.
And though environmental factors such as living in neighborhoods with lots of fast-food restaurants and no good 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 for the findings, the team said.
Fowler, speculating that friends could influence one another just by getting together once or twice a year, said, "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. This is not due to people eating or exercising together."
The researchers also found that one person's weight gain increased another's risk only if the second person considered the first to be a friend. If not, there was no effect. If each considered the other a friend, the effect was magnified.
"This shows that this is a social process that goes on here," Christakis said. "If it was because you had two people exposed to the same fast-food joint or there was something in the air, then the direction of the friendship should be irrelevant. The fact that it is relevant helps us to exclude spurious or confounding effects."
That was reinforced by the fact that people of the same sex influenced one another 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 sexes 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 have 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 healthful lifestyles.