Obesity can spread among a group of friends like a contagious disease, moving from one person to another in an epidemic of fat.
That's the finding of a novel study released Wednesday that reported that having close friends who are fat can nearly triple your risk of becoming obese.
The effect is so powerful that distance doesn't matter -- the influence is the same whether friends live next door or 500 miles apart, according to the report in the New England Journal of Medicine.
The study, conducted by Dr. Nicholas A. Christakis of Harvard Medical School and James H. Fowler of UC San Diego, is the first to document the spread of obesity through a social network -- a pattern of contagion most often associated with infectious diseases such as influenza and AIDS.
Instead of transmitting germs or viruses, people infected each other with their perceptions of weight. For example, a man attending a Thanksgiving meal may notice his brother has gained weight and conclude that it's OK to be heavier, Christakis said.
"It's about the spread of norms from person to person," said Christakis, a professor of medical sociology.
The phenomenon worked in the other direction as well. People who become thinner, increase the chances that their friends and relatives will lose weight too, researchers said.
The report added a new theory to help explain the remarkable increase in the rate of obesity, which has doubled in the U.S. over the last 25 years.
One-third of American adults are obese, and that proportion may increase to 40% in the next eight years, according to a recent Johns Hopkins University study. Many in public health describe obesity as an epidemic that has helped fuel a rise in diabetes, heart disease and other chronic conditions.
The trend has been linked to inexpensive fast food, a sedentary lifestyle and genetic factors. The new research suggests that those factors have a role but that their influence is amplified through social connections.
"This is a seminal study," said Richard Suzman, director of the National Institute on Aging's behavioral and social research program, which funded the research. "It takes what was seen as a noninfectious disease and shows it clearly has got communicable factors."
The report is the latest to apply network analysis -- a concept with roots in computer science -- to the study of human behavior. Instead of focusing on individual cases, researchers analyzed the spread of obesity through a network of 12,067 people over 32 years.
"It is very plain to those of us who work in community settings that health behaviors occur in the context of a social network," said Dr. Katherine Kaufer Christoffel, a childhood obesity expert at the Northwestern University Feinberg School of Medicine who was not connected with the research.
Researchers said the methodology could also be used to devise ways to break the social connections that feed smoking and drug addiction.
"People are interconnected, and their health is interconnected," Christakis said.
In the latest study, researchers used data from the decades-old Framingham Heart Study, which has collected information on health, diet, exercise, family ties and, to a limited extent, friendships among the residents of Framingham, Mass.
They used a standard measure, body mass index, to determine whether a subject was obese. BMI is a ratio of height to weight; a person with a score of 30 is considered obese. For example, a man 6 feet tall and 225 pounds has a BMI of 30.5.
Researchers looked closely at the influence of gender, smoking, socioeconomic class and geographic distance among participants.
They found that the influence of friends on weight gain was as powerful as the effect of genetics found in other studies.
Neighbors who weren't friends had no influence on each other, suggesting community characteristics often linked to obesity -- such as a lack of parks or a dependence on cars -- weren't as important as previously thought.
Overall, researchers found that if a person becomes obese, the chances that a friend will become obese rises 57%. Among siblings, the risk goes up 40%. Between spouses, the odds rise 37%.
Mutual friends -- study participants who identified each other as friends -- had the greatest influence. If one became obese, the risk skyrocketed 171%.
The gender mix in friendships played an important role. In same-sex friendships, the chance that a friend will become obese increases 71%. Friends and siblings of the opposite sex had no influence on weight gain, researchers said.
Researchers constructed diagrams of social networks and plotted the spread of obesity through chains of friends.
They found that a person who becomes obese increases the odds of obesity in about 100 people connected to one another though family or friendship.
"It is not only friends, but friends of friends' friends who are affected by this," said Fowler, a political science professor.