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.
