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People whose friends become obese have a greater chance of also getting too fat, a finding that suggests that obesity is "socially contagious," spreading from one person to another like a disease, according to a new study released Wednesday.

Geographic distance between friends doesn't matter — the influence of friendship is the same whether friends live next-door or 500 miles away, according to the report in the New England Journal of Medicine.

The study, conducted by Dr. Nicolas Christakis of Harvard Medical School and James Fowler of UC San Diego, was 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.

"People are interconnected and their health is interconnected," said Christakis, a professor of medical sociology.


The report offers a nuanced understanding of the forces driving the steady rise in obesity during the last 25 years — a trend that has been linked to cheap fast food, a sedentary lifestyle and genetic factors.

The study suggests many of those factors have a role but their influence is amplified through social connections.

"This is a seminal study," said Richard Suzman, head of the National Institute on Aging department that funded the research. "It takes what was seen as a non-infectious 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 human behavior. Other studies have demonstrated that smoking can ripple through groups of friends and depression can spread among family members.

"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.

The latest report suggests that diet and exercise plans focused on obese individuals won't be as effective as interventions aimed at networks of overweight family members and friends, she said.

"The truth is almost no one can do it own their own," she said.

The study analyzed social and family connections among 12,067 participants in the Framingham Heart Study, a decades-long epidemiological study of people from Framingham, Mass. The study collected data on health, diet, exercise, family ties and, to a limited extent, friendships. Every two to four years, participants had a physical exam.

Researchers looked closely at the influence of gender, smoking, socioeconomic class and geographic distance among participants.

They used body mass index, a ratio of height to weight, to calculate obesity. A BMI of 30 or more is considered obese.

Researchers found that the influence of friends on weight gain exceeded that of siblings and spouses, and 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 thought.

Researchers said the weight gains couldn't be attributed to smoking cessation, another explanation offered for the spread of obesity.

Specifically, 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%.

Although geographic distance between friends made no difference, researchers found gender had an important role.

In same-sex friendships, the chance that a friend will become obese increases 71%. Among brothers, the odds go up 44%. Among sisters, the risk jumps 67%.

Friends and siblings of the opposite sex had no influence on weight gain, researchers said. Gender may help explain why spouses have less influence than friends, researchers said.