Obesity can be contagious, finds new study

27 July, 2007:

Whoever thought obesity could be contagious! A report in the New England Journal of Medicine indicates that obesity can spread among a group of friends like a contagious disease, moving from one person to another in an epidemic of fat.

A new study conducted in the United States has showed that having close friends who are fat can nearly triple one’s risk of becoming obese. The effect is so powerful that distance does not matter; the influence is the same whether friends live next door or many miles apart.

The study, conducted by Dr Nicholas A Christakis, a professor of medical sociology at Harvard Medical School, and James H Fowler of University of California at 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 alright to be heavier, Christakis said. “it’s about the spread of norms from person to person.”

The phenomenon worked in the opposite direction, too. People who become thinner increase the chances that their friends and relatives will lose weight too.

The new report could help explain the significant increase in the rate of obesity, which has doubled in the United States over the last 25 years.

A recent study at Johns Hopkins University showed that one-third of American adults are obese, and that the proportion may increase to 40% in the next eight years. 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 blamed on low-cost 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.

Richard Suzman, director of the National Institute on Aging’s behavioral and social research program, which funded the research, described the study as a seminal one.

The report is the latest to apply network analysis – a concept based on computer science – to the study of human behavior. Instead of focusing on individual cases, researchers analysed the spread of obesity through a network of 12,067 people over 32 years.

Researchers said the methodology could also be used to devise ways to break the social connections that feed smoking and drug addiction.

Researchers who looked closely at the influence of gender, smoking, socioeconomic class and geographic distance among participants found that the influence of friends on weight gain was as powerful as the effect of genetics found in other studies.

Neighbours who were not friends had no influence on each other, suggesting that community characteristics often linked to obesity, such as a lack of parks or a dependence on cars, were not as important as previously thought.

Overall, researchers found that if a person becomes obese, the chances that a friend will become obese rises by 57%. Among siblings, the risk goes up by 40%. Between spouses, the odds rise by 37%.

Mutual friends – that is, study participants who identified each other as friends – have had the greatest influence. If one became obese, the risk jumped to as much as 171%.

The gender mix in friendships too played an important role. In same-sex friendships, the chance that a friend will become obese increases by 71%. Friends and siblings of the opposite sex had no influence on weight gain.

Researchers found that a person who becomes obese increases the odds of obesity in about 100 people connected to one another though family or friendship.