Obesity can be contagious

Do you have friends who are carrying extra weight? If so, there's a good chance you might be too. New research published in the New England Journal of Medicine finds that people in your social network, including friends, their friends and the friends of those friends, can strongly influence your chances of being obese.

U.S. Researchers studied a densely interconnected social network of 12,067 people between 1971 to 2003, assessing their body-mass index (BMI), a statistical measure of the weight of a person scaled according to height. They then used statistical models to examine whether weight gain in one person was associated with weight gain in their friends, siblings, spouse, and neighbours.

The results showed clusters of obese people, with a BMI of over 30, extended to three degrees of separation, meaning that even social interaction with a friend of a friend could have an impact on a person's weight. A person's chances of becoming obese increased by 57 per cent if theyhad a friend who became obese in a given interval.

Among pairs of adult siblings, if one sibling became obese, the chance that the other would become obese increased by 40 per cent. And if one spouse became obese, the likelihood that the other spouse would become obese increased by 37 per cent.

People of the same sex had a relatively greater influence on each other than those of the opposite sex. But neighbours who were obese had no effect on the study's participants.

Researchers theorize that as people watch others gain excess weight, they begin to perceive it as normal. "That tells us that in fact the social networks trump the genes or genetic influences," Dr. David Lau, president of Obesity Canada, told CBC News.

Experts say the study offers insight into why obesity is spreading rapidly. "As the proportion of people that are overweight or obese increases rather dramatically, then one could imagine this spread occurring almost to the entire population," Mark Tremblay, director of Healthy Living and Obesity Research at Children's Hospital of Eastern Ontario, told CBC News.

The answer, the study suggests, is to "harness this same force to slow the spread of obesity. People are connected and so their health is connected."

And there is a silver lining in the findings. The study found that if friends, siblings or partners lose weight, others tend to lose weight as well.