NEW YORK (Reuters):

Like the common cold, obesity can be spread from person to person, new research suggests.

A person's social network can influence his risk of obesity, according to new study findings reported in The New England Journal of Medicine. The results suggest that if you want to stay thin, you may not want to surround yourself with obese friends and relatives.

"It's not that obese or non-obese people simply find other similar people to hang out with," study co-author Dr. Nicholas A. Christakis, from Harvard Medical School in Boston, said in a statement. Rather, the one directly causes the other, he explained.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size. People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads," Christakis noted.

The findings stem from a study of 12,067 individuals who were part of densely interconnected social network and were evaluated from 1971 to 2003 as part of the Framingham Heart Study. Body mass index, the ratio of body weight t was determined for all subjects and complex statistical tests were used to determine how the weight gain of a friend, sibling, spouse, or neighbour might affect a person's own weight.

The researchers identified clusters of obese people in the social network that were apparent throughout the study period. These clusters extended to three degrees of separation, the authors note.

During a given time period, the likelihood that a person would become obese rose by 57 per cent if he or she had a friend who became obese. If a sibling or a spouse became obese, a person's risk of becoming obese increased 40 and 37 percent, respectively.

The risk of obesity was usually greater if the person's associate was of the same gender, the report indicates. By contrast, local environmental factors seemed to have little impact on person's risk of becoming obese. For instance, people with obese neighbours who were not in their social network were not a risk for becoming obese themselves.

"Social effects, I think, are much stronger than people before realised," co-author Dr. James H. Fowler, from the University of California, San Diego, said in a statement. "There's been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity; and what our paper suggests is that you really should spend time looking at the social side of life as well."

SOURCE: The New England Journal of Medicine, July 26, 2007.