Family, Friends May 'Spread' Obesity

In addition, a person's odds of becoming obese increased by 57 percent if he or she had a friend who became obese over a certain time interval. If the two people were mutual friends, the odds increased to 171 percent.

And if one adult sibling became obese, the chances that the other sibling would also become obese increased by 40 percent.

Marriage mattered, too -- if a spouse became obese, the other spouse had a 37 percent increased likelihood of following suit.

Social networks were more important than geographic location, the study found. In other words, who you tend to hang out with is more influential, weight-wise, than who your next door neighbor is.

People of the same gender had a greater influence on each other than people of the opposite gender, the team found.

Finally, the researchers found that the trend among Americans to quit smoking did not account for the spread of obesity within a given network.

According to the study authors, there's much more involved in the phenomenon than just similarities in lifestyle and environment, as evidenced by the fact that social networks are more important than geography. Two likely explanations are the spread of behaviors and the spread of social norms, with evidence pointing more to the latter.

"To a point, it reminds me of peer pressure -- we want to fit into this particular group," said Dr. Juan Castro, director of the Texas A&M Health Science Center Coastal Bend Health Education Center in Corpus Christi.

Christakis agreed. When overweight or obesity becomes normal in a given social circle, people may be more likely to become obese themselves. In other words, he said, "I see you gaining weight, so it's OK for me to gain weight."

That finding may support efforts to provide nutrition education in the workplace, where many people find their friends, Castro said. There is also value in targeting interventions at the person in a family in charge of food buying and preparation, he added.

More information

For more on the epidemic of obesity, visit the U.S. Centers for Disease Control and Prevention.

SOURCES: Nicholas A. Christakis, M.D., Ph.D., M.P.H., physician and professor, Harvard University, Boston; Julio Licinio, M.D., chairman, department of psychiatry and behavioral sciences, University of Miami Miller School of Medicine; Samuel Klein, M.D., director, Center for Human Nutrition, Washington University School of Medicine, St. Louis; Juan Castro, M.D., director, Texas A&M Health Science Center Coastal Bend Health Education Center, Corpus Christi; July 26, 2007,New England Journal of Medicine