"This reinforces the idea that because people are interconnected, their health is interconnected," said study author Dr. Nicholas A. Christakis, a professor at Harvard University. "It takes seriously the embedded-ness of people within social networks and gives new meaning to the concept of public health."

While this may cause people to look differently at their friends and acquaintances (both overweight and thin), the real value of the research is in pointing to new ways to combat the growing epidemic of overweight and obesity, experts said.

"Trying to address the problem on an individual level has been so hard, and it may be because we're not addressing the network, which could be family, neighborhood, community, school," said Dr. Julio Licinio, chairman of the department of psychiatry and behavioral sciences at the University of Miami Miller School of Medicine. "This is a fascinating way to look at the problem, and it may be a very good reason why treatments have been so difficult, because we're only addressing one member of the network."

"It brings up another component of our environment that influences obesity," added Dr. Samuel Klein, director of the Center for Human Nutrition at Washington University School of Medicine in St. Louis. "This would need to be proven, but it suggests that, to be effective in treating obesity, we have to not just treat the person who's obese but also the social network."

The study is likely to become a classic, experts stated.

The issue is of no small concern, since obesity has become a major public health problem in the United States and other Western nations. According to background information in the article, two-thirds of American adults are now overweight, while the prevalence of obesity has increased from 23 percent to 31 percent.

A variety of explanations have been put forth on the epidemic, including less exercising and more eating. But because the epidemic is so widespread and affects all socioeconomic groups, there are likely several social and environmental explanations for the phenomenon, experts said.

The authors speculated that the ubiquity of social networks, and the natural inclination of people to be influenced by the appearance and behaviors of those around them, suggest that weight gain in one person might encourage weight gain in others. Having social contacts of a certain bodily size may also cause you to adopt certain behaviors, or other people's behaviors may even stimulate certain parts of the brain, such as those related to eating food.

To investigate this theory, the authors of this study evaluated more than 12,000 socially interconnected people in New England who had participated in the Framingham Heart Study, a landmark initiative aimed at unearthing the causes of cardiovascular disease. As part of that study, the individuals had undergone repeated measurements including body mass index (BMI) over more than three decades. At the beginning of the study, participants had been asked to identify their friends for follow-up purposes.

The researchers found that thin and fat people tended to be clustered together, with the clusters extended to three degrees of separation. In other words, you're obese and so is you friend's friend's friend, or your friend's spouse's sister.

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.