Since 2006, Matt Ryerson and nine other friends in Chattanooga had always made time a few days a week to catch up over a hearty lunch.

But through the years, the trice-weekly meals, often large-portioned and greasy, quietly took their toll on the friends' waistlines. About a year ago, everyone in the lunch group was noticing, said Mr. Ryerson, who at the time was approaching 300 pounds.

"We were all kind of shaking our heads at how we had let ourselves go," he recalled. "If your circle of friends is generally overweight, you feel comfortable in that group. I think for lack of a better term, obesity runs in bunches."

So the 10 friends decided to make a change together. They began picking new locations for their lunch groups, and adding in early morning workouts at the local YMCA.

"Instead of our social networking at the mom-and-pop, grease-filled burger place, we started shifting to the restaurants that had some healthy choices," he said.

In 12 months, Mr. Ryerson has dropped 60 pounds, and most of his friends have made similar progress, he said.

As Americans' waistlines and obesity rates balloon across the country, experts are looking at the role social networks can play in spreading -- or by the same token, combating -- obesity.

"For many people, you simply eat and drink more when you're with other people doing the same thing," said Dr. Jean Cates, psychologist at the Chattanooga Lifestyle Center.

The up side of this argument is that, in the same way that weight gain seemed to be spread through friendships in Mr. Ryerson's case, healthy habits also can be socially contagious, she said.

Laura Barnes, the group fitness coordinator at Bradley Wellness Center at Hamilton Medical Center in Dalton, Ga., said a partner can help someone reach fitness goals.

"Everybody is programmed differently, but I think most people can benefit from the camaraderie of a classroom experience," said "A few years ago, I decided to start running, so I got a running buddy. I lost 30 pounds, she lost 20. We motivated each other and kept each other accountable."

Ms. Barnes said it helps to have someone checking on you

"I think when you know you are going to have someone ask you about your progress, it serves as confession time," she said.

Research published in the New England Journal of Medicine in 2007 found that close friends can have a big impact on each other's risk for obesity, even more so than one's spouse. This "person-to-person" spread of obesity leads to "clusters" of obesity linked by social networks and could be contributing to the nation's obesity epidemic, the authors concluded.

Researchers found that if a person's friend became obese, the person had a 57 percent greater chance of becoming obese.

For mutual friends -- those who both identified one another as close friend in the study -- the risk grew by 171 percent. Spouses only increased one another's risk by 37 percent.

The researchers attributed the change in risk to the strong influence of friends, particularly those of the same sex and even those who live far away, in determining one's perception of what is an acceptable weight, as well as the influencing behaviors directly.

CAUSATION OR CORRELATION?

The "obesity as contagious" line of research is not without its critics.

A study done in 2008 tried to replicate the work from the original contagiousness study, but found no significant effect from one's social network on obesity risk.

Yale University researcher Jason Fletcher, who co-wrote the follow-up study, said he believes the original study proved only a correlation, not a causal relationship, between friends' weights.

"Friends choose who they want to hang out with. It could be that obese people want to hang out with obese people," he said. Others "are friends because they are fit and share common interests."

Dr. Fletcher said he thinks a study of weight changes in randomly paired college roommates would be a more accurate look at how friends might directly influence one another's weight.

The 2007 obesity research sparked headlines in news coverage, which suggested that obesity is contagious, like a cold.

But that kind of language can spark negative attitudes toward people with weight problems and undermines an individual in his or her weight gain or loss, said Graham Brannan, social worker at the Weight Management Center at Memorial Hospital.

The idea evokes "some poor, healthy-weight person that's going to be exposed to all these self-indulgent fat people," he said. "If your friends are doing a particular behavior, is it going to be an influence to you? Yes. Is it contagious in the sense that you're going to be an innocent victim of this? I don't think so."

Instead, counselors must emphasize individuals' power to change behavior, without disregarding the role of environmental influences, such as living next door to a McDonald's or, alternately, having friends who go hiking every weekend, he said.

For Angie Lytle, 34, the company she keeps has had a significant and positive impact on her weight, which she says has been a source of struggle most of her life.

Since she started working as an administrative assistant at the Chattanooga Lifestyle Center in August, she said she's been surrounded by dietitians and exercise physiologists whose focus is health. The work refrigerator is filled with healthy foods, instead of junk, and she's joined in on clients' group workouts most evenings, she said. In a few months, she's lost 20 pounds, she said.

"It's always been easier for me to lose weight when your friends are involved with it," she said.

Staff writer Adam Crisp contributed to this report.