Having a circle of friends who are chubby just might make it more likely you will become overweight yourself. That's the interesting finding from a new study published in the New England Journal of Medicine.

The researchers found that if someone's friends become obese, that person's chances of becoming obese increased by 57 per cent. Siblings and spouses also have an influence, although a reduced one:

• People whose siblings became obese were themselves 40 per cent more likely to grow obese;
• People whose spouses became obese were 37 per cent more likely to be obese as well.

"This is really quite extraordinary because we also looked at family members and found that friends actually had a stronger influence on your own obesity than family members," study author James Fowler of the University of California, San Diego, told CTV News.

"Our study finds that obesity is socially contagious -- in other words, it tends to spread from one person to another."

Co-author Dr. Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston, says the findings reinforce the idea that obesity is a collective problem.

"People look around them and see people gaining weight and it might change their attitude about what constitutes an acceptable body size... You might say it's OK to be heavier," Christakis said.

Christakis and Fowler studied the records of 12,067 people living in the mostly white, middle-class town of Framingham, Ma. They had their health and habits regularly monitored, beginning in 1948. In 1971, children and spouses were invited to join the study, even if they had moved away.

Christakis and his team decided to look at the alternative contacts the patients had listed who could be called in case the patients themselves could not be reached for their appointments. In many cases, the alternative contacts were also participants in the study. So the researchers looked to see which friends and relatives were also in the study, and then looked to see who became obese and when.

They found that if someone became obese, their friends were 57 per cent more likely to become obese. If people named one another as contacts, they were 171 per cent, or more than double as likely to become obese if the other did.

It wasn't just direct friends affected: if a person became obese, their friends were more likely to become obese, as were the friends of friends.

"We found that one person's obesity actually influences other people in the network up to three degrees removed --- my friend's friend's friends," said Fowler.

"We were very surprised to see that the effect of one's obesity had such a wide impact on the network."

The findings were the same even if friends lived very far apart from one another. But on the other hand, having an obese neighbour did not affect a person's likelihood of becoming obese -- suggesting that common environments are not to blame.

"So what this is means this is NOT about friends eating together or exercising together; this is about ideas," said Fowler. "This is about the way we think about our bodies, about the way we think about healthy eating and healthy exercise behaviour and how we share those with people with whom we esteem."

"We speculate that what is really going on here is that people have conversations and people exchange ideas of what an appropriate body image is and what appropriate healthy behaviours are and that has either a conscious or unconscious effect."

Fowler notes they also found that not only is obesity "socially contagious," so is fitness.

"Thinness can spread from one person to another, so if you have a friend that becomes thin, that will also increase the likelihood that you will lose weight," says Fowler. "We are very excited by this particular finding because we think... we may have uncovered a tool to help us to reverse that trend."

Dr. David Macklin, the medical director of WeightCare, a group of weight-loss clinics in Toronto, agrees that friends and family can serve as a powerful influence to encourage one to slim down.

"We find that when people start eating better and losing weight, family members start eating better and losing weight. It's a trickle-down effect," Macklin tells CTV.

With a report from CTV medical correspondent Avis Favaro and producer Elizabeth St. Philip