Obesity — it's who you know

If you're putting on weight, you might want to take a look at who you're hanging around with.

A study — published in the July 26, 2007, edition of the New England Journal of Medicine — suggests that obesity may be "socially contagious." The study found that your chances of becoming obese are much higher if someone you are closely connected to becomes obese.

And it may not be because you're spending evenings sitting on the couch with your spouse or kids munching potato chips, eyes glued to the tube. Blame your friends.

The researchers found that if someone you consider a friend becomes obese, your chances of crossing 30 on the old BMI scale go up by 57 per cent. Among mutual friends, the odds go up by 171 per cent.

If your spouse becomes obese, you have a 37 per cent chance of doing the same. Among siblings, odds increase by 40 per cent.

On the bright side, that neighbour who loves to mow his lawn and show off his expanding gut won't have any effect on your odds of becoming obese.

The researchers studied data from the famed Framingham Heart Study, which began collecting data from residents of the Boston suburb back in 1948. For this obesity study, the researchers analyzed 32 years of data for more than 12,000 adults. When they stuck the data on a map, they were able to identify clusters of thin and heavy individuals. They also found that the whole network of people was getting heavier over time.

One of the lead researchers, Nicholas Christakis of Harvard Medical School, told a media briefing that the clusters of heavy people could not be attributed to heavy people deciding to hang out with heavy people.

People who lived hundreds of kilometres apart but who considered themselves close friends had as much of an impact on each other's obesity as friends who lived close by.

Christakis said that as people get bigger, they come to think that it is okay to be bigger because those around them are bigger. The researchers found the social network effects extend to three degrees of separation — to your friends' friends' friends.

But there is a second side to this coin. The researchers found that thinness is contagious, too. Help one person lose weight and you're helping several.

It's a big reason why people who get together with other people to exercise tend to have better results than those going solo.

It's called motivation.