The more obese friends you have, the more likely you are to become obese, a new study suggests. This confirms previous research that gaining weight may be socially contagious.
The research also shows that if nothing changes significantly in the environment and culture in the USA, about 42% of adults will be obese in about 40 years and then the obesity rate will level off.
About a third of Americans are obese - that is, roughly 30 or more pounds over a healthy weight. Those extra pounds increase their risk of developing type 2 diabetes, heart disease and many types of cancer.
After decades of skyrocketing growth in obesity, some experts have suggested that the rate may be leveling off. But Harvard researchers, in an effort to come up with a best-case scenario for the obesity epidemic, came to the 42% projection by applying an infectious-disease mathematical model to data from the Framingham Heart Study.
Their findings are reported online in the journal PLoS Computational Biology.
"We find that having four obese friends doubled people's chance of becoming obese compared to people with no obese friends," says Alison Hill, the study's lead author and a Harvard researcher.
About the rising incidence of obesity, she says, people have gotten better at gaining weight in recent years, but not at losing it.
David Rand, a Harvard research scientist who also worked on the study, says that the more obese people you have contact with, the more likely you are to become obese.
Researchers aren't sure why this is true. It may be that if you have a lot of friends with unhealthy eating habits, you wind up with similar eating habits, Rand says.
Of course, other lifestyle factors, such as diet and physical activity, have a huge impact on weight gain, Rand says, but the lesson of this research is "it's in your best interest to help your friends lose weight."
This latest research confirms a 2007 study by Harvard researchers and others that showed that one person's obesity can significantly increase the chance that his or her friends, siblings and spouse also will become heavy, suggesting that weight gain does spread through social networks.