Can having obese friends increase your risk of packing on pounds? A new study suggests it can, if you're a man.
The study, published Wednesday in the New England Journal of Medicine, finds that the obesity epidemic can spread like a virus through social networks. When a person becomes obese, his friends and siblings are likely to gain weight as well.
"We were stunned to find that friends who live hundreds of miles away have just as much impact as friends who are next door," said James Fowler, one of the study's authors and an associate professor of political science at the University of California San Diego (UCSD).
He adds that this may be due to the fact that friends may subconsciously share ideas about what constitutes a healthy weight.
The research was based on more than 12,000 people taking part in the three-decade-long Framingham Heart Study.
At each update in that study, doctors monitored participants' height and weight and also recorded information about their neighbors, friends and spouses.
Using new software, Harvard and UCSD researchers created diagrams that plotted obesity and relationships, mapping the past 30 years.
While mapping networks of neurons in the brain or HIV prevalence among different communities has been popular in the past, this is the first time that obesity has been put under the lens of social networks.
The researchers found that when a person becomes obese, the chances that a friend will become obese increases by 57 percent. Siblings of obese people have a 40 percent increased risk of obesity, and their spouses' risk increased by 37 percent.
On average, having an obese friend made a person gain 17 pounds, which put many people over the body mass index (BMI) measure for obesity.
Female friendships did not seem to be impacted by obesity. But the chances that a man might gain weight from having a fat pal doubled for so-called mutual friends -- friends who both listed each other as buddies.
"There is an important implication here for a broadening perspective on treatment for obesity," said Dr. Nicholas Christakis, the study's lead author. "Attitudes are changing about what constitutes an acceptable body size, more so than a sharing of behaviors.
"We don't think that this is the only cause of obesity. This is adding one additional factor or explanation."
Still, the research could have weighty implications in a society in which adult obesity rates have shot up from 15 to 32 percent over the past three decades. Currently, around 66 percent of adults are considered overweight.
One of the questions raised by the study was why women's weight didn't seem to depend on having obese friends.
"There is a strong social bias for women towards thinness," said Dr. Robert Kushner, president of the American Board of Nutrition Physician Specialists.
"Social norms may trump social networks here. Guys don't have the same social pressure. Men may be more influenced by their friends."
Other diet experts agree that the inner workings of male friendships may have a lot to do with weight gain.
"Current social stigma against obesity is greater among women, and women jointly discuss weight and support each other in dieting and exercising," said Jeffery Sobal, a professor of nutritional science at Cornell University. "Men may engage in joint activities that increase weight, such as consuming more calories or spending time in sedentary activities."