To avoid putting on weight, make sure your friends don't.
A new study reported in this week's New England Journal of Medicine found that obesity tends to spread among friends and family, which could go a long way toward explaining the national epidemic of extra pounds. For example, if a friend of yours becomes obese, the risk that you too will become obese in the next two to four years increases by 57%, the study found. The siblings of that friend have a 40% greater chance of becoming obese, and the spouse, 37%.
The risk among friends triples with best buddies: If one of the two becomes obese, the chances that the other will follow suit becomes 171%.
Framingham Heart Study
The good news? Those same percentages hold true on the reverse side. If a friend loses weight, your chances of losing weight increase by 57%. And a new friend who is already obese does not boost the other person's odds of gaining weight—only those friends who become obese over time have a substantial impact on their social circles. Also, neighbors who put on weight do not have a negative effect unless they are also friends.
The researchers behind the study, Nicholas Christakis of Harvard Medical School and James Fowler of the University of California in San Diego, say the findings were unexpected and they cannot explain with certainty why obesity spreads through social networks. The cause appears to be that the norm for acceptable body image changes. "If the people around you gain weight, your idea of what might be considered overweight changes," says Christakis.
The researchers based their study on extremely detailed data collected in the Framingham Heart Study, a famous public initiative started in 1948 in Framingham, Mass., that has been tracking the health and lifestyle habits of more than 12,000 people across two generations. The researchers, who examined those records going back to 1971, say they "lucked out" because the Framingham study asked participants to list close friends, many of whom were also in the study. They tracked the body-mass index (BMI) of 12,067 people from the study and observed a total of 38,611 social and family ties.
Three Degrees of Separation
The researchers also looked closely at the influence of gender, smoking, socioeconomic status, and geographic distance. After excluding those factors, they found that social connections were a key risk factor for obesity as measured by BMI, which is calculated by dividing a person's weight by her height squared (and, for pounds and inches, multiplying the result by 703). A BMI of 30 or above is considered obese, while above 25 is considered overweight.
Fowler and Christakis discovered that the closer two people are in a social network, the stronger the obesity effect. And geographic proximity wasn't necessary. If a person lived 500 miles away and gained weight, his or her friends were still at risk. The risk can extend to three degrees of separation—the friend of a friend of a friend of someone who becomes obese is also at greater risk of becoming obese, though to a lesser degree than the direct correlation among friends.
"One person's obesity can influence numerous others to whom he or she is connected, both directly and indirectly," says Christakis. "It is not that obese or non-obese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship," as a friend puts on weight.
Matthew Gilman, director of the obesity prevention program at Harvard Medical School, says the study may help to explain the explosion in obesity in the U.S., often referred to as an epidemic. Over the last 25 years the incidence of obesity among U.S. adults has shot up from 15% to 32%, and two-thirds of all U.S. adults are now considered overweight. "Many people think genes are the cause of obesity," says Gilman. "But genes really can't explain the obesity epidemic. This paper is about the genesis of that epidemic."
Clues to Treatment
The study also suggests, says Christakis, that obesity treatments may be more effective if people are treated in groups rather than individually. "There should be a broadening of perspective on treatment."
By the way, the researchers also stressed that people should not start dumping friends who become overweight. They note that numerous studies have pointed out the health benefits of strong social networks, which hopefully will outweigh the heightened risk of putting on extra pounds.