WASHINGTON – Obesity can spread from one person to another like the flu or a fad, researchers reported Wednesday in a first-of-its-kind study that helps explain – and could help fight – one of the nation’s biggest public health problems.
The study, involving more than 12,000 people tracked over 32 years, found that “social networks” play a surprisingly powerful role in determining an individual’s chances of gaining weight, transmitting an increased risk of becoming obese from wives to husbands, from brothers to brothers and from friends to friends.
The researchers found that when one spouse became obese, the other was 37 percent more likely to do so in the next two to four years, compared with other couples. If a man became obese, his brother’s risk rose by 40 percent.
The risk rose even more sharply among friends – between 57 and 171 percent, depending on whether they considered each other mutual friends. Moreover, friends affected friends’ risk even when they lived far apart, and the influence cascaded through three degrees of separation before petering out, the researchers found.
“It’s almost a cliche to speak of the obesity epidemic as being an epidemic. But we wanted to see if it really did spread from person to person like a fashion or a germ,” said Nicholas Christakis of Harvard Medical School, who led the study being published today in the New England Journal of Medicine. “And the answer is, ‘Yes, it does.’ We are finding evidence for a kind of social contagion.”
Christakis stressed that the researchers are not saying that obesity is literally caused by a virus or some other pathogen, or that factors such as poor diet, lack of exercise or a biological propensity are unimportant. Rather, the findings suggest that once a person becomes obese for whatever reason, it may make it more socially acceptable for people close to him or her to gain weight, and that new social norms can proliferate quickly.
“What spreads is an idea. As people around you gain weight, your attitudes about what constitutes an acceptable body size changes, and you might follow suit and emulate that body size,” Christakis said. “It may cross some kind of threshold, and you can see an epidemic take off. Once it starts, it’s hard to stop it. It can spread like wildfire.”
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