This is the VOA Special English Health Report.

When one person gains weight, their close friends often follow. Researchers have just offered evidence in a study that says obesity appears to spread through social ties. But the findings might also offer hope.

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If friends help make obesity acceptable, then they might also be influential in losing the fat. The researchers note that support groups are already an effective tool in dealing with other socially influenced problems, like alcoholism.

The findings appeared in the New England Journal of Medicine. The researchers used information collected from twelve thousand people. It was collected between nineteen seventy-one and two thousand three as part of the Framingham Heart Study.

The information was highly detailed. There was even contact information for close friends of the people in the study.

The researchers examined more than forty thousand social ties. They found that a person's chances of becoming severely overweight increased by fifty-seven percent if a friend had become obese.

A sister or brother of a person who became obese had a forty percent increased chance of becoming obese. The risk for a wife or husband was a little less than that.

Nicholas Christakis of Harvard Medical School was a lead investigator in the study. He says there is a direct causal relationship between a person getting fat and being followed in weight gain by a friend.

The study found that the sex of the friends was also an influence. In same-sex friendships, a person had a seventy-one percent increased risk of becoming obese. Men had a forty-four percent increased risk of becoming obese after weight gain in a brother. In sisters, it was sixty-seven percent.

The researchers also considered the effect of where people lived in relation to each other. James Fowler of the University of California, San Diego, was the other lead investigator. He says a friend who lives a few hundred kilometers away has as much influence as one in the same neighborhood. He says the study demonstrates the need to consider that a major part of people's health is tied to their social connections.

Both investigators say their research shows that obesity is not just a private medical issue, but a public health problem.

And that's the VOA Special English Health Report. For more health news, go to voaspecialenglish.com. I'm Steve Ember.