If your friends and family get fat, chances are you will too, researchers report in a startling new study that suggests obesity is socially contagious" and can spread easily from person to person.

The large, US-funded study found that to be true even if your loved ones live far away. Social ties seem to play a surprisingly strong role, even more than genes are known to do. "We were stunned to find that friends who are hundreds of miles away have just as much impact on a person's weight status as friends who are right next door," said co-author James Fowler of the University of California, San Diego. The study found a person's chances of becoming obese went up 57 per cent if a friend did, 40 per cent if a sibling did and 37 per cent if a spouse did. In the closest friendships, the risk almost tripled. On average, the researchers calculated, when an obese person gained 17 pounds, the corresponding friend put on an extra 5 pounds. Gender also had a strong influence.

In same-sex friendships, a person's obesity risk increased by 71 per cent if a friend gained weight. Between brothers, the risk was up by 44 per cent and 67 per cent between sisters. Researchers think it is more than just people with similar eating and exercise habits hanging out together. Instead, it may be that having relatives and friends who become obese changes one's idea of what is an acceptable weight. Despite their findings, the researchers said people should not sever their relationships.

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