Fat friends 'pose obesity risk'

Having an obese friend dramatically increases the risk of becoming similarly fat, according to a study.

Obesity is "socially contagious", spreading from person to person in a social network, researchers said.

The study, published in the New England Journal of Medicine, found that if one person becomes obese, those closely connected to them have a greater chance of becoming obese themselves.

Surprisingly, the greatest effect was seen not among people sharing the same genes or household but among friends.

When a person becomes obese, the chances that a friend of theirs will also become obese increases by 57%, researchers from Harvard Medical School and the University of California found. Their siblings have a 40% increased risk, and their spouse a 37% increased risk. However a neighbour is not at any increased risk, unless they are also friends.

The researchers analysed data over 32 years for 12,067 adults who underwent repeated medical assessments as part of a heart study. They were able to map the interconnected social network using information that listed participants' family changes and contact information for their closest friends.

The map, which included the adults' body-mass index (BMI), revealed the whole network grew heavier over time, consistent with other studies that have found an obesity epidemic in the US.

But it also immediately showed distinct clusters of thin and heavy individuals which could not be solely attributed to the forming of ties among people of comparable weights.

Co-author James Fowler, an expert in social networks from the University of California, said: "This is about people's ideas about their bodies and their health. Consciously or unconsciously, people look to others when they are deciding how much to eat, how much to exercise and how much weight is too much.

"Social effects, I think, are much stronger than people before realised. There's been an intensive effort to find genes that are responsible for obesity and physical processes that are responsible for obesity and what our paper suggests is that you really should spend time looking at the social side of life as well."

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