Fat friends make you three times more likely to be obese

Doctors claim the company we keep has a huge effect on our weight.

Those with fat friends are almost three times more likely to become obese themselves, they say.

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A 32-year study of thousands of men and women showed that friends influence weight more than family - suggesting friendships are more important than blood ties in the battle of the bulge.

The researchers, who describe obesity as "socially contagious", found the friendship effect so strong that it applies even when friends are separated by hundreds of miles.

Likewise, thin people tend to have a network of slender friends. The researchers, from the Harvard Medical School, said the phenomenon could not simply be explained by a tendency to feel most comfortable in the company of those of a similar build.

Instead, it seems that we take our cues as to what is an acceptable weight from those we like and trust the most.

Researcher Dr Nicholas Christakis said: "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.

"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size.

"People come to think that it is okay to be bigger since those around them are bigger, and this sensibility spreads."

The study, published in the New England Journal of Medicine, tracked the weight and friendships of more than 12,000 adults. The analysis revealed that if a person's friend becomes obese, their own chances of piling on the pounds are almost trebled.

Even separation of hundreds of miles does not alter the influence of friendship, with those who live 500 miles away having as big an effect on weight as those in the next street.

Family and marital ties have a smaller effect, suggesting there is more at work than genes and lifestyle.

Having an overweight sibling raises the risk by 40 per cent, while an obese husband or wife carries a 37 per cent risk.

Dr James Fowler, who helped write up the study results, 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.

"When we help one person lose weight, we're not just helping one person, we're helping many.

"And that needs to be taken into account by policy analysts and also by politicians who are trying to decide what the best measures are for making society healthier.

"It's important to remember that we've not only shown that obesity is contagious but thinness is contagious."