You're having a Sunday morning fry-up with friends when you turn the pages of a newspaper and the headline screams: "Are your friends making you fat?" It's enough to make you drop your sausage into your mug of tea.

So you surreptitiously look around at your friends, slyly looking at the size of their tummies, and furtively checking out their bulk bent over their full Irish. Are they fat, you ask yourself, willing them silently and desperately to pull their bellies in and sit up straighter.

It's the scenario which afflicted New Yorkers on a Sunday morning recently when the New York Times Sunday Magazine ran a story on growing trends in obesity under the headline: "Are your friends making you fat?"

And there is a scientific study to back up the theory of friends being a potential cause of weight gain, as well as the anecdotal stories about people who like to eat a lot of junk food being more likely to hang out with others who share this preference instead of people who like to nibble on salad leaves.

In a research paper published in the New England Journal of Medicine, researchers at Harvard University and the University of California, San Diego, revealed that if a close friend becomes obese, your likelihood of also becoming obese soars by 171pc. Oh dear. And it's not only your close friends who are in danger of making you fatter. If a casual friend or acquaintance puts on weight, the risk of you also putting on weight rises by a significant 57pc. Bucking that trend is chef Nigella Lawson, who has kept a watchful eye over husband Charles Saatchi's huge wieght loss.

By comparison, having an obese sibling increases your risk of weight gain by 40pc, and an obese spouse ups your risk by 37pc. All these risk calculations were carried out over a 32-year period.

"These findings reinforce the idea that obesity is not just an individual problem, but a collective problem," said Nicholas Christakis, a professor of medical sociology at Harvard Medical School in Boston. Adding: "It's not that people who are overweight or obese seek each other out, but rather that friends are growing fatter together.

"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," he said.

This "obesity contagion" is not merely a matter of like-minded people befriending one another. But common sense suggests that people choose friends with kindred vices. So, for example, if you enjoy skulling eight pints down the pub on a Friday, you're not going to hang out with a teetotaler in the local cafe.

So who are the friends who are most likely to make you fatter?

The Comfort Friend:

You get dumped, you're broke, or you are fired, and she feeds you. You only have to look a little down in the dumps and she is roasting you chicken and potatoes, and insisting you have a second helping of pavlova. A sweetheart, she will listen to you ranting and raving for hours, while passing the box of chocolates back and forth, and refilling your wine glass. Great for your soul, she is however lethal for your waistline.

One solution is to ask her to go for a walk with you, and insist she doesn't bring any snacks along.

The Party Friend:

She is always up for a good time, and tempts you into the pub with promises of hot gossip, and her rebuke that you don't have enough fun in life, and need to get out more. Before you know it, you're eating a bag of chips after several glasses of wine after a long day in the office, and the pounds are beginning to creep up on you.

One solution is to arrange to meet her once a week, and stay strong when she texts you pleading for company because she's had such a rotten day, or needs a wing woman because there's a sexy man she fancies in the pub. It's called emotional manipulation.

The Foodie Friend:

She loves cookery books, and trying out new recipes, and needs someone to help her eat the food, to appreciate her efforts, to clear their plate and rave about the flavours and texture, and how much they loved the dish. That's you! And when you see your friend chowing down on something fattening, it gives you permission to do so, too. You're often quite happy to oblige, and a free meal is a free meal after all. Yet all this obliging is making you fatter.

One solution is to tell her that you are going through a salad phase and would really love to sample any new salads she feels like trying.

The Restaurant Friend:

Trying the latest and newest restaurants in town is her idea of heaven, and she's always dragging you along.

When in Rome you believe in behaving like a Roman, and will order indiscriminately from the menu, and then finish everything on your plate because you are, after all, paying for it.

And you never ever succeed in resisting the dessert menu because you have a sweet tooth, and often end up finishing her dessert too, which is so unfair because you are more prone to putting on weight that she is.

Why does she order dessert if she's not going to finish it?

One solution is to eat a banana on the way to the restaurant because this takes the edge off your hunger, and stops you ordering the most fattening items on the menu.

- Anna Coogan