Joan Smith: Can't be too thin? Oh yes you can

When Samantha Cameron was pictured looking sombre during a show at London Fashion Week, insiders hastily explained that it's not the done thing to reveal emotion when you're sitting in the front row. A more obvious explanation for SamCam's expression is that she couldn't believe her eyes as the Swedish model Chloe Memisevic glided down the catwalk, looking like a woman who is barely acquainted with the concept of a proper meal. Apparently Memisevic is hugely in demand by labels such as Erdem, and that's more important than that she looks malnourished.

Of course the Prime Minister's wife might also have had the disconcerting sensation of being caught in a time warp. In 2011, another crop of stick-thin models is parading on catwalks and once again a very thin woman is about to marry into the royal family. No one is suggesting that Kate Middleton has an eating disorder but I experienced a sharp intake of breath when I saw her wearing that blue Issa dress in her engagement photo. Issa's designer, Daniella Helayel, says the inspiration for her latest collection is Wallis Simpson, Duchess of Windsor, otherwise known as the woman who told one of the biggest untruths of the 20th century: a woman can't be too rich or too thin.

The royal family has painful memories to prove how dangerous that myth is. Lady Diana Spencer's weight dropped alarmingly as her wedding approached in 1981, signalling the beginning of what turned out to be a lengthy struggle with an eating disorder.

Diana started out as a chubby teenager, recorded by a photographer in an unguarded moment at the nursery where she worked. Her struggle with bulimia was an open secret for years before she decided to talk about it publicly, leading to an increase in diagnoses of bulimia and anorexia. After her campaign to raise awareness about the effect of eating disorders, it's hard to imagine the late Princess being anything but horrified by the cult of near-starvation on display at London Fashion Week.

Chloe Memisevic is not the only model whose extreme body shape has caused astonishment. Martyna Budna's jutting shoulders and visible ribs put me in mind of parts of the globe where the population faces a daily struggle to eat. I know that the currency of fashion is fantasy, but the industry has become so divorced from reality that it's promoting a look that millions of starving people in developing countries would give anything to avoid. I've seen children with distended bellies in West Africa, and adults suffering the effects of poor nutrition in childhood, and I'm not amused by the spectacle of adult women being encouraged to impose similar privations on themselves.

It's important, I think, to lay the blame for this decadent phenomenon at the right door. I don't blame aspiring models, who look at the stars of the catwalk and get the message that high fashion doesn't want healthy women; I flinch when I see photographs from this year's shows, and wish that someone would persuade those women to eat a three-course meal. We know that anorexia kills: the US-based National Association of Anorexia and Associated Diseases estimates that up to 24m people have an eating disorder and one fifth will die from complications.

Last year the short life of Isabelle Caro, the French model who became the face of a controversial anti-anorexia campaign, came to a tragic end when she died at the age of 28. At one point in her life, Caro weighed only 55lb, and images from the campaign still have enormous power to shock. Another model, Ana Carolina Reston, died at the age of 21, while Luisel Ramos was only a year older when she expired, having reportedly lived on lettuce leaves and diet drinks. If aspiring to become a top model puts young women at risk of a near-death experience, I think we can understand why Mrs Cameron looked so glum.

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