Charities involved with eating disorders have called for tighter controls on the Internet after it emerged that popular social-networking sites such as MySpace and YouTube were being used to promote anorexia.

Pro-anorexia Web sites, on which girls exchange extreme dieting tips and view "thinspiration" videos featuring alarmingly thin women, have existed for some years. But they have always been difficult to find and the people posting on them have remained anonyomous.

Now pictures and footage of underweight teenagers are emerging on more mainstream sites, reaching a potential audience of tens of millions.

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On Facebook, some groups extol the virtues of anorexia as a lifestyle choice. MySpace's groups include one that has more than 1,000 members. Its rules state: "No people trying to recover. It ruins our motivation."

Other videos on YouTube, like this one, have come out against the trend, however. Here's one that's not so critical.

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Thousands of people have viewed film clips of emaciated looking teens and twentysomething women on YouTube which, along with the other networking sites, has rules against posting harmful content.

The two to 10-minute videos often feature the more slender celebrities such as Victoria Beckham and Kate Moss, neither of whom is anorexic. They also show images of underweight women in their underwear.

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