How about this for a weight-loss tip: "Eat six small meals a day spaced about three hours apart. At 50 calories six times a day, that is only 300 calories. Your body will thank you, and so will your metabolism!" Sound stringent? It is. Remember, "you can never be too thin". Because, after all, "being thin and not eating are signs of true willpower and success".

You may not be aware that these "thinspirations", and hundreds more, are now within a click of your teenage daughter's computer mouse. They form the basis of a new trend: websites that espouse anorexia and other eating disorders as a "lifestyle choice". Collectively dubbed "weborexia", these sites (there are currently about 400 of them) are, according to groups such as America's National Association for Anorexia Nervosa and Associated Disorders (Anad), created and maintained by women under the age of 20. They offer a dreadful glimpse into the mindset of eating disorder sufferers. And they could, experts agree, trigger the disease in vulnerable visitors.

Each site certainly provides enough material to keep any parent awake at night. Using their own cosy terminology ("Ana" is anorexia, "Mia" bulimia), the creators offer tips on how to starve and binge and how to conceal this from family and friends. They provide mottoes, quotes, "commandments" and "triggers" and seem to see themselves as an underground movement united against persecution in a unique quest for "perfection".

The "triggers" are perhaps the most immediately disturbing thing about these sites. These consist mainly of galleries showing emaciated celebrities and models in glamorous poses, some doctored to make them look even more Belsen-like, others (perhaps more worrying still) untouched: Gisele Bundchen and Jodie Kidd are two of the most popular "thinspirations". One site provides a long list of super-thin female stars ranging from Jennifer Aniston and Julia Roberts to teen "icons" such as Buffy actress Sarah Michelle Gellar and Kirsten Dunst. The supposed height, weight and body-mass index of each woman is given - no weight exceeds about 120lb (54kg). Veracity isn't the point. The point is that "beauty" and "success" equals emaciation. It's a familiar media message in stark terms, made all the more worrying when you remember that it is aimed at teenage girls (according to the Eating Disorders Association those most at risk of developing anorexia are girls between 10-19 years of age).

The confused messages they give mirror the conflicts of the illness. One homepage begins, "Anorexia is a lifestyle, not a disease" before warning that: "Anorexia nervosa is a serious, potentially life-threatening eating disorder." Many sites say things like: "If you do not already have an eating disorder, turn back now. If you are in recovery, turn back now. Anorexia is a deadly disease. It is not to be taken lightly." Such warnings, of course, are about as effective as holding out a bone to a starving dog and telling it not to bite.

Inside these sites your quest for self-annihilation becomes wholly reasonable. You are taught how best to starve (chew, but don't swallow; eat only sugar-free jelly; take up smoking; drink gallons of Diet Coke; punch your stomach when it rumbles). And how to hide that "perfection" (wear baggy jeans with stretch pants underneath; put rolls of pennies in your waistband). If you're wavering, the slogans will keep you on track. After all: "Nothing tastes as good as being thin feels."

The "chatrooms" are equally distressing. Conversations are divided between discussions on the calorific value of semen (is it protein or sugar?) and outbursts of despair, self-hatred and suicidal longing ("I don't want to die, I refuse to die fat, but I've lost the will to live. What's the point? I don't think there really is one"). Visitors are often self-aware ("If you don't have a good head of hatred, you probably don't have an eating disorder") and have eloquent names ( "lardass", "fatass", "willbewaif4ever"). They exchange agonies - and often genuine support - in subgroups: "Ana", "cutting" (self-mutilation); fasting, diet pills, "Mia". The tragedy of all this is possibly summed up best by the case of one American girl who established a "weborexic" site, then died because of her eating disorder. Nobody knew the password to close the site down, so it remained open.

Of course, these sites have quite a clientele. In Britain, the Eating Disorders Association (EDA) estimates the number of people diagnosed and undiagnosed with an eating disorder to be about 1.15 million. According to Anad, there are about 8m eating disorders sufferers in the US. Vivian Hanson Meehan, Anad's president, confirms that these sites can indeed be deadly: "Receiving or providing tips to promote thinness reinforces [the anorexia sufferer's] own negative and chaotic thinking, enabling them to increase their own efforts towards 'perfection'," she says. Many sufferers are highly competitive, showing the desire to be the "best anorexic" (or bulimic) - the thinnest. "Behaviours to achieve thinness become a game to win at all costs over physicians, therapists and family."

But could such mottoes as "It is far better to be thin and dead than fat and living, because to be fat is a fate worse than death" push a "normal" teenager over the edge? According to one recent EDA survey, if your teenage daughter is like 75% of her female classmates, she will be unhappy with her body. If she's like 20% of them, she'll actually be on a diet. And it is not just teenage girls who are vulnerable; Anad says the number of eating disorders in the eight-to-11-year-old age range is increasing rapidly. And about 10% of eating disorder sufferers are male.

Dr Sarah Beglin, a Cambridgeshire clinical psychologist specialising in eating disorders, explains the risks: "It would be simplistic to believe that such websites are causal. Eating disorders have many causes - family, personality, socio-cultural factors, maybe even genetic ones." However "Triggers such as these are definitely terribly unhelpful for the average young woman with poor self-esteem who feels she has no control over her life."

There is, Beglin continues, a "sub-group of anorexics who have actively chosen to become anorexic. They start off dieting to lose a bit of weight and make themselves feel better. In the early stages they may get plenty of admiration for their control, restraint, for how slim they are. They then decide that by becoming anorexic they will be even more 'special'."

So what can be done? Thanks to campaigns from the likes of Anad, internet portals such as Yahoo and MSN now take down these websites wherever they appear. But - understandably - those creating them are adept at dissemblance. Most now use addresses along the lines of Totally in Control or Living on Oxygen to conceal their "pro-Ana" purpose.

In general, though, the message from the professionals is simple: if you find that your child is visiting such sites, take it seriously. "It is important to have an open conversation about it," says Beglin. "Simply banning your child from looking at these websites will probably not help." After all, she says, "A comment in the playground, or simply opening a magazine, can be enough of a trigger to a vulnerable person."