Our children are surrounded by images of physical perfection, so it's little wonder that many of them develop eating disorders. But as Joanna Moorhead reports, the early signs of anorexia are often there to see - if you know what you're looking for

Afterwards, at supper, I mentioned her to my own two dance-school daughters. Who was that girl on the second row from the back? Did they know her? "Yes," they said. "Isn't she thin?" "A bit too thin," I ventured. "Well, being thin isn't bad, Mum," said my eldest, who's 13. "But I guess, now you mention it, she wouldn't want to be any thinner..."

With four daughters, I've always been aware of the dangers of eating disorders: two of mine are keen dancers, which was linked last year to an increased risk of developing anorexia nervosa. What's more, we do have women in the family who exhibit anorexic-like behaviour. I don't see any especially worrying traits in my two older girls, but when you've got daughters it clearly pays to be vigilant. This is the message that Dr Michael Strober, one of America's leading authorities on eating disorders, points up in his new book Just a Little Too Thin.

Strober's viewpoint is this: at puberty your daughter will get more aware of her body shape, and at precisely the same moment nature will intervene to flesh her out a bit. Some girls will respond by deciding to go on a diet to make themselves a bit thinner. And their diet might be OK: it might just be an attempt to shave off a few pounds of puppy fat and get into a new pair of drainpipe jeans. But for other girls, a diet will be the first of a series of events that eventually leads to a medical condition so serious that it kills more people than any other psychiatric disease. What's more, anorexia nervosa is notoriously difficult to identify and to treat, so being eagle-eyed about your daughter's behaviour really could pay dividends.

Equally crucial is that we as parents are aware of what Strober terms the "slippery slope" of adolescent dieters. At the top of the slope - stage one - is the "innocent but rigid" dieter. She's on a diet and sticks to it. She's starting to exercise, and she's proud of how healthy she now is. Stage two is the "exhilarated dieter": she's done well on the diet, and is getting lots of compliments. Maybe she can do better; so she starts counting calories more rigorously. When she eats, she can't wait to exercise it off. By stage three, our dieter is "obsessed and preoccupied": thinking about her eating strategy for the day as soon as she gets up. And all of a sudden the smell of food is a bit nauseating, and she wishes people would just leave her alone and maybe let her "eat" in her bedroom in peace.

Scary stuff - especially because, says Strober, one stage tends to merge into another and parents often fail to spot the transition. But what we have to hold on to is this: no matter where your daughter is on the slope, she can get off it, and with the right help, move back to firmer and healthier ground.

I say "she" because eating disorders, though they do affect some boys, remain overwhelmingly the province of females. It's difficult to be sure of the statistics, but probably around 1 per cent of women aged 15-30 are anorexic. And there are worrying indications that girls as young as six are now more aware and more anxious about the way they look than ever before.

So the big question is this: what can parents actually do? When I call Strober at his home in LA his advice is uncompromising - as a mother, he says, my role is crucial. As with most else in parenting, it's a question of "physician, heal thyself": there is no point at all in telling your adolescent daughter that fat is a non-issue while all the time moaning on about how you can't squeeze into your new trousers.

What's much more useful, says Strober, is to make very clear to your children what you value them for. It's not their body size, but nor should it be their straight A-grades either: girls who are prone to anorexia tend to be perfectionists and may have had pushy parents. What children need, says Strober, is a strong sense that they're valued because of everything they are: because of their kindness, their thoughtfulness, their sense of humour, their loyalty, their trustworthiness. "Feeling good about yourself doesn't come from looking good in a bikini - it comes from a sense of personal strength, resilience and pride."

What surprises me, though, is how much emphasis Strober puts on my husband's role. In many cases, he says, ideas about weight are passed on to daughters by their fathers. "It may be that fathers are very conscientious in stressing their own weight, and their own exercise regime," he says. "They have to be careful about that - maybe go to the gym when the children aren't around. But what's important, too, is that they're careful never to disparage others because of their weight - it's going to really make a difference to a girl if she hears her dad talking about other girls and women with an emphasis on how fat or thin they are. What you want is to give the message that a person's weight isn't particularly interesting, that it's not the thing about them you're really noticing."

One of the things I like best about Strober's book is that he's very realistic about the lives of teenage girls. Like me, he's quietly appalled by the tone of the magazines teenagers read today. But Strober knows our children are going to go on buying this unedifying nonsense whatever we do, so he suggests damage-limitation strategies rather than outright bans. So, he says, read the magazines with your children, and watch telly with them, too. And when you see some ridiculously thin or unbelievably beautiful model, laugh out loud. Introduce a bit of scepticism. "Say things like, 'those eyelashes are glued on for sure'. Or, 'you do realise that most of these photographs aren't entirely real? I've heard they can take inches off a model's body using computers.' Make it clear that a lot of what your adolescent is looking at is fake."

But if you can shore up your daughter's confidence by adopting good attitudes to body shape and counteracting media images, you still need to be on alert for the signs of an eating disorder. The current wisdom, says Strober, is that certain character traits will put her at far greater risk. "There's usually a tendency to extreme perfectionism, a difficulty with handling change, a rigid behaviour pattern. If you see that composite in your daughter, make a special effort to support her. Encourage her to do something different; encourage her, particularly, to take risks. Try not to take too much responsibility for her. Parents with anxious children often end up doing more and more for them, but the result is that their children become more and more helpless as a result."

One of the difficulties for parents today, I reckon, is the current debate over childhood obesity: it's hard to pretend no one gives a fig about weight when the media is crammed with features on how children are getting fatter, and what we should do about it. Strober agrees this is tricky territory. "If you have a child who is overweight, what you do is going to depend on whether the child herself is concerned. I'd say a sensible approach is getting her to eat a more balanced diet without scolding or ridiculing. Tread carefully."

'Just a Little Too Thin: How to Pull Your Child Back from the Brink of an Eating Disorder' by Michael Strober and Meg Schneider, (Lifelong Books, £14.99).

Food for thought: how to spot if your child has a problem

* Body dissatisfaction, particularly in girls who mature early ("I can't stand my hips. No one else has them!")

* Skipping meals ("I had a big snack at school")

* Weight loss to the very bottom of normal for her build and height. Slowly the weight drops into slightly below normal without worry or fear of health consequences ("I'm fine. I feel great!")

* Excessive exercising

* Preferring to eat in private

* A tendency towards perfectionism in everything she does ("I can't believe the teacher gave me an A-")

* Developing odd eating habits, such as cutting things into tiny pieces

* Eating only low-fat food, or suddenly deciding on vegetarianism

* Vomiting after a big meal ("Something must have been off" she might say if caught)

* Sudden bingeing ("See! I'm eating!").

* Episodes of overeating or undereating - bingeing and starving

* Studying food labels

* May feel uncomfortable both emotionally and physically after eating, due to anxiety over the calorific intake, as if one meal could catapult her into a size 18

* Begins to consider various diet plans and rigidly applies herself to them