JK Rowling is worried about her daughters. I can relate to that: I've got four of them myself, aged between four and 14 (Rowling's are 12 and one). I worry about mine all the time. What I can't relate to is the particular focus of concern that Rowling has chosen to share with us on her website. Because what she's saying is that she's worried about her daughters being thin. She's anxious about them growing up in a society that puts too much emphasis on size and shape. So much so, she suggests, that she'd prefer them to be too fat than too thin. "Is 'fat' really the worst thing a human being can be?" she asks.

Well, here's what I reckon. I reckon that Rowling wasn't a fat teenager herself. And I reckon that her older daughter (the baby is too young to be considered) isn't remotely fat herself.

I reckon these things because, when I was 10, I ballooned almost overnight from being quite a slim child into a very fat one. I now think a family trauma triggered my comfort eating - massive quantities of bread and jam after every meal. I stayed fat right through my teenage years; I was fat at secondary school, and I was fat (though slimming down) when I was at university. When I say fat, I do mean fat: there is a photograph in my parents' album of me aged around 13, with my baby brother on my knee. I am smiling, and he is smiling, but I am sitting with my feet tucked under my thighs, and it is my knees that dominate the picture. I have always hated looking at it: not only because it reminds me that I was so overweight, but because it reminds me of everything that went with being a fat girl growing up.

This was the 1970s: Susie Orbach, who has applauded Rowling's comments as ever so sensible, was writing Fat Is a Feminist Issue at exactly the same time. Alas, Orbach's words of wisdom about how it's the world that's out of kilter, not the fat people, never managed to filter through to my convent boarding school. It wasn't that we were bombarded with images of stick-thin models or that the skinny-obsessed culture had infiltrated our mindset: far from it, since the nuns banned us from watching TV and reading magazines. It was that everyone knew it was better to be slim than fat, and I was fat.

I wouldn't say being fat dominated my life, but it definitely wasn't the best backdrop to adolescence. At school I was always the worst person at gym and games: I remember how horrible I felt in the too-short skirts we wore for hockey, painfully aware of my thighs. And it wasn't just that I felt I looked horrible; it was that I was never able to do all that leaping over the horse in the gym. The only reason I ever got my basic gymnastics certificate was because nice Mrs Valentine, the gym teacher, took pity on me and said I'd done a neat forward roll - actually, I'd done an appallingly clumsy one.

Opportunities to go clothes shopping were few, but I do recall the occasional toe-curling trip to places such as Miss Selfridge with a group of friends. And I remember how self-conscious I felt: having to choose clothes so much bigger than everyone else's, having to strip off in the communal changing room. The other day a friend told me about a difficult and rather horrible day out with her 15-year-old daughter, buying a new dress for a special occasion. The daughter is 11 stone, as I was when I was 15. It is not a fun experience, shopping for a special outfit when you are 15 and 11 stone. I didn't like it, my friend's daughter didn't like it, and it's not something I'd wish on any of my daughters.

All four of my girls are slim: there's a slim gene on their father's side, and they all seem to have inherited it. Hurrah, say I. Of course I don't want anorexics for daughters - who would? - but I haven't seen anything to make me think there's any anorexia lurking. In fact, at a recent performance at their dance school, both my older girls, who are 14 and nearly 12, were shocked by the appearance of a fellow dancer who clearly does have an eating disorder. "Her shoulders were jutting out," said Rosie, my older girl. "It's not pretty, is it?"

Of course it isn't: and there are plenty of teenagers who are easily smart enough to recognise that. My older daughters, like most of my friends' daughters, watch far too much telly, by our evaluation, and spend far too long with their heads in dubious magazines that peddle exactly the sort of too-thin models that Rowling is so hot under the collar about. But that doesn't make my daughters want to be like those models, because it's what happens in their family that matters, not what they read in magazines or see on TV.

Ask any eating-disorder specialist, and they will tell you the same thing: what drives anorexia isn't so much growing up in a culture laden with pictures of rake-thin girls, but in an atmosphere of overambition where getting anything less than A grade after A grade counts as failure. Dr Michael Strober, author of Just a Little Too Thin: how to pull your child back from the brink of an eating disorder, says that the most important thing you can do to bombproof your child from disastrous thinness is simply to value her (or him) for the person she is, rather than for her achievements. Which is why the girl with anorexia is more likely to have had pushy parents than to have spent too long gazing at pictures of Paris Hilton.

Beyond that, it's parental attitudes to weight and exercise that are the other important influence on a growing girl and her attitude to body shape. Stephanie Pierson, co-author of You Have to Say I'm Pretty, You're My Mother, says that a mother's attitude towards her own body is crucial. What message are you giving your daughter if you've tried 10 diets in the last year, if you're forever checking the Weight Watchers calorie counter, and if you can bore for Britain about what you'd give to be a size 10?

Precisely because weight once loomed so big in my life, I've always been aware of how important it is to keep the messages subtle with my daughters. I've tried not to let food be a battleground: no child in my house has ever been required to eat everything on her plate, and despite my husband's protestations that one of them would surely die of undereating, none of them has. I never buy cakes or biscuits, although if anyone is desperate for a sugar rush they can walk down the road to the corner shop and buy some (my theory is that if they're in the cupboard they're just too available). I don't go on about needing cosmetic surgery in my daughters' earshot (or anywhere else, for that matter); I never go on a diet; and although I weigh myself a couple of times a week, I never talk about my weight with my daughters. And the truth is that, having been a size 16 when I was 18, I'm still rather thrilled to be a size 12 at 43. I don't have to pretend to be happy with my body because, in fact, I've never been happier.