Modern women spurn the word "diet". But that doesn't mean they don't cling to the idea that weight is something that could - and perhaps should - be controlled. In place of diets are "discipline", "healthy eating", "low-fat regimes" and punishing sessions at the gym. Such women see nothing strange in the fact that, over just a few months, their body weight might vary by as much as a stone. For women in the public eye, these fluctuations are plain for all to see: witness the yo-yoing forms of Kate Winslet, Kirstie Alley and Oprah Winfrey, as well as Dahl.
I know how they feel. I have a thin self and a fat self. My fat self is warm, cuddly and loveable, happily shovelling in apple crumble and roast potatoes, surrounded by a Ready Brek fuzz of contentment. My thin self is spiky and uptight, chain-smoking and more concerned with what others think of me than how I feel about myself.
Happily eating one day, the next heralds doom as my trousers warn I have gone too far. Waistbands are surreptitiously unbuttoned and I have to cover my bottom before leaving the house. "I love your (big) bottom," insists my boyfriend. Wise man. I hurry to the gym. I go twice in one week and feel incredibly virtuous. (Hopefully, I then put my back out so I can't return for at least a month). I eat salad like it's going out of fashion for a week and gaze longingly at Mars Bars when buying my morning paper. I drink black coffee instead of eating pudding, and have carrots for elevenses. But every single moment I think of food. The more I resist, the more I crave. The minute I stop this punishing regime I eat all the food I've been dreaming of, and put the pounds straight back on.
My most painful realisation was that there is rarely a meal that I don't feel guilty about - yet I love my food. I have set up a system of denial in my mind and the object of desire - food - is forbidden. So until I conquer the guilt, the yo-yo spins. Diets, we all know, just don't work - but once we start, it's very hard to stop. And it's where the trouble begins.
How many women like myself have surreptitiously scanned the many column inches heralding Xenical, the new anti-fat drug now coming to Britain? Yes, we know it's meant not for us but for the seriously obese. But which yo-yo girl, immersed in her own appetite-skewing logic, has not dreamed of being able to down unlimited amounts of fatty foods without having to bear the consequences?
At the age of 10 I would watch my mother weigh out her breakfast cereal to the nearest ounce. At dinner time, during her diets, she would count her every potato. Only on Saturdays were my sisters and I allowed sweets. At 12, with lunch money in my pocket for the first time, I would spend every penny on chocolate in the corner shop before I hit the school playground - and then starve through the lunch hour. I piled on pounds of puppy fat. At 15 I would read fashion magazines and gaze at skinny models.
So at 21 and finally, oh joyous day, underweight, when I would smoke 20 cigarettes and stand wistfully in front of my empty fridge, there was a very great element of control involved. If I was thin, which surely was meant to be, I would also be very happy. So I would eat seldom and very slowly. I would drink five cans of Diet Coke a day, filling up with fizz to prevent hunger. At meal-times I pushed my food around my plate until it became a scrappy mess and friends would scold me for being picky. But crucially, I could wear hot-pants to a club and not worry about my cellulite. Never mind that I was under-nourished, I was skinny. At 24, I rewarded myself for being so thin by eating all the food I had deprived myself of in the last three years. I comforted myself with food, I punished myself with food. I am a woman, I am emotional and therefore food, like sex, is intrinsically linked to my emotional well-being. A regime was born and as the yo-yo bounced, my self-esteem went with it.
Why do we equate happiness with being slim? The latest television icon is the positively twig-like Ally McBeal. At the Emmy awards last week Callista Flockhart, who plays Ally, was photographed in shimmering white. With scraped-back hair and big round eyes she looked ill - not supermodel, but super-alien. She is so thin we are scared she will snap, and she is not typical - yet she obviously feels that her her new cult status demands this. We are trapped in a vicious circle of contradictory images and it takes a strong woman to know her own mind.
Lisa Colles, author of Fat: Exploding the Myths (Carlton pounds 6.99), written to accompany the current ITV series Fat, points out that "whilst the Nineties have the thinnest beauty ideal ever and there is huge pressure to conform, we also live in a world where food is ever more accessible and people do less and less exercise."
Only five per cent of the female population are predisposed to have the current model shape, and models today are 23 per cent thinner than the average woman. So how dare the press carp about actress Kate Winslet's fluctuating weight? It must be the gratification of seeing the filmstar as real woman too, imperfect and not infallible. Rather than celebrate her size, we scorn her for her upwardly mobile dress-size at the premiere of her latest film - whilst secretly starving ourselves. Although not exactly Titanic (note that the size of increasingly large male star Leonardo di Caprio goes totally ignored), Kate was then reported to have put herself on a Brussels sprout diet. Thank god the girl saw sense - last week showed Kate in full swimsuited glory, romping in the waves with her boyfriend, voluptuous (a much nicer word than thin) and obviously happy. The key, perhaps, to this yo-yo dieting - happiness does not come from a dress size, but from ourselves.
Step forward Mia Tyler. Little but larger sister of Liv, at 19 she is making a fortune from out-size modelling. She talks of how her model mother sent her to weight-loss camp at the tender age of 12. "I lost 30lbs in six weeks... and put it all back on again... pigging out and eating ice-cream just to spite everybody." Mia yo-yoed for years until "I'd tried soup diets, bingeing, diet pills... then I realised I had to be myself". We pray she retains her integrity. Sex kitten Drew Barrymore has recently gained a few stone, and claims she has never felt happier - the question is, do we really believe her? For us up and downers, fatter naturally equals more defiant.
The downfall of the yo-yo is that what goes up must come down. However happy we are when at the upper end of our scales, something will still snap inside us. As Lisa Colles says, "until we embrace a broader ideal of beauty, women continue to aspire to unrealistic goals."
Dr Jane Ogden, senior lecturer in health psychology at King's College, London, warns that we must accept that dieting is counter-productive. "Undoubtedly our norms of aesthetic beauty are being diverted by the media. Celebrities are under pressure to maintain their image as it is constantly commented on, and as film-stars, models and even news readers look increasingly similar there is no range for 'normal' women to judge themselves against." Dr Ogden emphasises that the trick for yo-yo dieters is to realise that "you're not the only one who doesn't look like Kate Moss".
In the bestseller Bridget Jones's Diary, Bridget changes weight daily, obsessively recording it. At one point she loses half a stone and goes to a party, feeling fantastic and full of herself (if not of food). "You look absolutely terrible," comments a male friend. When my best friend told me the same thing during my thinnest period, I put it down to jealousy, because I couldn't believe that I wasn't now at my most stunning. Yo-yo dieters are also very likely to think their men prefer them thinner, but Dr Ogden quashes this theory. "Women are far more critical of themselves than their male partners are - we project our own beliefs onto the men. And remember, only one per cent of dieters maintain their weight loss."
My mother eventually gave up dieting. She simply realised that it wasn't worth constantly hating everyone else in the whole world who wasn't dieting. The minute she abandoned the diet, food no longer held the same connotations and it was no longer an emotional issue. She took up art instead and lost the weight by lugging her easel around. Personally I banned scales from my house when I gave up being very thin. What you don't know doesn't hurt you. And anyway, how can we possibly be happy when we're counting every calorie? Get off the treadmill and have a cream cake on me.