Are you letting the side down if you stop being fat? Is it somehow "unfeminist" to concentrate, sometimes obsessively, on what you eat (or don't eat)? Is a preoccupation with weight loss equal to wearing a placard round your neck announcing that you are vain and shallow? I have to say, these are some of the most asinine and women-loathing questions I've heard in a long time, but they have been levelled at me recently, and at other women I know who have lost significant amounts of weight. Dieting is for dweebs, apparently - thick dweebs, if you please: stupid, misguided women who don't know any better.

Two years ago, I was a size 22. I was obese - morbidly, no less. It was absolutely horrible in every single respect. Having subsequently lost five stone, I'm really quite hard pushed to see how regaining control of your life, and not wishing your thighs to rub together when you walk, instantly turns you into a simpering air-head. One of the things about being fat - and I'm talking about being stones overweight, not about "needing" to shrink from a size six to a size two - is that, after a certain point, it makes you invisible. It's hard to understand how this might be considered any kind of achievement, feminist or otherwise.

You may occupy a great deal of physical space if you're very fat, but in everyday life, it's as though you weren't there. Sales assistants stare blankly through you. Men pretend you don't exist, or start calling you "mate". You wonder whether your children are embarrassed to be seen with you in public (the answer to that one is yes, probably). You wish you could go for a bike ride with them, but you're too self-conscious, because you look like a potato balanced on an ant. You can only buy clothes in specialist shops, and these clothes are as undesirable as you have started to feel. Your self-esteem - well, I was going to say "plummets", but it's hard to plummet when you've reached rock bottom.

You develop a whole fraudulent persona to go with your weight: you become a "jolly" fat person with a nifty line in self-deprecating jokes, expertly - and viciously - insulting yourself before anyone can insult you. And all of it feels suffocatingly miserable, every single day, but you squash it down and box it away, and try and live your life. Which you can't, properly, since you are miserable; and then the misery impacts on others close to you - partners, relatives, colleagues, friends, children. Still, you can always comfort yourself with the fact that you're une femme serieuse, not some weight-obsessed ninny. Well, whoop-de-do.

I've written a diet book explaining how I lost my five stone, and my co-author and I recently started a blog and forum to accompany it. The latter currently gets between 25,000 and 30,000 hits a week and is populated by incredibly articulate, intelligent, successful, funny women, most of whom have between two and six stone to lose. I check in a couple of times a day, and sometimes I sit at my keyboard reading people's comments and find myself welling up. The self-loathing, the sense of shame, the insecurity, the guilt, the issues around sex, the despair, the ugly, violent language women use about their own bodies, and the timid longing - "Can I really do it? Will it really work?" - to remedy all of the above by losing weight can be overwhelmingly affecting.

As I'm writing this, the members of our forum who are following our diet have collectively lost 905lbs, which is roughly two sumo wrestlers. And try as I might, I can't fathom out in what conceivable way they have done themselves any kind of disservice by confronting and finally addressing something that has blighted every aspect of their lives, sometimes for decades on end, sometimes from childhood.

So how come some people have been critical of me writing a diet book? Writing in this paper, under the headline, "You're Vain and Stupid", Zoe Williams posited - with specific reference to my book - that weight was "a funny old thing for an intelligent woman to be concerned with", adding: "The real mystery is how people get away with fixating on themselves like this without relinquishing their right to be taken seriously." And so on. this is a peculiar but not entirely unique view: many "intelligent", "feminist" women who have dropped significant amounts of weight frequently get the accusation of frivolity (and the rest) levelled at them by other women, as though having arms that look like whole hams were a feminist triumph, and as though despair were a state we should all aspire to.

It is clear that, by losing some of the weight, the women on our forum have regained some of their self-esteem and, by extension, regained their sense of self - which, like it or not, is for the majority of women tied to appearance as well as achievement. It's all very well bleating about how this is wrong, but actually I don't see that it's wrong at all. Why is it wrong to like what you see in the mirror, or to like your body? Why is it good to be pleased that you look like a pig? I believe unhappily fat women are doing themselves an injury - literally, in health terms, but also emotionally. And I don't think them wanting to stop injuring themselves is weird, or naff, or vain, or self-obsessed. I think it is triumphantly life-affirming. Heroic, too. I see the middle-aged woman on our forum, asking for tips about how to apply eyeliner because she's feeling better about herself, her trousers are looser, and she thought she might investigate the possibilities of make-up, and I cheer for her, just as I cheer for the woman whose husband puts her and her weight down every single day. One of these days, he's going to have to stop. One of these days, she and her new-found confidence aren't going to take it any more.

There exists a very bizarre, inverted kind of feminism (invoked by critics of dieting) that isn't about what you can achieve, but what you mustn't achieve. It's about not being things - not making any effort to improve yourself, not celebrating, or even noticing, what you look like and what your body can do. Its adherents write and speak as if being a woman consisted of being under constant siege from the male gaze (yeah, right - maybe one day, eh?), which rather misses the point that many of us dieters aren't particularly thinking about the male gaze. We are thinking about our own gaze, about what we want, and about what it does to our sense of ourselves to want things - weight loss, in this instance - and not to blame or punish ourselves for wanting them.

The women who habitually crucify other women in the name of sisterhood have become the enemy they prided themselves on having identified four decades ago. They gain strength of a vampiric kind from policing other women - policing their desires, their sense of self, their weaknesses, anxieties and insecurities - and love nothing more than to tell them how criminal and silly they are for caring about themselves, for - ew! - going on a diet. All I can say is, that's never been my kind of feminism, and those women are not my kind of women. The women on my blog, on the other hand, make me proud of my gender. They are real, human, fallible, kind to each other, supportive, and I admire them more than I can say.

What of other dieters - those joyless women picking at salad because they're three pounds overweight, "forgetting" to eat, more preoccupied than they might be with fitting into children's clothing? Can we despise them instead? No. Why should we? We can despise a society that fetishises eating disorders, and a male-dominated media that projects its screwed-up desires onto Ms 2.4, causing her to believe that she would be happier being skeletal, orange, plastic-chested, nail-and-hair extensioned, but where's the point in making fun of her? Wouldn't it be kinder to help create a world where she can be comfortable being her natural 14 size? How? Well, women not bullying other women might be a start. And it might also be a step forward for those "critics" of dieting to control the urge to judge and dismiss something that they clearly don't (or can't) understand. Because you do begin to wonder: whose side are they on?

· Neris and India's Idiot-Proof Diet by India Knight and Neris Thomas is published by Penguin Fig Tree, £14.99. The forum is at www.pig2twig.co.uk.