It is January, as you probably already know, but unlike 82% (or thereabouts) of the adult population of the UK, I will not be starting the year with a diet. I will not be detoxing, or raw juicing, embracing a new exercise regime, or doing anything even vaguely weight loss-y. This is because I am already thin. How thin, exactly? Thin enough to worry my mother; thin enough to enjoy shopping for jeans (although annoyingly few stores stock down to a 24-in waist currently); I am thin enough to inspire Observer readers to send me thin-related hate mail on a regular basis (hi, fans!). I am definitely thin.

My story so far: four and a half years ago, I was slim. A size 10-going-on-12, nine-ish stone. I'd never dieted. I wanted to be a bit thinner, in an abstract sort of sense; I had a vague notion that it would be good to be half a stone or so lighter - doesn't everyone? But I never did anything about it. I thought that the constant background hum of dissatisfaction I felt about my weight came as part of the package of being a 21st-century bird.

Although officially, I pitied those who dieted. Officially, I 'knew' that being thin couldn't make a girl happy. But then, bingo! A freakish combination of grief, stress and heartbreak conspired to make me drop a shade under two stone over the course of four months. There's no diet in the world as effective as abject misery; and oh! I was miserable! During a routine shopping trip (an attempt to buy myself happy, naturally) I discovered that, in dress-size terms, I was much, much smaller than I had been pre-misery. A size 6/8 in fact. And furthermore, I liked it! Once the sadness subsided, I chose to maintain the thinness. I kept to the grief-enforced eating regime (of really Not Much), and I stayed skinny. Four years ago, I had the audacity to write about it, which was when the hate mail started. I was genuinely surprised to discover that thin is a very thorny issue indeed, and people choosing to endorse it are not admired - unlike people who are fat, because they're making some manner of brave statement; and it isn't 'I Like Cake'. Since then, thinness has escalated into something of an international row. It reached new heights last year, with the rampantly over-subscribed size 0 debate, (a weird time for me because suddenly, in relation to super-thin celebrities like Nicole Ritchie, I didn't seem that skinny after all).

Anyway. Four and half years into my tenure of being thin, I have substantial experience of it. I know stuff. I know, for example, that being thin is not the same as having a good body. I know a girl's arse can sag even when she's hardly got one. I know that problems arise when one looks 15 years younger from the back, than from the front - as the lecherous youngster who expressed disappointment in getting a close-up of my 35-year-old face, will testify. (Oh, how frantically he back-pedalled on all the chat-up lines he'd been directing toward the back of my head only moments earlier!)

But mostly, I am very familiar with the kinds of things that thin people say, and what they - we - really mean:

I just plain forgot to eat lunch today!

You're mad if you believe that. Mad. No one who isn't a) heartbroken or b) in the throes of a nervous breakdown, forgets to eat an entire meal. But, by surfing my hunger pangs with clever use of fragments of oatcake, and distracting my taste buds with vast amounts of Diet Coke, I just about managed to scrape by without lunch today.

You don't think I'm too thin, do you?

Please say yes! Please say yes!

I have a very fast metabolism

I metabolise at the same rate as everyone else; but I mainline black coffee so I'm pretty much speeding all the time.

I don't miss carbs

I miss everything about carbs! Everything! I miss white bread and the multiple forms in which potato comes (but especially roast). I miss crisps and pies and scones and sponge cake and fusili and sometimes - quite a lot, actually - I dream about rice.

I never diet

I never eat.

Beth Ditto's so cool

Beth Ditto's so fat

I eat whatever I like

It's just that I've retrained my palate to 'like' anything low fat, high GI, and carb-lite. And actually, sometimes, in restaurants, I'll order dishes I don't especially like, because I know I'll eat them slowly, and probably won't finish them. I do at least get drunk very quickly, so I'm not a complete loss socially. (I don't think.)

I'm completely stuffed!

The more psychotic edge has been taken off my hunger, because I've eaten a whole bit of sashimi and a Jaffa Cake. But completely stuffed? Ha! Not since the summer of 2002.

I wish I could be bigger. I'd love to feel all curvy, sexy and womanly

Ah ha ha ha ha haaa!

I am perfectly well-adjusted about food

I am bonkers.
Polly Vernon

Last year, I was 39, and a size 22. This is seriously big - huge, actually. Hippopotamoid. If you're short, you look like a ball of dough, with two titchy currants for eyes; if you're tall - I'm 5 ft10 - you look like a wrestler, or a drag queen gone to seed. This year, I'm 40 and I am a size 14 on top (unshiftable bosoms, the boredom) and a 12 below. I've lost five stone. I am unbelievably pleased with myself, though of course it's quite odd that a size 14 should strike me as the acme of perfection, size-wise, and strike other women as being about an inch away from monstrous heiferdom. But I can live with that. The reason I got so fat - well, one of them - in the first place is that I had, and continue to have, an absolute horror of those women who push a leaf around a plate and call it lunch, or who 'forget' to eat (yeah, right) or - worst of all - who use inappropriate adjectives to describe food. Deliberately tripping up someone is 'naughty'; stabbing your boyfriend in the head for a laugh is 'sinful'. Chocolate cake is neither.

But I've learnt that not wanting to turn into one of those women doesn't mean I have to go to the other extreme, and make some kind of weird, messed-up virtue out of having thirds. I've learnt a lot, in fact. Like: every seriously overweight woman who claims to be happy that way is either lying, or in denial. I was in denial for years about how fat I'd got - I genuinely believed I was just freakishly unphotogenic. When it struck my friend Neris and I - we've written a diet book together - that it might be an idea to illustrate said book with un-retouched shots of us wearing leotards, tights and no make-up, we were so deep in denial that we thought we looked OK - quite nice, in fact. A year on, those pictures - 2 x 16 stone poured into Lycra - strike me as terrifying. They would put off the most ardent chubby-chaser (that's another thing - imagine making yourself deliberately attractive to weirdos with a sexual interest in the morbidly obese).

Of course, it takes quite a lot of unravelling to get to the point when you know you are deluding yourself weightwise. What I said, or thought, as a fat person, bore absolutely no relation to reality. Thus:

I am weak with hunger

This actually means 'I feel very slightly peckish'. It can also mean 'I am bored', 'I am sad' or 'I am happy' - which is to say it means, 'the fridge is my friend'. It isn't.

The thing is, I don't actually look that fat

This means you have not looked in a full-length mirror for some time, and are a dab hand at avoiding your reflection in shop windows.

My size is a sign of my joie de vivre

This one is actually true up to a point - I'd rather be fat again than become a neurotic food bore, and I do think that flesh is sexier than bones. However: this may be true of a well-proportioned size 16, but there is nothing sexy or life-loving about having thighs that rub together, or giant bingo wings.

You can get nice clothes in large sizes nowadays

Again, up to a point. I mean, Boden goes up to a size 20. Personally, I'd rather not look like the lone freak Sloane who ate all the pies.

My husband/boyfriend loves me just as I am

Listen - just because he doesn't actually retch as you sink like a bollard onto the mattress doesn't mean he wouldn't prefer to get his arms round your waist.

This is all so shallow. It's about what's on the inside

It's not shallow - it really matters. Losing large amounts of weight can change your life. Besides, the insides aren't looking so pretty when you're carrying around the equivalent of half another person in excess weight.

We live in a tyrannical society when it comes to women and their looks

I agree. It's very wrong. I'm not suggesting we all shrink to nothing. I despise body fascism and I have no interest in being a size 2. All I'm saying is, if you're over a size 16, unhappy about it, unable to shop in normal stores, and bewildered as to where you go from here, there's something you can do about it. I did. And if I did, so can anyone.
India Knight

· Neris and India's Idiot-Proof Diet (Penguin, £14.99). To order a copy for £12.99 (free uk p&p) go to observer.co.uk/bookshop or call 0870 836 0885