I didn't mean to get so thin. I liked food, especially supermarket top-end ranges and expensive freebie work lunches. And I didn't have an eating disorder. Just the usual understated body image issues; that low-level discontent most women experience on watching the stars on a glossy Channel 4 American import go about their whip-thin, fragrant business. Aka Sex and the City syndrome.

But thin wasn't something I actively pursued. Thin happened to me. Firstly, there was some heightened life turmoil. A new job, a very ill mother, general unease about where I was going now I had hit my thirties. None of which made me inclined to eat. Secondly, I became 'accidentally Atkins'. I fell spontaneously out of love with carbohydrates - with the pasta and bread that used to make up most of my meals - and in love with protein. I discovered fish, properly, for the first time ever. Sea bass, tuna steak, sword fish, yes!, went a bit yoghurt-based for breakfast, carpaccio of beef and rocket for lunch, fancy wilted spinach salad for dinner. In six months, I lost nearly two stone. On a clothes shopping expedition, I realised with vague interest that I'd gone from being a size 10 going on 12, to a size six going on eight. This tipped me and my five-five frame into the realm of what's technically referred to as 'probably a bit too thin, lady'.

What I didn't realise was how massively Thin would impact on my life. Beyond the predictable scenarios (mother fretting, lover wondering out loud if I was 'bulmenic' [sic]), other things changed. Rather excitingly, I was embraced by a fast and glam super-thin super-class. I was summoned for a drink by a group of women I used to work with on Vogue, a place where thinliness is not just next to godliness, it rates way, way above it. They practically applauded when I entered the bar. 'How did you do it?' asked the people who eat nothing (give or take the occasional pistachio nut - which, apparently, are the thin person's preferred bar snack because the time it takes to open them, multiplied by the possibility that you might break a nail on the shell, means you won't consume nearly as many as you might if they were, say, cashews). 'Stress, grief, a bit of abject misery, accidental Atkins,' I told them. 'Wow,' they said. 'Brilliant.'

We spent the night drinking cinnamon Bellinis, the Diet Coke of the cocktail world. We waved away the canapès on offer. We didn't eat: caramelised red onion and soured cream blinis, mini portions of sausage and mash on big china spoons, California rolls, mini lemon mousses in shot glasses. Everyone hung on my every word because I was newly, truly thin, and therefore authoritative.

Not everyone was so impressed. Old friends started watching me when we lunched together. I could see them thinking: why doesn't she have any bread? And why is she only having a starter? That's the oldest trick in the book, that is! They stealth-ordered extra portions of fat chips for me, in the vain hope I wouldn't notice. I did.

Men, in general, do not approve. Builders accost me on the street, telling me I could be a 'good-looking bird, if I ate a bit more'. They wave their sandwiches at me, in case I am a bit hazy on what food looks like. When I was loudly declaring my love of dark choc Baci in my local Italian deli, I heard the man in the queue behind me mutter, 'Yeah, right.' In short: you get thin, you become public property.

Our culture is obsessed by thin and by The Thin, by the obesity versus anorexia debate, by our (women's, obviously) constant taboo pursuit of thinness. A celebrity who isn't incredibly thin isn't generally considered 'A-list'. It's not right, but there it is. Everybody wants to be thinner. Whatever they say. And now I am, and I love it. I lost weight by accident, but I could have put it all back on again by now if I'd wanted to. But I don't.

Contrary to popular belief, being thin has made me happy. I've spent probably 16 years wondering if I would look better if I were skinnier, and now I know for sure. I do. But then almost everyone does. I like the way clothes fit me. And I like having cheekbones (although I admit I can look a touch gaunt in harsh lighting). As one of the most astute women I know said to me recently: 'You've lost too much weight. God, I love it when people say that to me.'

And weirdly, I like food more, now that I am untroubled by Sex and the City syndrome. I certainly think about it a lot less. I don't waste time calculating that day's fat intake, totting up calories consumed absent-mindedly on the corner of a Post-it. I don't need to. I know it's not that much: that's sushi for you.

My friends have re-christened me Thinny Girl, and I love it. Thin is part of who I am now and I thoroughly intend to make the best of it. With no apologies.

· Mimi Spencer is on maternity leave.