Occasionally, when I need to remind myself of what I have done, I pick up my four-year-old son. Daniel is a big sturdy lad for his age, and weighs around three stone. I can carry him for about five minutes before my arms give up, a little longer if I put him on my shoulders. Which is interesting because, until six months ago, I was carrying those three stone, plus another four pounds, all the time. That is what I have shed since the day, last September, when I walked into the newly opened gym just two minutes walk from my south London home and signed up for membership.

For a year or so before that moment I had been travelling the world, working on a book about the globalisation of luxury restaurants and my search for the perfect meal therein. In Moscow I had eaten carp, while its siblings swam about beneath me under a glass floor, and been served venison by a bunch of Cossacks at Boris Yeltsin's favourite country inn. I had eaten Gordon Ramsay's food in Dubai and New York, tasted Robuchon in Las Vegas, London and Tokyo and been treated to sushi in the smallest high-end restaurant in the world. It was laid for just one person: me.

To finish the book I decided to test the luxury-restaurant experience to destruction by doing the high-end Super Size Me. Where Morgan Spurlock ate in McDonald's every day for a month, I would eat in a Parisian Michelin three-star every day for a week. If I was invited to take the tasting menu I would have to say yes. Seven days; 21 Michelin stars. Like Spurlock, before and after I had a medical.

And so, the day after my return from Paris - my 41st birthday - I found out exactly what I weighed. At the time I was so horrified I could not bring myself to put the number in the book. Now, having done the work, I can. It was 132 kgs or - in language you understand - a shade under 21 stone. Cue sharp intakes of breath.

The problem was I had become used to my size. I have never been thin. There is a picture of me taken shortly before my sixth or seventh birthday party, all toothy grin and flowing cravat, and looking at it I can see my weight was probably about normal. But I don't recall feeling normal, even then. I did not come from a family of normal people. Normal people were thin and we were certainly not that.

I would like to attribute this to something hidden in the genes: a couch potato of a metabolism specifically engineered to cope with the harsh winters of eastern Europe whence we had come, by storing as fat any of the scarce calories that came our way. As various members of my immediate family have struggled with their weight over the years there might indeed be something in this, but the truth is far simpler. We overate. Arguably my mother [Claire Rayner] put too much food on the table at times, but she was pushing at an open door. In my family, when she lifted the serving spoon we all lifted our plates.

I recall dieting for the first time when I was about eight. At 12 I went on an egg diet which made my breath stink. Only when I was 16, and my waist had topped 44 inches, did I achieve any measure of success, though not with a carefully designed programme. I simply ate less. But of course the weight crept up again and, over the years, I would have to intervene with my body much as the United Nations has had to intervene in chronic, intractable civil wars.

Then, nine years ago, I became a restaurant critic. You don't need a degree in physical fitness and nutrition to recognise this as a very bad idea. It was like putting a smack addict in charge of the medicine cabinet. I spoke to one of my predecessors who said he had put on two stone in three years, and attributed it all to the desserts. I knew another restaurant critic who did it for a dozen years and put on almost eight stone. I comforted myself with the thought that these were previously thin people who, like curious virgins unacquainted with the clap, had no idea of the consequences of the world into which they were diving. I was different. I was already promiscuous at the fridge. I knew what eating could do, understood the mathematics of calories, and was determined not to let the bitter calculus of food get the better of me.

My solution was exercise. For a while I belonged to a gym, but I didn't have enough time to get there. So I cancelled my membership and bought a huge Nordic Cross Trainer, a great chromium thing of handles and paddles with a digital screen that pulsed out flashing digits in diodes the colour of blood. I positioned it in my office at home behind my desk, as an encouragement. I asked my accountant if I could claim the substantial cost against tax.

'I'm afraid not,' he said. 'The truth is you don't need to be thin to do your job.'

'Perhaps not, but I do need to be alive.'

My accountant laughed. 'The Inland Revenue does not care whether you live or die.'

My cross-trainer work was admirable but, in truth, the two or three workouts I managed a week served only to stabilise my weight. On my 41st birthday I realised that something more drastic was necessary. It seems someone was listening for, two days later, a fancy new gymnasium opened just two minutes' walk from my house, at Brockwell Lido, a lovely old Thirties red-brick open-air swimming pool which had just finished a massive refit at a cost of £3 million. Most London gyms are cramped, sweaty, low-ceilinged boxes. This was bright and airy, with a calming view of the rippling waters through the windows.

I did my usual workout - 40 minutes on a cross trainer, until I was an ugly mess of sweat, snot and hair - and loved it. So I went back the next day. And the next. And the day after that. It seemed that, as well as stabilising my weight, those two workouts a week had gifted me something else. I may have been grossly overweight - read obese - but I was fit. In short I discovered that, if I could find the time, I could work out five or six times a week without crumbling. Bit by bit the regime expanded, from the massive thrash on the cross trainer, to include running and eventually weights. I lift things. I push things. I pull things. I wear a headband and look like a twat, but I don't care. If you can't look a twat in the gym where can you?

On top of that came a change in diet. I stripped out most of the carbs - potatoes, pasta, rice, bread; oh, how I miss good sourdough with a smear of salted butter - leaving just enough to give me the energy needed to do the workouts. (A breakfast of All-Bran and yoghurt since you asked.) When I go out to review restaurants I eat normally; after all, I still have the day job to think about.

This took an enormous amount of time but I decided to regard it as a job of work. As far as I could see, if I went out on the promotional trail to sell a book called The Man Who Ate the World looking like I had literally eaten a fair slab of Kansas, nobody would want to buy it. So now the writing work stops at 3pm and the workout begins. Keeping up with the regime has been tough and I don't think I would have managed it without the staff of the gym who became my cheerleaders. Three weeks after joining I was stopped by the manager, a wry, sharp Australian woman called Sophia who said simply, 'We're watching you,' and strode away. Two weeks later she stopped me again to announce I had a nickname among the staff. 'We call you the candleman.' Why? 'You're melting.'

It was true. When I started my waist was about 43 inches, my 42-inch jeans leaving a deep red welt about me. Within a month I was buying new jeans in a 40-inch waist. By Christmas they were being replaced by 38s. In February I started buying 36s. I have recently been considering the 34s, a size I have never worn in my adult life. Numerically my weight will always be high. I suspect getting below 16 stone is an impossibility. It's the way I'm engineered. Still, I have become aware of muscles I didn't know I had, of shapes I had never seen before. I discovered that I didn't actually have a massive arse but, relative to the rest of me, quite a small one. My wife who, bless her, has made a point of never commenting on my weight unless she could sense that I had become concerned, made no secret of the fact that she was pleased. After 20 years together almost any change can be a positive, and this one certainly has been.

And yet, while I am proud of what I have done, I have not completely vanquished my appalling body image. This is something men are not supposed to talk about, unless it is to explain how they obtained ripped abs in just six weeks. The notion that we can fret over the sag of our stomach or the immovability of our love handles or a thigh full of cellulite as much as any woman is not discussed. Certainly I didn't want to discuss it; writing about my own body was not my idea and I had to be convinced to do so. (And don't even get me started on the warped psychodrama of being photographed.) A lot of what I see in the mirror intrigues and pleases me, but some of it I still find appalling. I don't think that will ever change, however much I lose.

For, while I have filleted my wardrobe of three sackfuls of clothes that no longer fit, I am haunted by a ghost. That ghost is me, from six months ago, the one the clothes belong to. It is said that inside every fat man is a thin man trying to get out. Well inside this thinner man, is that appalling fat man, casually studying his fingernails until his moment comes again. I may have worked out six times a week for the past six months. I may have denied myself all manner of lovely things to eat, and run myself to within five years of a double knee replacement but the job is not done. The truth is the job will never be done. So please excuse me. I'd love tell you more, but I have an appointment at the gym. And I can't afford to miss it.

· The Man Who Ate the World is published by Headline at £16.99. To order a copy for £15.99 with free UK p&p go to observer.co.uk/bookshop or call 0870 836 0885