My diet invention - the Roberts Plan

I have the Evening Standard to thank for the fact that I have such a beautiful physique, one that - though I wear baggier swimming trunks than him - tends to remind people of those recent photos of Prince William.

For a decade or so, Londoner's Diary described me as "pinguid", which I had always thought meant "penguinlike" and was therefore meant rather affectionately. Six months ago a helpful friend, who'd gone to the bother of looking it up, told me that since it was coined in 1635 pinguid has in fact meant "fat, oily and greasy".

What's almost worse was that, according to one specialist etymological search engine, "pinguid" has only been used 16 times in the English-speaking world in the past eight years, nine of them by Londoner's Diary to describe me. Successive editors of the Diary have carefully passed the soubriquet "pinguid" on to each other as the only possible way to describe me, as though it were some valuable dynastic heirloom. I decided to deal with my pinguidity.

It became something of a quest, a healthy obsession that, to be frank, has led me to adopt one of the most bizarre diets I've ever come across. The Roberts Plan.

I've been on diets before - almost all of them. I've weighed nearly 13 stone since coming down from university in 1985, which at 5ft 6in counts as fat.

First came the slightly scary Rosemary Conley Plan, with rigorous checks on calorie content, which I pursued with vigour for three days until I realised I simply couldn't handle the daily calculations. Of course I've tried the Atkins (lots of fat, no carbohydrates) but I must confess after a week of meat I got rather bored with it and, besides, I hated being part of a massmarket phenomenon. Then there was the Fixx Diet, which demanded that I cut out all carbohydrates, though more worryingly it also involved a vast and compulsory amount of jogging to support the food plan. That one lasted around 48 hours.

Finally, I opted for a Harley Street clinic where a charming man gave me incredibly expensive blue pills. I think they're still gathering dust in a cupboard somewhere.

I'd been a member of the Sheraton Park Tower gym (great view), No 1 Cadogan Gardens gym (beautiful women), LA Fitness South Kensington (superb equipment), Nell Gwynne House gym in Sloane Avenue, and three other places. I joined, got bored, stopped going.

Although I enjoyed listening to CDs on the running machines, I resented the hours it took out of my life - both at the time and every afternoon when I had to sleep due to the exhaustion.

It helped when I started to go out with a gymaniac, biographer Leonie Frieda, whose tennis, running and workout schedules would make Royal Marines sergeants quail and who was recently told by her personal trainer that she had - unlike anyone he'd ever worked with before - a perfect physique with zero per cent body fat. But although Leonie told me what I mustn't eat - everything I loved - even she couldn't train my taste buds.

But then something clicked, something so ridiculously simple, so pathetically obvious, that I only wish I could copyright it. I started to check out the nutrition information on the back of foods in my local Knightsbridge supermarket, specifically the fat values.

That was it. I discovered that foods I thought were good for me were swimming in fat. Checking every label became something of an obsession. And that was when I fell hopelessly and headlong in love with - and this will start sounding like an ad, but it isn't - M¸ller Light yoghurts. With a fat content of 0.1g per 100g and fewer than 105 calories per pot, it was as if my new quest had unearthed the dieter's Holy Grail.

I should know; they're virtually all I've eaten for six months now, for breakfast and lunch when I'm at home, and maybe as a little snack in the afternoon, too. Those and Kallo Organic Slightly Salted Rice Cakes (2.8g fat per 100g), larded with Marmite (also only 0.1g per 100g) and steamed broccoli and mangetouts for all the other necessary vitamins.

The sweet taste of the Muller Lights contrasts delightfully with the Marmite and salted rice cakes, therefore I recommend you eat them in that order. As well as experiencing a taste sensation, I've lost over a stone in weight. And, bizarrely, I don't feel hungry.

I've not given up on everything, of course. I'm still a sucker for smoked salmon (without all of that bread I used to indulge in) and the odd glass of champagne, and I eat pretty normally in the evening (pasta, chicken and the like with less of the fatty addons), but the thing is I'm far more conscious about what I'm buying and eating. I don't take anything for granted any more.

Now, it's not to everyone's taste, I know, and I'm sure there's some nutritionist out there who'll tell me I'm destroying my intestines, or something, but I don't care. It works. And all it took was a ridiculously sensible approach, none of those overpaid diet gurus ordering my life according to a series of ever more complex rules. I actually enjoy my Roberts Plan Diet and haven't felt healthier in 20 years. In 1982 I returned from a three-month diving expedition in the Caribbean with an Adonic figure, which over-indulgence at university ruined. Now I feel I'm back on track to recapture it.

Of course business lunches and official dinners are a problem. As an author, I deliver about 30 or 40 speeches a year, usually after-dinner ones. The key is to lay off the extras, the things you don't really need or have to eat.

I get on the scales every morning - just like the fitness gurus advise you not to (but as the Roberts Plan proves, what do they know?) - and plot the relentlessly downward line in red biro on the graph I've hung on the wall. Sixteen pounds off so far. Inside the fridge is a photo taken of me on the diving expedition, with the command: "STOP!", in case I reach for the beer I keep for visitors. People mention how much better I'm looking, and I now know they're not saying it to be nice. Hallelujah!

Memo to Londoner's Diary: how about "formerly pinguid", "nolonger-pinguid", "the ex-pinguid" Andrew Roberts, or something along those lines?

Andrew Roberts's latest book, What Might Have Been, is published by Weidenfeld & Nicolson (£12.99). It is not a diet book.