Why Dieting will end up Making you Fatter than when you Started.

For years - decades, if you want to know the truth - I suffered from a fluctuating weight problem.

At first, the fluctuations were minor. In my teens, I veered from slim to slightly plump and back again. In my twenties, I went from slim to plump to podgy, and repeated the process. When I turned 30, I made a gargantuan effort to lose weight, and was slim for two years.

But then something happened; I lost control again, and my weight exploded. I was plump in no time, then frankly fat. It was horrible. And something, I knew, didn't quite make sense.

Of course, on one level, it made perfect sense. I was fat because I was greedy, or lazy, or both. To get technical, I was fat because I had disturbed my "calorie balance" - I had been consuming more calories than I'd been expending.

Conversely, when I lost weight, I had successfully reversed the process. But why, when I got fat again, was I always fatter than I'd been the time before? And why, when I got slimmer, was I never as slim as I'd been the time before?

In early 2003, I weighed about 240lb - at just over six feet tall, this made me close to obese.

I was, I could see, taking a similar trajectory, and at the same time, as Kirstie Alley, Sam Malone's love interest, Rebecca, in Cheers, who had been slim, and then plumpish, and was now gaining girth at a huge rate.

I wondered if there was any way back for Alley. There certainly didn't seem to be for other celebrities, such as Robbie Coltrane, Dawn French and Johnny Vegas. Were they stuck? Was I?

There comes a point when you lose heart - when a diet seems like the only answer. But could it be that dieting had made me fatter? Or, at the very least, that it had been a contributing factor?

According to research published by scientists from the University of California this week, the answer is: absolutely.

The Californian team analysed more than 30 studies of diets; one study found that a group of dieters ended up fatter than a control group who hadn't restricted their food at all.

One of the researchers, Dr Traci Mann, says: "You can initially lose 5-10 per cent of your weight on any number of diets. But after this honeymoon period, the weight comes back."

Over the years, I have tried just about every diet I could find. I started in the 1970s; low-carb diets had been popular, but low-fat was making a comeback. For a while, I trimmed fat off my steaks, and removed skin from chicken legs. I felt perky and trim.

Later, I did the F-Plan diet, which involves eating loads of fibre (it fills you up, but passes right through.)

Later still, I did the Hay diet, in which you avoid mixing carbohydrates and proteins.

Each diet seemed to be a trick to get you to consume fewer calories. And each diet worked - for a time. It felt great, to be fixing myself. And so quickly! But the overall problem was getting worse and worse.

There was a pattern - the process of getting fat was governed by a stronger force than the process of getting slim. Thinking about this, I remembered a book with the improbable title of Dieting Makes You Fat, written by Geoffrey Cannon and Hetty Einzig in 1983.

I'd flicked through this book in a bookshop during a slim period, and, for the 15 or 20 minutes I spent with the text, it seemed to make perfect sense. But it didn't stop me dieting.

When you begin a diet, you're never quite in your right mind. You feel fat, ashamed, desperate, and guilty. In this state of mind, punitive self-deprivation seems like a good remedy.

Years later, I read the book again, and again it made sense. "Dieting," said Cannon and Einzig, "creates the conditions it is meant to cure." When you diet, something funny happens to your metabolism - it gets better. Better, that is, at making you fat. To see why this should be the case, you have to think like a Darwinian.
Genetically, we are, to all intents and purposes, exactly the same as our Stone Age ancestors, who were threatened, above all, by starvation.

To survive, and reproduce, they had to have a metabolic system that would enable them to deal with periods of scarcity. And we, of course, are the same. Except we don't have periods of scarcity - we have diets.

What happens when the body is given less food than it needs? In the short term, it lives off its own reserves of fat. It gets thinner. But another mechanism comes into play: it also gets better at getting fat. When you diet, your mind wants to lose weight, but your body does not. When you diet, your body thinks you are unable to find food. You think: diet. Your body thinks: famine.

In the Stone Age, your fat-packing genes made you better at both survival and reproduction. Now, in this time of great abundance, they make you worse at both - more prone to heart attacks, and less attractive to the opposite sex.

And crucially, the more diets you go on - the more famines your body is exposed to, in other words - the better you become at getting fat.

What had happened to me, across the decades of dieting, was that I had developed a talent for getting fat.
Thinking about this in 2003, at my fattest, it all seemed logical. But still, I wasn't done with dieting. I decided to eat lots of protein and green vegetables, and cut down on carbs and sugar.

I thought I'd found the answer. I stopped eating pies, croissants, bagels, cakes and chocolates. Instead, I ate fish, chicken, steak and vegetables.

Did I lose weight? Sure. But then something weird happened. I started drinking heavily. Pretty soon, alcohol had replaced food as my big problem.

I thought I'd been unhappy because I was fat. It turned out that I'd got fat because I was unhappy. And now I was drunk for the same reason.

In the end, there are two important truths about diets. The first is that they can never work in the long term, which means that they can never work.

If you restrict calories, you'll just get better at getting fat. If you restrict unhealthy foods, and never eat them again, you might lose weight. But losing weight probably won't cure your real problems.

Just look at some people who have successfully lost weight: Lindsay Lohan, Nicole Richie, Victoria Beckham. Have they dealt with their real problems?

The other big truth about diets is this: they are hugely attractive because, on a diet, you don't have to confront your problems.

On one level, I love diets. I can't help myself. Neither can Kirstie. Just look at her now.
• 'The Hungry Years: Confessions of a Food Addict', by William Leith, is published by Bloomsbury, price £5.91.