Beth Porter plans to lose five stone in a year. Can she really win the battle of the bulge? We'll follow her every month, as she tests various regimes
Month One. I'm on a treadmill going nowhere. My T-shirt is soaked, and sweat stings my eyes. I've just pedalled for 20 minutes on an exercise bike. At this point all I can feel is a throbbing calf muscle threatening to secede from my leg. I'm here to fight that flab, to take it off and keep it off. I'm here to lose it.

It's Week Two of my latest get-thin mission. But this time I'm at a psychological crossroads. Having shed three stone about six years ago, I've been steadily gaining the weight back over the past three years. Now I'm so gross, either I take a step toward health or surrender forever to fat.

Losing weight is like murder. You need motive, means and opportunity. As an actress, I had to keep in shape or wave bye-bye to my career. Now I have to motivate myself. It came recently glancing in a mirror and seeing my once lovely, forever fat mother looking right back at me.

So: will I ever be thin again? I will if Chrissie Mullings-Lewis has anything to do with it. She is my personal trainer for a month. She leads me to the scale: "Tis a far, far better thing I do now..." I weigh 141/2 stone. And I'm five foot two inches.

I'm aerobicising in the exercise room of London's Jubilee Hall gym. The place is filled with toned torsos, taut tummies. I've opted for sweat pants and a large T-shirt to hide my wobbly bits. No skin-tight cycling shorts.

First time I tried the treadmill, Chrissie set the pace at 2.7. I could hardly huff and puff my way through 10 minutes. Today, it's at 3.5. And I'm striding.

Having abjured exercise for more than two years, my life now revolves around hour-long gym sessions every other day. Nearing the end of Month One, 40 minutes of aerobic pedalling and brisk treadmill walking actually feel invigorating. And the circuit training and series of stretches and sit-ups, for strength and muscle tone, are paying off.

First time on the bench press I pleaded after five repetitions, "I can't do any more." "Yes, you can," Chrissie said. And I did! Now I complete two sets of 15 reps with comparative ease. And on to conquer the quad extender and pec-deck. My reward is a sauna.

OK, what's for lunch? I consult the book Fit for Life, by the Californian gurus Harvey and Marilyn Diamond. The cover proclaims it "the internationally acclaimed weight-loss plan that proves it's not what you eat, but when and how!" Which is a slight fib; you're urged to give up caffeine and dairy products. And I don't notice any recipes for chocolate cake.

As "diets" don't work, the vegetarian Diamonds have devised one of those food-combining programmes, meant to change actual eating patterns. No calorie-counting or measuring: the key rule is avoiding any form of protein at the same time as carbohydrates.

Bye-bye Big Mac. I've spent a month savouring the "properly combined sandwich", variations of which all include wholemeal bread filled with enough leaves to qualify me as a silkworm. I've also gone through more fruit than Carmen Miranda. Luckily, the Diamonds encourage improvisation around their theme, and the book is full of helpful recipes.

A typical day begins with a huge glass of fruit juice. I process my own; grape, pear and mango. Lunch can be more fruit, a veggie sandwich, or home-made soup thick with fresh vegetables. You round off the day with a major mixed salad, punctuated by either a protein (chicken) or a carbohydrate (rice).

This stuff works because overall you consume fewer calories. Since 80 per cent of your intake is fruit and veg, to be blunt, you also spend a lot of time in the loo. It should be here noted that the Diamonds are both fascinated and repelled by physiology. Milk is demonised as "a major mucus producer" and much attention is paid to what is euphemistically called "elimination".

After a month, how do I feel? I feel healthy. I've worked out, I've eaten sensibly, I've resisted temptation. Friends comment on how well I look. My heart/lung capacity's increased from 55 per cent to 75 per cent. And the scale says it all. I've lost... 121/2 pounds. Yes, a lot of that is water. It's also a start. I celebrate - with a sauna instead of a sundae.

Next month, I continue. I'm going to combine Slimfast with a walking regime. Care to join me?

Chrissie Mullings-Lewis teaches classes and gives personal training at the Jubilee Hall, Covent Garden, London (0171-836 4007). 'Fit For Life', Bantam, pounds 5.99, from the Index Bookcentre, 28 Charlotte Street, London W1.