New diet fads constantly offer hope to the unhappily overweight, before fading away, leaving only disappointed expectations and stubborn flab.

The more extreme the eating plan, the more keenly it's adopted - until its followers realise that measuring portions with a thimble isn't sustainable in the long term.

But there's a new diet trend which claims dizzyingly high success rates, promises painless life-long commitment and allows dieters to eat anything they want.

Faith-based diets take the principles of Christianity and apply them to our overwhelming craving for chocolate, chips and cheese. Advocates say dieters learn to fill the spiritual hole inside themselves with something more powerful than saturated fats.

The basic principle common to the U.S. programmes Christian Weigh Down and Thin Within ('Helps you grow in faith while shrinking your waistline'), and the British equivalent Fit For Life Forever, is that dieters need to identify the deeper reasons why they over-eat, before they can hope to lose weight and keep it off permanently.

The trend began in America in the Eighties, but it's finally taking hold here, with Christian weight-loss groups springing up, and dramatically increased sales of 'spiritual dieting' books such as What Would Jesus Eat?, Hallelujah Diet and The God Diet.

The most famous remains The Weigh Down Diet, a programme launched in Tennessee in the late Eighties by Christian fundamentalist and nutritionist Gwen Shamblin.

Her unique method of cheerleading support, underpinned with the threat of God's displeasure, has encouraged hundreds of thousands of Americans to throw off their excess flab and praise the Lord.

There are more than 30,000 Weigh Down groups in America, and the concept has just crossed to Britain.

Shamblin is a perma tanned, frosted-blonde size 6, given to homey pronouncements such as: 'God created the wonderful flavours of blue cheese dressing, pepperoni pizza and chocolate brownies. He wants us to enjoy them - within His boundaries!'

She describes any desire to eat, apart from physical need, as 'head hunger', which, she insists, can be solved by an open heart rather than an open fridge.

Evangelical testimonials litter her website - 'Praise God for removing 300lb from my morbidly obese body!' cries Maggie Sorrells of Texas, as if God is some sort of celestial liposuction surgeon. Weigh Down doesn't advocate particular foods, but suggests that, once participants  learn to fill their spiritual hole by other means, they'll rejoice in being able to 'stop eating that candy bar halfway through'.

But Dr Elisabeth Weichselbaum, of the British Nutrition Foundation, is concerned by the lack of specific dietary guidance.

'If you follow this diet, you could end up eating nutritionally unsound foods,' she warns. 'Choosing meals that don't satiate your hunger means you could still consume too much energy.'

She is much more impressed by the What Would Jesus Eat? diet, whose author, Dr Don Colbert, has identified a whole new angle on the Mediterranean lifestyle.

Colbert explains: 'Jesus ate primarily natural foods in their natural states - lots of vegetables, especially beans and lentils. He would have eaten wheat bread, fruit, drunk a lot of water and also red wine. And he would only eat meat on special occasions, maybe once a month.'

'This diet takes a very sensible approach,' says Dr Weichselbaum. 'The staple food would be bread, and food from plant sources, with moderate amounts of proteins.

'This is exactly our advice - eat anything in moderation. It also recommends asking yourself: "Why am I eating this?" It makes you aware of what you consume and encourages you to think about it.'

Experts have less praise for the best-selling Hallelujah Diet, the work of Reverend George H. Malkmus, who advocates eating only plant-based foods, arguing 'the Lord gave us everything we need in the Garden of Eden: fruit, vegetables, nuts and seeds.'