Gwen Shamblin has no muscle mass. She's 5"4', 109lbs, and an American size 6 - but it's not down to slimming pills. It's certainly not down to exercise. It's down to God. Shamblin received her Master's Degree in Nutrition from the University of Tennessee but - more importantly - she has loved God since she was a girl. He is her nutritional foundation. "We are created with two empty, needing-to-be-fed holes in our body" she says. "One is the stomach, and one is the heart. Only God can bring us to a peace with food. Binge out on God." Shamblin's Weigh Down Diet is now the fastest-growing weight loss programme in the world. God is, it seems, moving in mysterious ways.

The idea of religious diets isn't new. After all, original sin was a form of gluttony. There's 'More Of Jesus, Less Of Me' and 'Help Lord - The Devil Wants Me Fat'. They are your basic low-fat diets with Bible Study supplements. There's the Hallelujah Diet, advocating raw fruit and vegetables like those found in the Garden of Eden. And Jehobics, a routine for Christians to exercise away unwanted pounds. But only the Weigh Down Diet lets God-fearing men and women eat what the hell they want and still lose weight. Maybe that's why it's proving so popular. Shamblin has built an empire of 24,000 classes in only six years. And now it's in more countries than Weightwatchers.

Shamblin had problems with her weight at college. She looked like "a potato with toothpick legs", and took to fastening her trousers with safety pins. One day she was in McDonalds with a thin friend. The friend left half her Big Mac because she was, well, full, and Shamblin set about studying the eating habits of thin people. "My highest goal to start with was to be a dietician in a hospital. Then I realised that food was being my God. My comfort. I knew God, but He wasn't everything. He was a portion. I enjoyed Him, but I loved His food more. Some friends in my counselling class said 'This message doesn't belong here. It belongs in church'. So I rethought everything."

The resulting Weigh Down Diet was common sense. Learn to distinguish false head hunger from true stomach hunger - eat only when you're physically hungry. Drink three ounces of orange juice to raise your blood sugar before you pick up your knife and fork. Take smaller bites. Eat your favourites first - that way you're more likely to abandon the rest when you're full. But the common sense was underpinned by an unshakable belief in God. He can help, if you're eating between meals and ruining your appetite. "Whenever people get tempted to snack" says Shamblin, "I say 'Chew on this' - and I hand them a Bible. Run to the Lord - not the refrigerator."

To begin with, the church elders objected. They didn't like Shamblin being on such close terms with their God. "Sure" she says, "some didn't like it - the people who were 'religious', but didn't have a real relationship with God. You have to know Him well enough to come up with the concept that God loves brownies. That was revolutionary to America. The God I know is the God who makes the chocolate and throws in the fat. He stirs in the sugar. Luckily, I'm female, and I'm non-threatening. Nothing about me looks religious, and I believe I got in through the back door of the churches because of that. Then they saw the fruit of Weigh Down, and they wanted me to stay."

The Diet turns thinness into a sign of spiritual vitality. It's already a social pressure to be svelte - now it's a spiritual pressure as well. But Shamblin denies that Weigh Down is judgemental. "There are no weigh-ins" says Shamblin. "There are no measurements. In the first week I say, don't judge the person that's overweight because you yourself may have a love of cigarettes, or a love of soap operas. We've all been in Egypt. [She compares dieting to the struggle to escape slavery in Egypt].We've all been enslaved. Religion is what you adore. Religion is what makes your pulse go up. Is it the buffet lunch at the restaurant? Or is it the Lord God Almighty?"

America's fat-acceptance lobby - the people who insist that they're both fat and fit - aren't convinced. But Shamblin doesn't seem to care. "When these people have tried everything - except God - and they can't manage to lose any weight, they give up. Then they start saying the rest of us have got to change. 'I'm going to force the stores to make bigger clothes. I'm going to force the airplanes to fit bigger seats. I'm going to make the world accept me, because I'm not going to change. I'm going to make my husband like me the way I am, and you'd better like me or else'. We're talking about some very intimidating people here. We've got to help one another to see the truth."

Shamblin was the cover-girl on Today's Christian Woman. She's written recipes for Exodus. Now she's more USA Today and the Washington Post. "I'm certainly happy with the reception we're getting on television. Current Affair [a tv news series] said, 'Move over Jane Fonda. Move over Richard Simmons. Make way for the new kid on the block'. I thought, 'Oh no. It's going to be me'. Then they showed a picture of this beautiful church. The new kid on the block was God Almighty! I loved that." Her most recent success was Larry King Live. It prompted 50,000 e-mails. Now Ruby Wax wants to film her. "We're still praying about that" says Shamblin.

"I feel like the media are actually behind the times" she says. "They are uneducated. They don't really have a finger on the pulse. New York and Hollywood didn't realise that Touched By An Angel [a tv drama series that emphasises the spiritual aspect of life] would be the number one show of all time. So they're scrambling to figure out what the people really want. New York's flying to Nashville, picking up all these authors. And they're finally realising that country music is popular. It's about down-to-earth, clean lives, and that's the basis of America. All the newspapers are suddenly doing a Religious Section. Their motives might be mercenary, but may Christ be preached."

Weigh Down is Shamblin's way of serving His house. But it's a profit-making organisation, and at $103 (£65) for the 12-week course, it's serving Her house quite nicely as well. "God has lead me to not live like a pauper" she says. "Or take the vow of poverty. He has lead me to red meat, sour cream, and brownies. I said 'God, in the 1990s, righteousness and having things don't go together'. Nothing about me looked righteous. I didn't home school my kids. I didn't have all my hair cut off. I wore make-up. I was worried. But He was saying 'Look, not only am I filthy rich, I live in a big house'. I've told people before - God probably wears designer clothes." And drives a Lexus. Just like hers.

She drives me to J Alexanders, a dark-lit restaurant next to Taco Bell, for some lunch. Shamblin is wearing red. He decided she should wear it. "Well" she says, "it happens every morning. I don't have an alarm clock. God wakes me to see his sunrise. I go 'That's a good one, God'. I look outside the window - I say 'You put that grass colour with that grey and cream sky. You're the genius behind colour. You pull together my outfit.' I dress for him." Today He's gone for a red two-piece with contrasting gold necklace. It's smart but casual - just right for a lunch. It's no surprise to find that He also chooses what she eats. Steak, fries on the side, and the chocolate cake to follow.

God will take away the sins of the earth - and Shamblin's desire to eat the second half of that steak dinner. All she has to do is ask. The Weigh Down Diet is about building that sort of personal relationship with God. It's about feeling happy to trouble him with the details. It takes the mundane problem of weight, and turns it into an opportunity for spiritual growth. Eat only when you know you're really hungry - it's called 'the growl' - and stop when you're full. Therein lies the problem. Most of us can't stop. "Leave the table and go into another room and pray for God to remove the desire to eat another bite" she says. "Give Him a few minutes, and He will answer the prayer."

Her steak looks good. Better than my salad. "God did not put sour cream, or blue-cheese dressing down on earth to torture us" says Shamblin, with the enthusiasm of the cheerleader she once was. "He put them on earth for our enjoyment." Shamblin eats whatever she wants, whenever she wants, as long as it's "between the green line of hunger and the red line of fullness". She doesn't do Lite - although she does wash everything down with Diet Coke. Shamblin picks at her steak. It's not joyous to watch. It's certainly not sensuous. She only eats about one-tenth of what's one her plate, and gets the remains 'to go'. This is food as fuel.

We drive back to the Weigh Down headquarters, on a concrete business park outside Nashville. It has the look of a crematorium. This is where Holland Baker, Weigh Down's outreach director, and her 34 phone operatives answer up to 2,000 calls a day. "Inevitably each call goes through to the right operative" says Baker. "That's down to the hand of God. If a call is going to take ten minutes or less, it's for Outreach. More than that, it's for Counselling. A couple of people in Counselling do have degrees, but we prefer mature Christians with life experience. Then we pray together down the phone line."

Shipping is next to Outreach. It's where the fridge-magnets, the audio cassettes and the Weigh Down t-shirts (up to a XXXL) are freighted out around the world. Distribution Manager Dave Taylor used to work for McDonalds. But then he discovered the Weigh Down Diet. He lost 50lbs, hung up his stretch pants once and for all, and came to work for Shamblin. He's gone from managing stock in a two-car garage in Cookville, to this - a 40,000 sq ft warehouse. With his own team of co-workers in matching blue polo shirts. It's the tail-end of the New Year's resolutions, and Weigh Down's busiest time of the year. But Taylor is likely to get busier.

PART TWO

The Otter Creek Church of Christ is a warm, welcoming place. Okay, so we are covered in the blood of Jesus, but the pews are still padded, and the hymns are nicely projected onto the wall. Shamblin worships here whenever she can. We sing four hymns, back-to-back, then it's off to nightclass. Churches in these parts are doing their best to look relevant, and offer Divorce Recovery on Monday, Faithbuilders on a Tuesday, and Weigh Down Workshop on a Wednesday.The hands-on approach has invigorated organised religion, and brought new people through the front door. The hope is that they sign up for Bible study before they leave.

Tonight's Weigh Down testimonials are as enthusiastic as any SlimFast commercial. Debbie Collier used to take slimming pills. She smoked heavily. Now she's saved her marriage and lost 7lbs. "But I've lost a total of 100lbs off my evil heart". Dave and Joan Campbell have lost 50lb between them. This is the fifth time they have done the class. They don't need to lose any more weight - they just like the Bible study, and the sense of community. Now they bring their daughter, Alison. "Before Weigh Down, Rollaids [ant-acid tablets] were just like candy to me" she says. "This has brought me back to God. It's an amazing way to spread the gospel."

Dani Waites was uncomfortable with the whole idea to begin with. "I didn't like calling on Him personally. It seemed too charismatic, too Holy Roller. I wasn't raised to go to Him, no matter how small. But I'd tried everything. I remember watching these overweight people in an infomercial. They were on a cruise. He was real excited. 'Do you want to lose weight?' he asked her. 'Yes' she said. 'Do you want to eat more food?'. 'Yes' she said. 'Do you want to eat more often?'. 'Yes' she said. It was about eating more often to speed up your metabolism. But like the rest of us, I just needed to eat less." She found Weigh Down, and lost 33lbs. Now she runs her own Workshop.

The class starts by sharing, and externalising. "I'm having trouble distinguishing 'full' and 'satisfied'" says one workshopper. Only the Lord can really help you distinguish. The Lord can do anything. To illustrate the point, a young girl stands up. She really wanted to hear My Girl. Then it came on the radio. "Then I was watching Designer's World. I thought 'Wouldn't it be cool if it was the one where all they went to Japan?' And it was." Dani smiles. It just shows God's attentiveness. Then we watch one of Shamblin's videos. Tonight she is in Egypt. Literally. For seven days she filmed her own version of Exodus, using the pyramids as a backdrop. Now she's ready to free Britain from slavery.

The first British Weigh Down Workshop opened two years ago. In Ilford. There are now over 30 classes in the UK. One participant, Carol, was initially reluctant to leave food on her plate. She found it easier when she decided that the extra food belonged to Jesus. The British Weigh Down market, however small, does exist, but they better understand the language of salvation in America. They aren't frightened by talk of the Holy Spirit. And there are Christian radio and tv stations in every city, aggressively preaching the gospel. In Britain, we worship very differently. If the Weigh Down Diet - like I Prayed Myself Thin - isn't going to end up in the remainder bin, Shamblin needs to understand that.