Fergie does it. Helen Mirren does it. Koo Stark does it. Food combining - the 'healthy eating system' based on the principle that starches and proteins don't mix - is the diet a la mode. Even though it means, shockingly, no bread and cheese, no muesli and milk, no pasta and pesto.

The guru with two books in the bestseller lists (The Food Combining Diet and Food Combining In 30 Days) - despite the outrage of the nutrition establishment - is 43-year-old Kathryn Marsden. The diet, which works on the principle that if you digest food more efficiently you will lose weight, has clearly worked for her. Almost too well. A shade under 6ft, she weighs just 9 1/2 st and her velour tracksuit hangs from all the sharp points. On the other hand, as a one-time sufferer from acne, eczema and a severe hormone imbalance, which resulted in a hysterectomy at the age of 24, her eyes are now luminous and her skin clear.

Ms Marsden is no zealot. 'Nothing is banned,' she says, ordering scones (buttered) and coffee (caffeinated) with cream. 'Strict? Oh heavens, no. People always imagine I live on lettuce and lentils. I eat the same as everybody else. I just combine it more carefully.'

Common sense appears to be her favourite subject. 'Don't you think we've become thoroughly obsessed about what we put in our mouths?' she muses, with no apparent irony. 'Food is a social activity, apart from just being fuel for the body. We get together with our friends and our families. We don't want some social bore sitting there saying I can't eat this and I can't eat that.'

Ms Marsden even confesses to a weakness for banana fritters. 'They are the most taboo combination you could think of. You've got milk and eggs as protein, you've got flour as starch and you've got fruit in the banana and you stick them all together.'

The one thing that gets her goat is the hostility of the medical establishment. 'Food combining?' says Alison Black, a dietitian at the Dunn Nutrition Unit in Cambridge. 'In a word, rubbish. The human body is perfectly adapted to digesting protein and starch at the same time. In any case, all foodstuffs - except highly refined ones such as sugar and oil - contain both starch and protein. Bread, for example, contains between 7 and 9 per cent protein. Every diet is a gimmick to make people eat less.'

Professor John Garrow, of the St Bartholomew's Hospital department of human nutrition, believes food combining is worse than a gimmick. 'Kathryn Marsden's book is an absolute disgrace,' he says. 'Obesity is a major health problem. But the more people talk nonsense, the more difficult it is to help people rationally. To digest food better cannot make you thinner - quite the reverse, in fact.'

'Although the orthodox people love to say there's no scientific proof,' counters Ms Marsden, 'there's actually masses. It's just that they haven't bothered to look.' Where is it? 'Loads in America,' she says, vaguely. 'I'm actually working at the moment on something myself. The early results look extremely promising.'

The problem with Ms Marsden's theory is this: food combining suggests that a diet of chips for breakfast, fried chicken for lunch and chocolate for supper would help one to lose weight. But her books are full of instructions to eat more greens, less fat and less sugar - the standard low-calorie diet.

Ms Marsden was originally not a diet guru but a hotel manager who became a specialist in the treatment of digestive disorders after her husband was diagnosed with stomach cancer in 1984. As she explains at the start of The Food Combining Diet: 'He underwent two major operations, one for the complete removal of his stomach and a second which divorced him from his spleen. Kept alive by a drip for about five weeks, he was discharged into my care with the words that I should not be too hopeful of any recovery.'

Thanks, she believes, to food combining and a regime of vitamin and mineral supplements, Ralph, now 61, has a good appetite, excellent muscle tone and a fresh head of hair. He has also been cured of

the diabetes that he developed in middle age.

Food combining seems to me not as pernicious as many diets; it simply provides a psychological technique for refusing to eat things that are fattening. And it provides an alternative to the Weight Watchers' principle of weighing every little ounce of food. Faced with a plate of steak and no chips, diners will vary the monotony with greens. Instead of filling up on bread, they have no option but to take a second helping of salad. No cakes. No fish in batter. And no banana fritters.

Kathryn Marsden is not the original food combining 'expert': the Beverly Hills diet preached the principle of food combining with pineapple on the side. Ms Marsden herself claims to be following Stone Age hunter gatherers (wild hog today, berries tomorrow); an early Jewish sect; and a Victorian doctor called William Howard Hay, who believed that mixing proteins and starches led to indigestion. His disciples Doris Grant and Jean Joice also have a book in the Top 10, but since they are, respectively, 89 and sixty- something, the future of food combining is in the hands of Kathryn Marsden.

'If you've suffered for years with bloating or indigestion or three stone extra that you don't want to carry around, it's worth giving it a try,' she says. 'We're not saying it works for everybody, but it certainly seems to for the vast majority.'

So if some people can face a future of baked potato without grated cheese, fish without chips and roast beef without roast potatoes, where's the harm? Ms Marsden claims not to be making a mint from others' gullibility.

'While it's hugely enjoyable,' she says, 'from a financial point of view it's a waste of space. But my husband and I love each other very much, and he nearly died. The fact that I've got him back, from the jaws of death, is reward enough for me.'

Meanwhile, she eats only one bite of her scone, complaining it's too sweet.

(Photograph omitted)