Dr Gillian McKeith, food guru extraordinaire and presenter of the Channel 4 series You Are What You Eat, is not the kind of person who ever has a sudden urge for a Mars bar. 'Never! she cries. Her favoured treat is a carob fudge brownie delight. You can find the recipe for this delicacy on page 225 of the You Are What You Eat Cookbook. The basic ingredients are: dates, raisins, flax seeds and carob powder. You whizz everything together in a food processor, spread the mixture over a large baking tray, then freeze it for an hour. Mmm, delicious. But where's the fudge? 'It's amazing,' she says, looking at my face, which is puckered with displeasure. 'You'd never know it's not chocolate, yet it's raw. Wonderful!' So what's for dinner tonight? 'A black-eyed bean stew, and an amazing miso soup with kohlrabi and pak choi.' Hmm. In this country, kohlrabi is usually fed to cattle.

The good doctor is chomping her way steadily through a plate of crunchy salad, and all the while reading my food diary, a list of everything I have eaten in the last week. While her idea of a treat is generally brown and chewy-looking, mine is more along the lines of a nice pile of fresh seafood with maybe a few chips and a little mayonnaise, followed by - ooh, let's see - a slice of lemon tart, or a gooey wedge of brie. 'Let me look at your tongue!' she says. She peers at it, while I worry whether my breath smells of garlic. Apparently, it is quivering unhelpfully. Worse, there is a line on it that suggests I have something of a problem with the booze. 'When I look at your tongue, I can see that your organs are stressed. I can see the stagnation that is on your liver. All this wine you're drinking is making it worse. I would want to work on your liver, and I would want to stick a good colonic tube up your backside. How are your bowels? Are you bloated? I'd imagine you would be farting after all this.'

McKeith giggles. Perhaps she thinks I find a little wind embarrassing. But the truth is, I don't. Everyone does it - particularly, I would be willing to bet, after one of her aduki bean concoctions (McKeith is just mad about aduki beans; according to her, they remove 'unwanted mucus, congestion and stools'). In any case, for all that my diet is replete with the things she despises - butter, eggs, meat, bread - I have a hunch that it is doing me far more good than her own. In person, McKeith, who is in her mid-forties, looks oddly emaciated. She is a famously tiny woman, but even so, there is something papery about her skin; when she shakes my hands, I feel bones wrapped in parchment. She would no doubt laugh out loud at the suggestion that she is absolutely famished but, beyond all the wild enthusiasm and the preaching and the 'positive energy', this is precisely what she radiates. She looks hungry.

But back to the diary. 'I could clear up the bloating, gas and flatulence,' she says. 'I could also improve your energy levels. You seem stressed! All this wine you are drinking! I would recommend milk thistle for the liver, and nettle tea.' Nettle tea is Dr Gillian's number one herbal brew (though it's a close-run thing with dandelion). It is, she believes, loaded with iron, which makes it an excellent 'blood-builder' and, although it is not a laxative, three cups a day can 'really get your bowels going'. The fact that it tastes as bitter as a witch's tit is neither here nor there. Does she never long for a crisp glass of sauvignon blanc? 'Maybe on New Year's Eve. But I would only drink a quarter of the glass.' McKeith gazes at me, her blue eyes brimming with sympathy. Obviously, I am in denial. Later, she turns to me and says: 'I wish we could be friends, Rachel. Then I could help you with all your problems.' As a sales technique, this is surprisingly effective. But it is also, I think, just a little bit sinister.

Dr Gillian McKeith burst on to our television screens two years ago, when Channel 4 began screening a series, made by the independent company Celador, called You Are What You Eat. Even by the uniquely peculiar standards of TV makeover shows, it makes for extraordinary viewing. McKeith, a Mrs Pepperpot-sized woman with a strange accent (it's Perth by way of Beverly Hills), travels the land in search of fatties and, once inside their homes, plays a kind of nutritional David to their Goliaths, wildly berating them for their grotesque diets. Having raided their fridges and examined their stools (she is, she cheerfully admits, obsessed by 'poo - even as a child, I would always look'), she then sticks them on one of her regimes, which tend not to involve the kind of foodstuffs you can pick up down at Budgens: quinoa, seaweed, miso, millet and lots of aduki beans. It goes without saying that, when McKeith revisits these fatties some weeks later, they are invariably thinner, happier and more full of energy. Gillian's tempeh with kale: 2. White bread and butter: 0.

Like most of those who make a living advising people on their diet, McKeith thinks we eat far too much sugar, salt and fat and not nearly enough fruit and vegetables. However, weight loss for its own sake does not interest her. Rather than urging people to go on 'fad' diets, McKeith would like to see a wholesale revolution in the kind of food we eat. Broadly speaking, her philosophy is that certain foods have extraordinary and hitherto unrealised properties - that they can literally heal (in the foreword to the book of the TV series, she tells the story of a woman whose terminal cancer was apparently cured by a macrobiotic diet). She is keen on raw vegetables - aka 'living' vegetables - and 'superfoods' such as blue-green algae and alfalfa (the latter, which she calls the 'father of all foods', can 'enrich the liver' and 'purify the blood'). If all this sounds outlandish, well, a lot of people seem to be willing to buy into it. According to her publisher, Penguin, You Are What You Eat - ' the plan that will change your life' - has sold more than a million copies.

Has she been surprised by her success? 'I wasn't surprised. I knew the country was ready. My results [with patients at her clinic in north London] are so fantastic - I'm sorry to sound immodest, but it's true - I wanted to take that to the masses. This woman - me - who is completely, brutally, totally honest. People like that. I'm real.' But aren't her methods unnecessarily complicated? The cases she encounters on You Are What You Eat are so extreme - in one programme, a young man would get up in the middle of the night in order to make himself, from scratch, a shepherd's pie of football pitch-proportions - that simply telling them to eat less and move more would probably suffice. Suggesting that they drink miso soup for breakfast rather than, say, a bowl of Special K is likely to have them dashing straight for the eggs and bacon once the cameras stop rolling. 'To make miso soup, all you have to do is open a packet,' she says (I love miso, but not while I'm listening to the Today programme). 'So "no" is the answer to that question.'

As you may have noticed, McKeith has a kind of messianic zeal for her work, and a sense of self-belief that will see her through pretty much anything, including the scepticism of other people and, in particular, their attacks on her somewhat - how shall I put this? - sketchy CV (to which we shall return). She spent years trying to get on to our television screens, eventually landing a slot on Richard and Judy's old ITV morning show called 'Feel Fabulous Forever'. 'I thought I'd get my own TV show after that,' she says. 'Nothing. Nothing happened. People would say: 'You're great on TV, you're phenomenal on TV, but this is not a primetime idea. Forget it.' Once, in the days when she did a health slot on Richard and Judy's BBC rivals, Good Morning with Anne Diamond and Nick Owen, she managed to present a few sections of the show proper. 'Anne was ill, so I sat in for her. Nick said to me: 'We're good together.' That was all I needed. I went to the producer and said: 'Whenever Anne is ill, I want to host the show.' They said "no". I don't know what was wrong with me that day, but I wouldn't take no for an answer. She walked away from me, and I ran after her. 'Why not? Why can't I host the show? I know I'm the health person, but there's more to me than that. Don't you run away from me. You stop.' She was never asked back.

McKeith was born in Perth in 1959, and attended Edinburgh University; her degree was apparently in languages and business. She is vague about events and dates thereafter. First, she lived in Spain, where she gorged on paella and sangria. Then she moved to America, where she worked, she tells me after some hesitation, 'in marketing and international business'. Precisely what happened next I find impossible to pin down, even with the woman sitting opposite me. McKeith suffered from a whole series of ailments, including a migraine that simply would not go away, none of which any doctor was able to help her with. There were times when she felt so desperately ill, she was virtually unable to get out of bed. At some point, however, she had a Damascene conversion to healthy eating, and was cured. How did this come about? First, a boyfriend took her to a macrobiotic centre, where she was taught 'everything about how to get into the kitchen'. Second, while presenting her own health-related radio show, a member of the public called in and told her about their miraculous experiences with blue-green algae.

'Here's a thing. I somehow started to get interested in health because I'd been going through this desperate struggle. I couldn't work at my normal thing any more. It all occurred around the same time. It got to the point where I thought: I just don't want to live any more. Someone said I had chronic fatigue, but that was never verified. Someone else said I had a brain tumour. A woman came on to the radio show who could see into the body - I thought that was ludicrous at the time - and she told me that I didn't have a brain tumour; I had a vitamin deficiency. She was like an angel. I remember saying to [her]: "I'm going to die young. Help me die nicely."' In fact, McKeith does not seem to have appeared on any 'nationally syndicated' American radio show. One of her programmes aired on a small Christian station where she paid for air time. Her claim, following this, that for years she reported on the diets of the rich and famous in Hollywood for The Joan Rivers Show, is also questionable. A spokesman for the show said that 'she was certainly not a regular contributor' - rather she presented a slot on a less high-profile gossip show hosted by Rivers.

Anyway, McKeith began training as a nutritionist, and eventually moved back to London, where she began working with private patients. What year did she do that? 'I think it was about '92. I was already doing things in nutrition, then I came back to London and continued...' She frowns, drifts off. 'I think maybe [it was] more about '95.' Later, she starts worrying about this, and says: 'I hope I'm accurate with dates. I don't want to give you wrong information. While I was ill, I was investigating health. You've got to understand. I hope all my dates aren't wrong. You know what it is? I never think about dates, or age, or anything. I'm nervous that you're going to write dates that are wrong. I can't remember them at the moment.' .

So where did she train? McKeith's CV has been the subject of some debate in the press. Her PhD, for instance, was gained via a distance learning programme at a non-accredited college, the American College of Holistic Nutrition, now known as the Clayton College of Natural Health. The fact that this college is non-accredited means that the US secretary for education does not recognise its degrees for the purpose of educational grants. In some states, the holder of a degree from such an establishment would not be allowed to practise as a clinical nutritionist. McKeith has never published any properly evaluated scientific research, not even her PhD thesis. As the journalist Ben Goldacre pointed out in his Bad Science column in the Guardian, McKeith's much-vaunted certified membership of the American Association of Nutritional Consultants is also a peculiar boast. Ben Goldacre managed to buy the same membership for his dead cat via the internet for the bargain price of $60.

McKeith claims not to read these attacks on her, though her American husband, Howard, a lawyer, is highly protective (McKeith refuses to discuss this man, or her two daughters, who are five and 10, but I gather that her wedding cake was made of carob and that Howard makes an 'incredible quinoa porridge'; she also tells me, with a wide smile, that it is extremely 'handy' that he is a lawyer). 'There's always going to be people who try to bring someone down who is speaking in a certain way. I expect to be criticised. But, at the end of the day, my results are phenomenal.' But what about her qualifications? What does she have to say about those? 'I know who I am. My education is stellar. I never stop learning. I've been on so many courses. I have had thousands of patients over the years who would rave about me. I get letters from medical doctors all over the world raving about my information.' OK then, so which of the many courses she has done proved the most useful, or interesting, or revolutionary? Where did she learn the most? She thinks for ages. Literally, a couple of long minutes. Then she says: 'My training with food in the early days: my own personal experience.'

Of course, none of this would matter if all the advice that McKeith was handing out was based on scientific fact. But this is not always the case. Much of what she says is patent nonsense. In the past, she has informed us that a seed contains 'all of the energy necessary to make a fully grown plant', that 'chlorophyll is high in oxygen' which means eating green leaves will 'really oxygenate the blood', and that 'the colours of foods represent vibrational energies... foods which are orange in colour have similar vibrational energies and even similar nutrient make-up'. A cursory glance through her books reveals that she is no scientist. I opened You Are What You Eat at random, and read the following: 'Floating stools that will not flush show a liver imbalance.' Catherine Collins, chief dietician at St George's hospital, London, points out that: 'it is impossible to diagnose medical conditions from looking at a normal brown stool.' Even a persistent floater? So it seems.

Some experts consider her advice is potentially dangerous: for instance, her advocacy of colonic irrigation. 'Colonic irrigation is not to be undertaken lightly,' says John Garrow, professor emeritus in human nutrition at London University and the chairman of Healthwatch, a charity that promotes better understanding by the public of the importance of clinical trials in medicine. Garrow is one of McKeith's most severe critics. In 2000, when McKeith published her book Living Food for Health, in which she boasted of a 'living food powder' she had developed in order to help reluctant patients ingest the enzymes contained in raw foods, he urged her to subject one of her treatments to a simple clinical test. 'I told her that if she could prove it worked, I would pay her £1,000. Of course, she proved totally resistant to this idea, and her husband phoned and said I was defaming her. I do not believe there is a scientific basis for the things she claims.' McKeith says she has never spoken to Garrow and his idea is 'preposterous. Health and wellness must not be trivialised.'

But why should she care too much about her critics? The cash must be rolling in: the TV series, the books, the food products (her Living Food Love Bar, it is claimed, will 'nourish libido energy' and is yours for just £1.69, which is far cheaper than a plate of oysters down at J Sheekey). This summer, you can even tune into a Celebrity You Are What You Eat, in which she will transform the diet of Pop Idol's hefty Michelle McManus. She honestly seems to believe that this is what she was put on earth to do. Aged 12, she had a vision. She was on stage, talking to hundreds of people. On buses, she has to force herself not to stare at people, looking for their symptoms. 'I like it when people come up to me and ask me questions,' she says. 'The most popular one is: "Would you look at my tongue?" Oh, I love tongue analysis. Love it, love it, love it. And I'm really good at it.'

McKeith is Tiggerish, and quite warm, in a steely kind of way - assuming, of course, that you don't mind her squeezing your vital organs as if they were a couple of old bean bags. Nevertheless, being with her is weirdly dispiriting. This isn't down to all the bad science (though that is wearying enough). No, the real problem with her is that she is so anti-life. Food is about history, and culture, and ritual. But not for her the artisan cheese-maker, or the fifth-generation baker, or the man with an ancient vine. Many of the foods she recommends are not even indigenous to these islands; flying them here literally costs the earth. Most of all, though, it bothers me that there is so little that is celebratory - or even vaguely pleasurable - about her regimes. Feeling glum, I tell her this. I should have saved my breath. 'You can have fun foods!' she says. 'My pizza has a crust of sweet potato!'

Gillian McKeith's tongue test

Can your tongue really your reflect your health?

McKeith believes it can. Take her simple test ...

1. Does your tongue have a line down the middle?
2. Does your tongue have teeth marks round the side?
3. Does your tongue have a bright red tip?
4. Is your tongue sore?
5. Does your tongue appear dotted all over?

If you answer yes to a question, it could mean:

1. Weak digestion. You may feel bloated and suffer from gas and indigestion. A strong digestive system is important for nutrient absorption. I would suggest food combining for a eight weeks. Keep meals simple, and eat ingredients that are particularly good for you - brown rice, avocado and tofu.
2. Spleen weakness and nutrient deficiencies. Your spleen is your energy battery and may not be taking up nutrients as effectively as it should. Symptoms can include feeling tired all the time, gassy and bloated.
3. Emotional upset or bodily stress. Either way, you will need B vitamins to calm it all down.
4. Vitamin B6 deficiency, and low niacin and/or iron levels. Drink nettle or dandelion teas and eat foods rich in vitamin B6, including sunflower seeds, brown rice, buckwheat and avocados.
5. Liver stagnation. In western holistic medicine, we think of the liver as the organ of detoxification. When it is overworked, it may perform sluggishly. Foods that support the liver include kohlrabi, broccoli, cauliflower, flax seeds, hemp seeds and sunflower seeds. Nettle and dandelion teas can help, too.

· Taken from Gillian McKeith's You Are What You Eat Cookbook (£14.99, Penguin)