Some celebrity gurus are coy about their clients, but not Nish Joshi. Princess Diana, Cate Blanchett, Juliette Binoche, the Sugababes... the names fall from his lips like coins from a slot machine. 'Gwyneth!' he purrs delightedly, when I mention, in passing, Gwyneth Paltrow, who once appeared in a backless dress that revealed the circular marks where Joshi had applied hot glass cups to her skin to 'boost her qi' and 'purify' her blood. 'Bless her! She came because three of the ladies at her baby shower were patients of mine. So sweet.' Of course, Joshi owes the rich and famous such a lot. It's not only that gossip about his relationships with them helped to put his Wimpole Street practice on the map, or even that their testimonies were so useful when he came to write his book, Holistic Detox ('He sent me a medicinal drink for a bad bronchial infection I was suffering while I was performing Coriolanus in Tokyo,' says Ralph Fiennes, on page 114). In the case of Diana, Princess of Wales, he owes his very career to her; it was she who urged him to look to the East, to the Ayurvedic medicine on which his Indian family had relied for generations, for solutions to common Western ailments: '"Go back to your roots," she said. She believed so much in holistic medicine, in giving patients what they need rather than masking their symptoms.'

He smiles at me beatifically. This takes a little digesting: that Joshi, who is always at pains to point out that he is a highly qualified osteopath, was willing to take advice from the notoriously daffy and faddish Princess Diana. But we must move on. The big news is that Joshi is now giving his clients something back - something other than the love and care that they receive from him already. He has opened a Wellness Centre: a gym and yoga studio in which they will be able to stretch their (not very) wobbly thighs in total privacy. Programmes will be tailored to their health needs. Isn't that what all personal trainers do? Apparently not. 'Let me give you an example. If your sugar levels are out of control, exercise could make you feel even more tired. We'll tell the trainer: here is this lady, make sure that before exercise she has an isotonic drink. We'll work on postural training, and the session will finish with a shiatsu body stretch to dissipate tension.' So, in addition to the stuff that Joshi ordinarily recommends to his exhausted, stressed-out clients (diets, colonic irrigation, acupuncture) they'll now be able to pick up a prescription for exercise at the same time. 'It's about giving people tangible help,' he says.

Joshi and I are sitting in the Wellness Centre right now, perched on two high stools in an area where people will soon be drinking their energy-boosting shots of wheatgrass juice. He is a disarming man. He has a camp voice and an unctuous, calculatedly soothing manner that I find mighty annoying, but which neurotic celebrities no doubt love, and he answers questions in sprawling sentences which do not always contain the facts I am after. He is also undeniably plump. Does the guru actually practise what he preaches? 'Yes,' he says, smoothly. 'I don't eat fish, meat, eggs or dairy. I'm a Hindu vegetarian. I have a purely Ayurvedic diet. My meals are cooked for me by my housekeeper. I don't do well with croissants. I feel very bloated. Cheese makes me mucusy.' Exercise? 'I do my sun salutations every day, and I work out with a personal trainer twice a week.' However, I hear from a top source that Joshi loathes the gym; faced with a punchbag, his wrists droop like sunflower stalks in a heatwave.

The Wellness Centre, in Marylebone, is a glamorous place: in the changing room, there are towels so fluffy that to wrap yourself in one is almost comic - as if you're auditioning for lead sheep in a nativity play. The yoga studio is light and airy. To become a member is not cheap (£125 a month; yoga classes cost £25 on top, and personal training sessions £60), but membership is, by necessity and design, destined to be exclusive. It's another outward sign, then, of the success of the Joshi brand. His rise has been rapid and somewhat eventful.

Joshi grew up in north London. After school, he studied for a medical degree in India and then, having left after only three years, he trained at the British School of Osteopathy. After he qualified, he ran a practice treating dancers until, in 1993, while he was working at the Hale Clinic, he met Diana, who demanded to know what he was giving his patients, and could she please have some. Soon, he was a regular visitor to Kensington Palace (you can just see him, sweeping through the gates, the back seat of his car laden with tofu and tincture of liquorice), where he taught her to make pilau rice. He helped her through her bulimia - 'You saw how she flourished; she started to glow!' - grew more interested in alternative medicine, and finally, with financial help from his parents, set up his practice in Wimpole Street.

The question is: what sets him apart from the other celebrity gurus? Is his Wellness Centre, for instance, really any different from all the other boutique gyms that have lately sprung up to cater for the anxious rich? Joshi insists that his success is down to the fact that his treatments really work, so he gets great word of mouth. But he would say that, wouldn't he? I wonder out loud if the Joshi phenomenon isn't an example of the placebo effect. His remedies are surely not so important as the fact that he spends time with his patients, smiling and soothing and calling them 'dear'. He disagrees. 'You can see a private GP, and get more time with your doctor. Lots of my patients have private GPs.' OK, then. But does he worry that some patients are fixated on dramatic weight loss which they dress up as a desire to be 'well'? 'Completely! I'm trying to encourage people to have a healthy relationship with their diet.' So if Cate Blanchett came in and wanted to lose a stone... 'No, no, that's ridiculous. My programme doesn't allow you to lose weight unless your body requires it.' I'm not absolutely convinced by this, but the upshot is that I ask Joshi if he will treat me as a patient for three weeks so that I can attempt to discover his secret for myself - and he, very kindly, agrees.

A week later, I rock up for our first meeting (an initial consultation with Joshi is £180, and appointments thereafter cost £120 - but some patients fly him across the Atlantic or to Dubai if they can't make it to London). His clinic, I can't help but notice, is almost next door to the consulting rooms of Jan Stanek, the cosmetic surgeon who regularly appears on the Channel 4 show 10 Years Younger. In reception, I fill in a questionnaire. It asks me about illness in my family, whether I suffer from a range of ailments from insomnia to anxiety and, more bizarrely, about the length of my fingers. Joshi appears and talks to a waiting patient. He is all twinkly - almost giddy-seeming at finding her there, just beside the fridge that is stocked with his own brand of gluten-free loaves. She flirts with him a little, and says something about her father, who is also feeling ill. Joshi asks what star sign he is.

'Is he a Pisces?'

'Oh, Dr Joshi!'

I feel quite left out.

Oh, well. It's my turn now. We go up to his consulting room: big desk, couch, photographs of Joshi on the mantelpiece. I hand over my forms, but he barely looks at them. I tell him, truthfully, that I am tired, that I get headaches, and that my sleep seems to do me no good. I find the gym exhausting; I worry I will brain myself with a dumb-bell simply for want of some strength in my wrists. He tells me, without examining me, or taking any samples, that he thinks I have a hormonal imbalance, that my adrenal glands are not functioning properly, and that my bowel is lazy. I also have a chromium deficiency. He is going to put me on a diet and give me supplements that will sort these things out. He is also going to refer me to the Wellness Centre, where the trainers will work with me in a way that is not exhausting. Now, would I hop on the couch? I do, and he starts sticking acupuncture needles into my temples, wrists and ankles. Why? 'To reboot your computer,' he says, lightly. But what will they do? 'I already told you,' he says. 'They will reboot your computer.' He is going to leave me to relax for 20 minutes; I may feel a little sleepy.

After a while, the door opens and a young man in white overalls comes in. He sits down and begins insistently rubbing the arches of my feet - they supposedly connect to the relevant bits of my insides - which is quite painful. He is from Poland. In Poland, he says, the people in the villages are properly rustic, and thus honest and true: if they like you, they welcome you; if they don't, they will 'hit you'. This is not a particularly relaxing thought.

Finally, Joshi comes back. He puts two tiny needles into my ears - to keep cravings at bay, OK? - and then he takes me downstairs to make an appointment at the Wellness Centre. He also hands over seven bottles of supplements and a diet sheet on which is a list of good foods, of forbidden foods, and of all the pills I should take, and when. The forbidden list includes alcohol, bread and gluten products, dairy products, sugar, tea, coffee, chillis, fizzy drinks, red meat, all fruit (except bananas), vinegar, cucumber, courgettes, and the deadly nightshades: i.e. potatoes, aubergines, tomatoes and peppers. But I am allowed eggs, salad, dark green vegetables, white meat, soya and tofu, pulses, fish, brown rice, seeds, rice milk and live yoghurt. Where does spaghetti carbonara fit in?

Last of all, he sends me back upstairs for colonic irrigation. He doesn't explain why, but I know that Diana was a fan, so maybe that's it. It's horrible: uncomfortable and strangely humiliating, like being a giant baby. The therapist who performs this procedure - what a job! - tells me that I will have a colonic once a week for the next three weeks, but even as she announces this, I know I won't. I don't think you should shove plastic pipes up your backside without a very good reason and, at this point, I can't fathom what that reason might be.

So, now I'm on my 'voyage of discovery'. The diet is a nightmare: boring, restrictive, unsatisfying. Also, Joshi's 'Metabolic Detox' pills, of which I'm supposed to take four a night, seem to be giving me diarrhoea. I look at my sheet. It warns that they may cause cramps and loose stools, adding that: 'they are not laxative - they maintain the water content of your food to improve elimination of toxins'. But when I look on the side of the bottle, the first ingredient listed is senna, a bowel irritant widely used in the treatment of constipation. I stop taking them. The good news, however, is that the gym is a revelation. Joshi's trainers, James and Ross, are fantastic: knowledgeable, enthusiastic, motivating. They know about posture and about muscles, which means that they notice and know how to care for my tight calves. If I were rich, I would see them every day for the rest of my life.

After a week, I see Joshi again. How am I? Well, I say, those detox pills: are they laxatives? He insists not, but adds that he remembers 'school biology lessons' in which he learnt that it should take food an hour to pass through the system: I should be going to the loo three times a day, after every meal. But they're giving me the runs! 'So, take two instead of four.' Today's session is brief: more acupuncture, and some manipulation of my neck. It all seems rather random: a bit pick and mix. He doesn't seem to focus on what I'm saying, or make an effort to answer my questions. Nor does he mention the needles he stuck in my ears a week ago (in fact, I've already removed them; they were sore). I have enough pills, so when I leave, all I take with me is an appointment to see a masseur, Tommy (a former dancer from Sweden, he turns out to be excellent, and gives me the most expert massage I've ever had).

I persist with my sessions at the gym but, though I pop the other supplements dutifully - this takes minutes - I again cease taking the Metabolic Detox pills. I try hard with the diet, but it's difficult when there is your other half to consider - soya milk, darling? - and, if I eat only rice cakes and salad for lunch, I feel so hungry I can't work. I'm due to see Joshi one more time but (I admit this is not very journalistic of me) I duck out at the last minute, largely because I gather he is going to insist on that plastic pipe going back up my bottom. How do I feel after three weeks? Good; I've got more energy. But I think this is mainly due to the brilliant James and Ross, and their boxing regime. I've lost a couple of pounds, but anyone would with a diet this restricted and, since that wasn't my aim anyway, I'm wondering... where did I hide those biscuits?

It is good fun being treated by Joshi - if nothing else, I got to see a world I could never afford to visit in real life - but I can't work out what his diagnosis is based on (he's taken no blood or urine), nor can I see the logic in his detox diet (it's contradictory unless, as I suspect, it is simply about reducing calorie intake: for instance, dairy products are forbidden, while cottage cheese is acceptable). Question his ideas and Joshi's first line of defence is that Western medicine has always pooh-poohed that of the East, and that many of his treatments - from Ayurvedic herbs to Gwyneth Paltrow's favourite, cupping - have been used for thousands of years. This isn't good enough. He should be able to explain both why treatments are suitable, and how they work. The testimonies of other patients - 'Joshi is a miracle worker,' says Patsy Kensit - are not enough for me, nor should they be. 'People go to the doctor because they feel ill,' says John Garrow, professor emeritus in human nutrition at London University and vice-chairman of Healthwatch, a charity that promotes the better understanding of the importance of clinical trials in medicine. 'More often than not, the next time they go to the doctor, they will be feeling less low. These things are cyclical.'

Colonic irrigation has certainly been practised for thousands of years, but its benefits have been neither proved nor discredited scientifically. However, most doctors think it is, at best, unnecessary. Holistic practitioners seem to believe that the colon is a sewage system, and that if it isn't properly cleaned out, toxic substances will leak into the body. 'It's now recognised that the contents of the colon are not rubbish,' says Garrow. 'Fibre that is not digestible by enzymes is metabolised by the bacteria there, and turned into fatty acids which make a very small contribution to your energy.'

I show Joshi's diet to Catherine Collins, chief dietician at St George's Hospital, London. She, too, finds it inconsistent. Is it a diet aimed at avoiding common allergens like milk protein? If so, why am I encouraged to eat live yoghurt? 'Red meat is a valuable source of iron for pre-menopausal women,' she says. 'The lack of fruit is not great, especially since it will be difficult to compensate by eating lots of vegetables because he has excluded so many of those, too.' Why might foods from the deadly nightshade family be forbidden? 'There is evidence that they are a potential irritant, but the research is old and mostly relates to those with arthritic conditions. I work in arthritis, and I've never found a patient who has benefited from these exclusions. So I'm not sure why he forbids them. I'm afraid this diet is typical of those recommended by self-styled nutritionists. It is nutritionally compromised, and its inadequacies need to be offset by vitamin supplements.'

Ah, yes. My supplements. Of the seven white bottles Joshi gave me, the one labelled Super Liver Cleanse is just a vitamin and mineral supplement in disguise. The one called 5HTP is too, though it also contains the controversial amino acid 5HTP which, in some countries, is marketed as an anti-depressant (it can have serious side effects, though not in the amount contained here). The others are: Metabolic Detox; PH Balance; Digest-Aid; Kidney Support; and Essential Chromium. Collins is unimpressed by one of the primary ingredients of the Metabolic Detox tablets: senna. She wouldn't recommend senna for constipation except as a last resort (and I never said I had constipation), and 'anything that results in diarrhoea is not recommended'. PH Balance contains magnesium, sodium and potassium - salts that are found naturally in the bloodstream. Digest-Aid contains a number of enzymes, but enzymes are proteins, which are destroyed by the stomach acid before they are able to reach the site of absorption in the small intestine. Kidney Support contains vitamin C and cranberry juice, plus other plant ingredients like horsetail. Cranberry juice has been shown to have a borderline effect in the treatment of cystitis, but in the quantities contained here it is unlikely to be effective. What about my chromium deficiency? 'The body only needs chromium in tiny amounts,' she says. 'There's very little research to show than an ordinary member of the public would need any extra.' Collins is bewildered as to how Joshi came to his diagnosis, especially of my below-par adrenal glands (the adrenal glands regulate the body's stress response through the production of hormones such as cortisol and adrenaline). 'If something were wrong with your adrenal glands, it would be a major medical event. You'd have loads of symptoms. You'd be very ill. I'm concerned that he can presume a diagnosis of what would be a serious complaint without any physical examination, or analysis of blood biochemistry.' After I speak to Collins, I telephone Joshi and ask him how, exactly, he diagnosed all my problems. 'I looked at your symptom picture, to what you told me about your periods of tiredness, your headaches, your sugar levels dropping.' But what about my adrenal glands? What him made him think they weren't functioning properly? 'It's all clinical experience. I could have done some tests that confirmed this, but that would have been very expensive. I see this symptom picture over and over again. This is what I deal with a lot of the time.'

I am sure that Joshi means well. He certainly works hard. As he says to me with a weary but satisfied smile: 'There are so many ill people who need to be made well.' But if it is anything like the information he gave to me, a lot of what he tells his patients is, at best, confused and somewhat vague - and, irrespective of how rich and famous they are, those patients put their trust in him. He styles himself Dr Joshi but, as he also admits to me on the telephone, he never completed his medical degree, which is why he is not registered with the General Medical Council - and when I ring the General Osteopathic Council, with whom he is registered, a spokeswoman me that they actively discourage members from using the title 'Dr'. Oh, well. You can join his Wellness Centre without even having to see him - and I certainly recommend that. Although I flush his supplements down the loo, and go back to eating bread, red meat and - oh no! - fresh fruit, I must admit that I haven't had thighs as firm as this for ages.

The Joshi Clinic Wellness Centre, York Street Chambers, 20 Enford Street, London W1. 020 7723 2444. joshiclinic.co.uk