DOCTOR, doctor, I've got a problem: I don't know who to trust. There are all these people giving out advice on how to stay healthy, eat right and live well, but some just aren't what they seem.

Take that fearsome Gillian McKeith, who goes on television staring at people's stools and terrifying the, ahem, life out of them. You Are What You Eat is the name of her book, and at least a million people agree enough to have bought it.

She is a doctor, after all - it says so on the cover of the book and no fewer than 36 times on the home page of her website, drgillianmckeith. com. But last month the Advertising Standards Authority (ASA) in Britain told her to remove all mention of her title in print ads for future products.

"Gillian McKeith has never claimed to be a medical doctor, " said a statement from her company McKeith Research.

"[She] holds a doctorate degree, " (achieved by distance learning through an American college) "having earned a PhD in holistic nutrition after four years of study, coursework and dissertation, which does certainly entitle her to use the denotation of 'Dr'."

On paper anyway. If she ignores the usual convention that only practising academics or medics use the title (which must have led many readers to assume that someone who writes about food and health and calls themselves doctor must be a GP, work in a hospital or have a psychological qualification).

Dr McKeith has been on holiday for the past week. The office of Max Clifford, the PR guru who has been speaking for her, said she "wanted to get away from it all" and had left her mobile phone at home. So where to go for a doctor? Just look down the best-seller lists. To do so is to understand why the [British] government is expected to publish, in a white paper this week or next, proposals for the regulation of nutritionists and alternative therapists.

"Want more love/sex?" asks the Barefoot Doctor, offering his manifesto. The book "shows you how to get exactly what you want out of life, with ancient Taoist visualisation, positive thinking and relaxation techniques". Stephen Russell, to use his real name, used to practise as a private healer. He has reportedly denied allegations of sexual overtures to women he was treating. Besides books and TV shows, he puts his name to health drinks, lotions and potions. So is he really a doctor? Er, no, says his publisher. "Not a doctor per se." He follows the oriental tradition of wandering healers who are referred to by that name, apparently.

What about the Diet Doctors? These two women have a show on Five and a book of the same name promising: "We can make you slimmer and healthier - and you'll feel fantastic."

Wendy Denning appears on the cover with a stethoscope around her neck. She is a GP. The other one, Vicki Edgson, is not. She trained at the Institute for Optimum Nutrition (ION) set up by veteran TV expert Patrick Holford, who has sold a million copies of his books worldwide.

The latest, Food Is Better Medicine Than Drugs, says on the cover: "Don't go to your doctor before reading this book."

The back warns of the dangers of prescription drugs. So is Patrick Holford a doctor? No. He holds a BSc in experimental psychology, an honorary diploma from his own institute and an honorary fellowship of the British Association of Nutritional Therapists.

The Food Doctor promises you will "get off the diet treadmill for ever" with the best-selling Everyday Diet ("as seen on Richard & Judy" ). The name on the cover is Ian Marber, another holder of a diploma from the Institute for Optimum Nutrition. So he must be a doctor, right? Wrong.

"The Food Doctor is a brand, " he says at his clinic of the same name above a Starbucks in Notting Hill, west London. "It's certainly not me."

Mr Marber is studying for a master's degree in food labelling at Middlesex University and hopes one day to get his doctorate, but it may take a long time: life is busy for a presentable young TV expert who has sold 250,000 books and has people waiting to enter his consulting rooms, labelled Vitamins A, B and C. ("We thought it would be funny. We call Starbucks Vitamin D because of my coffee habit.") Hang on though: didn't he just introduce himself as "Ian Marber, the Food Doctor"? Absolutely not, apparently. "I said, 'from the Food Doctor'.

We are very careful to do that." For good reason, maybe, in view of what has happened to Gillian McKeith. "I was always very uncomfortable about what she did, " he says. "She created her own downfall."

He also had dealings with the advertising watchdog, five years ago, when the Nutrition Society claimed the name Food Doctor was misleading. But the complaint was rejected. The ASA was persuaded by the argument that it was a well-known trademark using the term "doctor" in the sense of someone who gives advice. "Like the Juice Doctor, " he says, " or the Kitchen Doctor."

When he studied at the ION there was no other college offering a course in nutritional therapy; now the one he took is accredited as a BSc and has many rivals, of varying quality. "Many of my fellow practitioners are excellent and hard working, " says Mr Marber. "Other people who claim to be nutritionists are. . . well, they belong in Hogwarts."

The trouble is, there is no legal definition of a nutritionist and no exam you have to pass. There are a couple of registers, but many of the estimated 100,000 people practising as nutritionists in the UK are not even on those.

"You don't even have to go on a course, " says Dr Frankie Phillips, a registered dietitian. "If you read a book you can call yourself a nutritionist. Anyone can." She speaks for the British Dietetic Association, which licenses dietitians.

"Sometimes the diet doctrines dictated by self-styled nutrition gurus and 'experts' either seem too good to be true - eat all you want and still lose weight - or simply beyond the realms of possibility for most people, " says Dr Phillips.

Sprouting your own seeds every day may not help, for example. Taking large doses of supplements could be dangerous.

Cutting out wheat and dairy for no good reason might be disastrous. "We certainly know of children who have been put on inappropriate diets and then turned up in hospital."

So that, doctor, is the problem. What do you prescribe? A 'doctor doctor' to register all the nutritionists and set a minimum qualification? The British government may be thinking the same. And this piece of paper you are handing me now, is it your doctorate? Oh no. It's your bill for this private consultation. Thank you 'doctor'. That's enough to make anyone feel sick.

IS THERE A DOCTOR IN THE HOUSE?

Gillian McKeith Best known for super-seller Yo u Are What You Eat (estimated sales 500,000) and current C4 series Gillian Moves In. Is she a doctor?

Has PhD in holistic nutrition but has agreed with the Advertising Standards Authority to stop using 'Dr' in print ads.

Allen Carr Made famous by his Easy Way to Stop Smoking (7m sales). Died in December, aged 72, from lung cancer.

Was he a doctor? No. Former accountant had no medical training, but estimated he had helped 10 million people quit. Richard Branson is a fan.

Dr Atkins Robert Atkins devised low-carb New Diet Revolution (1m sales). Celeb fans included Robbie Williams.

Was he a doctor? Yes, a cardiologist. Reportedly clinically obese when he died after a fall in 2003. The popularity of the diet has crashed in recent years.

The Greek Doctor Fedon Alexander Lindberg is in charts with new book (21,000 sales) advocating "slow carb", Mediterraneanstyle eating: olive oil, fish, veg, beans, lentils and fruit.

Is he a doctor? Absolutely.

Specialist in internal medicine and obesity in Norway.

Rick Gallop Inventor of GI Diet loved by celebs from Kylie to Steve Redgrave (over 1m sales). Is he a doctor? No. Former ad man who ran Canadian Heart and Stroke Foundation. But wife and fellow author Ruth is a doctor specialising in psychological health.

Paul McKenna Hypnotist with multiple books, including I Can Make You Thin (1m sales). Is he a doctor? Won libel case against newspaper that claimed PhD from La Salle University, USA, was "bogus". Got British PhD with thesis on neuro-linguistic programming.

The Barefoot Doctor Taoist therapist Stephen Russell fronts TV shows, and sells lotions, perfumes and books, including Manifesto (estimated sales 50,000). Is he a doctor? Not in the medical/academic sense, say publishers, but in "the oriental tradition" of wandering advisers.

Arthur Agatston Inventor of the South Beach Diet (estimated sales 1m), which has its own range of foods. Is he a doctor? Yes, a preventative cardiologist, associate professor of medicine in Miami Beach. Arthur Agatston MD devised threestage programmes.

Beechy Colclough Therapist to stars including Elton John, author of The Effective Way to Stop Drinking (est imated sales 100,000). Is he a doctor? Not known. Lost membership of British Association for Counselling and Psychotherapy over claims of sex with patients.

The Diet Doctors Wendy Denning and Vicki Edgson present show for Five, with book (78,000 sales). Are they doctors? Dr Denning is a GP. Ms Edgson's CV cites Institute for Optimum Nutrition, London School of Naturopathic Medicine and study of applied kinesiology.

Patrick Holford GMTV regular famous for New Optimum Nutrition Bible (1m sales). Is he a doctor? Never claimed to be, but Food Is Better Medicine Than Drugs says: " Don't go to your doctor before reading this book." Fellow, British Association of Nutritional Therapists.

The Food Doctor Richard & Judy regular Ian Marber's best-seller is The Everyday Diet (250,000 sales). Is he a doctor? No. Studying for masters in food labelling. But ASA agrees Food Doctor is a trademark brand name using "doctor" in the general sense.