Call her the Awful Poo Lady, call her Dr Gillian McKeith PhD: she is an empire, a multi-millionaire, a phenomenon, a prime-time TV celebrity, a bestselling author. She has her own range of foods and mysterious powders, she has pills to give you an erection, and her face is in every health food store in the country. Scottish Conservative politicians want her to advise the government. The Soil Association gave her a prize for educating the public. And yet, to anyone who knows the slightest bit about science, this woman is a bad joke.

One of those angry nerds took her down this week. A regular from my website badscience.net - I can barely contain my pride - took McKeith to the Advertising Standards Authority, complaining about her using the title "doctor" on the basis of a qualification gained by correspondence course from a non-accredited American college. He won. She may have sidestepped the publication of a damning ASA draft adjudication at the last minute by accepting - "voluntarily" - not to call herself "doctor" in her advertising any more. But would you know it, a copy of that draft adjudication has fallen into our laps, and it concludes that "the claim 'Dr' was likely to mislead". The advert allegedly breached two clauses of the Committee of Advertising Practice code: "substantiation" and "truthfulness".

Is it petty to take pleasure in this? No. McKeith is a menace to the public understanding of science. She seems to misunderstand not nuances, but the most basic aspects of biology - things that a 14-year-old could put her straight on.

She talks endlessly about chlorophyll, for example: how it's "high in oxygen" and will "oxygenate your blood" - but chlorophyll will only make oxygen in the presence of light. It's dark in your intestines, and even if you stuck a searchlight up your bum to prove a point, you probably wouldn't absorb much oxygen in there, because you don't have gills in your gut. In fact, neither do fish. In fact, forgive me, but I don't think you really want oxygen up there, because methane fart gas mixed with oxygen is a potentially explosive combination.

Future generations will look back on this phenomenon with astonishment. Channel 4, let's not forget, branded her very strongly, from the start, as a "clinical nutritionist". She was Dr Gillian McKeith PhD, appearing on television every week, interpreting blood tests, and examining patients who had earlier had irrigation equipment stuck right up into their rectums. She was "Dr McKeith", "the diet doctor", giving diagnoses, talking knowledgeably about treatment, with complex scientific terminology, and all the authority her white coat and laboratory setting could muster.

So back to the science. She says DNA is an anti-ageing constituent: if you "do not have enough RNA/DNA", in fact, you "may ultimately age prematurely". Stress can deplete your DNA, but algae will increase it: and she reckons it's only present in growing cells. Is my semen growing? Is a virus growing? Is chicken liver pate growing? All of these contain plenty of DNA. She says that "each sprouting seed is packed with the nutritional energy needed to create a full-grown, healthy plant". Does a banana plant have the same amount of calories as a banana seed? The ridiculousness is endless.

In fact, I don't care what kind of squabbles McKeith wants to engage in over the technicalities of whether a non-accredited correspondence-course PhD from the US entitles you, by the strictest letter of the law, to call yourself "doctor": to me, nobody can be said to have a meaningful qualification in any biology-related subject if they make the same kind of basic mistakes made by McKeith.

And the scholarliness of her work is a thing to behold: she produces lengthy documents that have an air of "referenciness", with nice little superscript numbers, which talk about trials, and studies, and research, and papers ... but when you follow the numbers, and check the references, it's shocking how often they aren't what she claimed them to be in the main body of the text. Or they refer to funny little magazines and books, such as Delicious, Creative Living, Healthy Eating, and my favourite, Spiritual Nutrition and the Rainbow Diet, rather than proper academic journals.

She even does this in the book Miracle Superfood, which, we are told, is the published form of her PhD. "In laboratory experiments with anaemic animals, red-blood cell counts have returned to normal within four or five days when chlorophyll was given," she says. Her reference for this experimental data is a magazine called Health Store News. "In the heart," she explains, "chlorophyll aids in the transmission of nerve impulses that control contraction." A statement that is referenced to the second issue of a magazine called Earthletter.

To me this is cargo cult science, as the great Professor Richard Feynman described Melanesian religious activities 30 years ago: "During the war they saw aeroplanes with lots of good materials, and they want the same thing to happen now. So they've arranged to make things like runways, to put fires along the sides of the runways, to make a wooden hut for a man to sit in, with two wooden pieces on his head as headphones and bars of bamboo sticking out like antennas - he's the controller - and they wait for the aeroplanes to land. They're doing everything right. The form is perfect. It looks exactly the way it looked before. But it doesn't work. No aeroplanes land."

McKeith's pseudo-academic work is like the rituals of the cargo cult: the form is superficially right, the superscript numbers are there, the technical words are scattered about, she talks about research and trials and findings, but the substance is lacking. I actually don't find this bit very funny. It makes me quite depressed to think about her, sitting up, perhaps alone, studiously and earnestly typing this stuff out.

One window into her world is the extraordinary way she responds to criticism: with legal threats and blatantly, outrageously misleading statements, emitted with such regularity that it's reasonable to assume she will do the same thing with this current kerfuffle over her use of the title "doctor". So that you know how to approach the rebuttals to come, let's look at McKeith's rebuttals of the recent past.

Three months ago she was censured by the Medicines and Healthcare Products Regulatory Agency (MHRA) for illegally selling a rather tragic range of herbal sex pills called Fast Formula Horny Goat Weed Complex, advertised as shown by a "controlled study" to promote sexual satisfaction, and sold with explicit medicinal claims. She was ordered to remove the products from sale immediately. She complied - the alternative would have been prosecution - but in response, McKeith's website announced that the sex pills had been withdrawn because of "the new EU licensing laws regarding herbal products". She engaged in Europhobic banter with the Scottish Herald newspaper: "EU bureaucrats are clearly concerned that people in the UK are having too much good sex," she explained.

Rubbish. I contacted the MHRA, and they said: "This has nothing to do with new EU regulations. The information on the McKeith website is incorrect." Was it a mistake? "Ms McKeith's organisation had already been made aware of the requirements of medicines legislation in previous years; there was no reason at all for all the products not to be compliant with the law." They go on. "The Wild Pink Yam and Horny Goat Weed products marketed by McKeith Research Ltd were never legal for sale in the UK."

Now, once would be unfortunate, but this is an enduring pattern. When McKeith was first caught out on the ridiculous and erroneous claims of her CV - she claimed, for example, to have a PhD from the reputable American College of Nutrition - her representatives suggested that this was a mistake, made by a Spanish work experience kid, who posted the wrong CV. Except the very same claim about the American College of Nutrition was also in one of her books from several years previously. That's a long work experience stint.

She even sneaked one into this very newspaper, during a profile on her: "Doubt has also been cast on the value of McKeith's certified membership of the American Association of Nutritional Consultants, especially since Guardian journalist Ben Goldacre managed to buy the same membership online for his dead cat for $60. McKeith's spokeswoman says of this membership: "Gillian has 'professional membership', which is membership designed for practising nutritional and dietary professionals, and is distinct from 'associate membership', which is open to all individuals. To gain professional membership Gillian provided proof of her degree and three professional references."

Well. My dead cat Hettie is also a "certified professional member" of the AANC. I have the certificate hanging in my loo. Perhaps it didn't even occur to the journalist that McKeith could be wrong. More likely, of course, in the tradition of nervous journalists, I suspect she was hurried, on deadline, and felt she had to get McKeith's "right of reply" in, even if it cast doubts on - I'll admit my beef here - my own hard-won investigative revelations about my dead cat. I mean, I don't sign my dead cat up to bogus professional organisations for the good of my health, you know.

But those who criticise McKeith have reason to worry. McKeith goes after people, and nastily. She has a libel case against the Sun over comments they made in 2004 that has still not seen much movement. But the Sun is a large, wealthy institution, and it can protect itself with a large and well-remunerated legal team. Others can't. A charming but - forgive me - obscure blogger called PhDiva made some relatively innocent comments about nutritionists, mentioning McKeith, and received a letter threatening costly legal action from Atkins Solicitors, "the reputation and brand-management specialists". Google received a threatening legal letter simply for linking to - forgive me - a fairly obscure webpage on McKeith.

She has also made legal threats to a fantastically funny website called Eclectech for hosting a silly animation of McKeith singing a silly song, at around the time she was on Fame Academy.

Most of these legal tussles revolve around the issue of her qualifications, though these things shouldn't be difficult or complicated. If anyone wanted to check my degrees, memberships, or affiliations, then they could call up the institutions, and get instant confirmation: job done. If you said I wasn't a doctor, I wouldn't sue you; I'd roar with laughter.

If you contact the Australasian College of Health Sciences (Portland, US) where McKeith has a "pending diploma in herbal medicine", they say they can't tell you anything about their students. When you contact Clayton College of Natural Health to ask where you can read her PhD, they say you can't. What kind of organisations are these? If I said I had a PhD from Cambridge, US or UK (I have neither), it would only take you a day to find it.

But McKeith's most heinous abuse of legal chill is exemplified by a nasty little story from 2000, when she threatened a retired professor of nutritional medicine for questioning her ideas.

Shortly after the publication of McKeith's book Living Food for Health, before she was famous, John Garrow wrote an article about some of the rather bizarre scientific claims she was making. He was struck by the strength with which she presented her credentials as a scientist ("I continue every day to research, test and write furiously so that you may benefit ..." etc). In fact, he has since said that he assumed - like many others - that she was a proper doctor. Sorry: a medical doctor. Sorry: a qualified conventional medical doctor who attended an accredited medical school.

Anyway, in this book, McKeith promised to explain how you can "boost your energy, heal your organs and cells, detoxify your body, strengthen your kidneys, improve your digestion, strengthen your immune system, reduce cholesterol and high blood pressure, break down fat, cellulose and starch, activate the enzyme energies of your body, strengthen your spleen and liver function, increase mental and physical endurance, regulate your blood sugar, and lessen hunger cravings and lose weight."

These are not modest goals, but her thesis was that it was all possible with a diet rich in enzymes from "live" raw food - fruit, vegetables, seeds, nuts, and especially live sprouts, which "are the food sources of digestive enzymes". McKeith even offered "combination living food powder for clinical purposes" in case people didn't want to change their diet, and she used this for "clinical trials" with patients at her clinic.

Garrow was sceptical of her claims. Apart from anything else, as emeritus professor of human nutrition at the University of London, he knew that human animals have their own digestive enzymes, and a plant enzyme you eat is likely to be digested like any other protein. As any professor of nutrition, and indeed many GCSE biology students, could happily tell you.

Garrow read the book closely, as have I. These "clinical trials" seemed to be a few anecdotes in her book about how incredibly well McKeith's patients felt after seeing her. No controls, no placebo, no attempt to quantify or measure improvements. So Garrow made a modest proposal, and I am quoting it in its entirety, partly because it is a rather elegantly written exposition of the scientific method by an extremely eminent academic authority on the science of nutrition, but mainly because I want you to see how politely he stated his case.

"I also am a clinical nutritionist," began Professor Garrow, "and I believe that many of the statements in this book are wrong. My hypothesis is that any benefits which Dr McKeith has observed in her patients who take her living food powder have nothing to do with their enzyme content. If I am correct, then patients given powder which has been heated above 118F for 20 minutes will do just as well as patients given the active powder. This amount of heat would destroy all enzymes, but make little change to other nutrients apart from vitamin C, so both groups of patients should receive a small supplement of vitamin C (say 60mg/day). However, if Dr McKeith is correct, it should be easy to deduce from the boosting of energy, etc, which patients received the active powder and which the inactivated one.

"Here, then, is a testable hypothesis by which nutritional science might be advanced. I hope that Dr McKeith's instincts, as a fellow-scientist, will impel her to accept this challenge. As a further inducement I suggest we each post, say, £1,000, with an independent stakeholder. If we carry out the test, and I am proved wrong, she will, of course, collect my stake, and I will publish a fulsome apology in this newsletter. If the results show that she is wrong I will donate her stake to HealthWatch [a medical campaigning group], and suggest that she should tell the 1,500 patients on her waiting list that further research has shown that the claimed benefits of her diet have not been observed under controlled conditions. We scientists have a noble tradition of formally withdrawing our publications if subsequent research shows the results are not reproducible - don't we?"

This was published in an obscure medical newsletter. Sadly, McKeith - who, to the best of my knowledge, has never published in a proper "Pubmed-listed" peer-reviewed academic journal - did not take up this offer to collaborate on a piece of research with a professor of nutrition.

Instead, Garrow received a call from McKeith's lawyer husband, Howard Magaziner, accusing him of defamation and promising legal action. Garrow, an immensely affable and relaxed old academic, shrugged this off with style. He told me. "I said, 'Sue me.' I'm still waiting." His offer of £1,000 still stands; I'll make it £2,000.

But, to me, it's tempting to dismiss the question of whether or not McKeith should call herself "doctor" as a red herring, a distraction, an unnecessary ad hominem squabble. Because despite her litigiousness, her illegal medicinal products, her ropey qualifications, her abusiveness, despite her making the wounded and obese cry on television, despite her apparently misunderstanding some of the most basic aspects of GCSE biology, while doling out "scientific" advice in a white coat, despite her farcical "academic" work, despite the unpleasantness of the food she endorses, there are still many who will claim: "You can say what you like about McKeith, but she has improved the nation's diet."

Let me be very clear. Anyone who tells you to eat your greens is all right by me. If that was the end of it, I'd be McKeith's biggest fan, because I'm all in favour of "evidence-based interventions to improve the nation's health", as they used to say to us in medical school.

But let's look at the evidence. Diet has been studied very extensively, and there are some things that we know with a fair degree of certainty: there is convincing evidence that diets rich in fresh fruit and vegetables, with natural sources of dietary fibre, avoiding obesity, moderate alcohol, and physical exercise, are protective against things such as cancer and heart disease.

But nutritionists don't stop there, because they can't: they have to manufacture complication, to justify the existence of their profession. And what an extraordinary new profession it is. They've appeared out of nowhere, with a strong new-age bent, but dressing themselves up in the cloak of scientific authority. Because there is, of course, a genuine body of research about nutrition and health, to which these new "nutritionists" are spectacularly unreliable witnesses. You don't get sober professors from the Medical Research Council's Human Nutrition Research Unit on telly talking about the evidence on food and health; you get the media nutritionists. It's like the difference between astrology and astronomy.

These new nutritionists have a major commercial problem with evidence. There's nothing very professional or proprietary about "eat your greens", so they have had to push things further: but unfortunately for the nutritionists, the technical, confusing, overcomplicated, tinkering interventions that they promote are very frequently not supported by convincing evidence.

And that's not for lack of looking. This is not about the medical hegemony neglecting to address the holistic needs of the people. In many cases, the research has been done, and we know that the more specific claims of nutritionists are actively wrong.

I've got too much sense to subject you to reams of scientific detail - I've learned from McKeith that you need theatrical abuse to hold the public's attention - but we can easily do one representative example. The antioxidant story is one of the most ubiquitous health claims of the nutritionists. Antioxidants mop up free radicals, so in theory, looking at metabolism flow charts in biochemistry textbooks, having more of them might be beneficial to health. High blood levels of antioxidants were associated, in the 1980s, with longer life. Fruit and vegetables have lots of antioxidants, and fruit and veg really are good for you. So it all made sense.

But when you do compare people taking antioxidant supplement tablets with people on placebo, there's no benefit; if anything, the antioxidant pills are harmful. Fruit and veg are still good for you, but as you can see, it looks as if it's complicated and it might not just be about the extra antioxidants. It's a surprising finding, but that's science all over: the results are often counterintuitive. And that's exactly why you do scientific research, to check your assumptions. Otherwise it wouldn't be called "science", it would be called "assuming", or "guessing", or "making it up as you go along".

But don't get distracted. Basic, sensible dietary advice, that we all know - be honest - still stands. It's the unjustified, self-serving and unnecessary overcomplication of this basic sensible dietary advice that is, to my mind, one of the greatest crimes of the nutritionist movement. I don't think it's excessive to talk about consumers paralysed with confusion in supermarkets.

Although it's just as likely that they will be paralysed with fear, because McKeith's stock in trade is abuse, on a scale that would have any doctor struck off: making people cry for the television cameras, I assume deliberately, and using fear and bullying to get them to change their lifestyles. As a posture it is seductive, it has a sense of generating movement, but if you drag yourself away from the theatricality of souped-up recipe and lifestyle shows on telly, the evidence shows that scare campaigns tend not to get people changing their behaviour in the long term.

So what can you do? There's the rub. In reality, again, away from the cameras, the most significant "lifestyle" cause of death and disease is social class. Here's a perfect example. I rent a flat in London's Kentish Town on my modest junior doctor's salary (don't believe what you read in the papers about doctors' wages, either). This is a very poor working-class area, and the male life expectancy is about 70 years. Two miles away in Hampstead, meanwhile, where the millionaire Dr Gillian McKeith PhD owns a very large property, surrounded by other wealthy middle-class people, male life expectancy is almost 80 years. I know this because I have the Annual Public Health Report for Camden open on the table right now.

This phenomenal disparity in life expectancy - the difference between a lengthy and rich retirement, and a very truncated one indeed - is not because the people in Hampstead are careful to eat a handful of Brazil nuts every day, to make sure they're not deficient in selenium, as per nutritionists' advice.

And that's the most sinister feature of the whole nutritionist project, graphically exemplified by McKeith: it's a manifesto of rightwing individualism - you are what you eat, and people die young because they deserve it. They choose death, through ignorance and laziness, but you choose life, fresh fish, olive oil, and that's why you're healthy. You're going to see 78. You deserve it. Not like them.

How can I be sure that this phenomenal difference in life expectancy between rich and poor isn't due to the difference in diet? Because I've read the dietary intervention studies: when you intervene and make a huge effort to change people's diets, and get them eating more fruit and veg, you find the benefits, where they are positive at all, are actually very modest. Nothing like 10 years.

But genuine public health interventions to address the real social and lifestyle causes of disease are far less lucrative, and far less of a spectacle, than anything a food crank or a TV producer would ever dream of dipping into. What prime-time TV series looks at food deserts created by giant supermarket chains, the very companies with which stellar media nutritionists so often have lucrative commercial contracts? What show deals with social inequality driving health inequality? Where's the human interest in prohibiting the promotion of bad foods; facilitating access to nutrient-rich foods with taxation; or maintaining a clear labelling system? Where is the spectacle in "enabling environments" that naturally promote exercise, or urban planning that prioritises cyclists, pedestrians and public transport over the car? Or reducing the ever-increasing inequality between senior executive and shop-floor pay?

This is serious stuff. We don't need any more stupid ideas about health in the world. We have a president of South Africa who has denied that HIV exists, we have mumps and measles on the rise, we have quackery in the ascendant like never before, and whatever Tony Blair might have to say about homoeopathy being a fight not worth fighting for scientists, we cannot indulge portions of pseudoscientific ludicrousness as if they don't have wider ramifications for society, and for the public misunderstanding of science.

I am writing this article, sneakily, late, at the back of the room, in the Royal College of Physicians, at a conference discussing how to free up access to medical academic knowledge for the public. At the front, as I type, Sir Muir Gray, director of the NHS National Electronic Library For Health, is speaking: "Ignorance is like cholera," he says. "It cannot be controlled by the individual alone: it requires the organised efforts of society." He's right: in the 19th and 20th centuries, we made huge advances through the provision of clean, clear water; and in the 21st century, clean, clear information will produce those same advances.

Gillian McKeith has nothing to contribute: and Channel 4, which bent over backwards to dress her up in the cloak of scientific authority, should be ashamed of itself.

'With all due respect, you're wrong': When McKeith put a cabbie in his place

Here is a bizarre story, which McKeith is evidently proud of, because not only does she recount it in her book, she has also recounted it in other published articles. She is in a cab, and the cab driver has spotted her, and tries to spark up a conversation:

"As I sat down to enjoy the ride and sighed a sense of relief in honour of some quiet time, I barely heard some mumbling from Harry to break a much cherished silence. Ignoring it to soak in the rapidly moving scenery, I heard it again ... 'You know, fish has more omegas than flax,' he stated. 'I beg your pardon,' I said. 'I said that fish has more omegas than flax seeds,' he re-stated. The only thing I could think of was: 'Why was this invasive, somewhat jovial, but truly kind man, talking about flax ...' 'In all due respect, you're wrong, Harry.

Flax seeds contain far greater levels of the healthy oils (omega-3 and omega-6) in a properly balanced and assimilable form,' I explained. 'No, I disagree,' he argued. 'What do you mean, you disagree? Have you spent years conducting clinical research, working with patients, lecturing, teaching, studying the omega oils in flax, obtaining worldwide data, compiling one of the largest private health libraries on the planet, and writing extensively on the topic?' I asked. Not to mention writing this very article on this very day.

'No,' Harry feebly replied. I wondered, 'Are you a scientist, a biochemist, a botanist, or have you spent a lifetime studying food and biochemistry as I have done?' 'No,' he again replied. 'So, where do you get such stuff? Where is your scientific authority?' I demanded. Harry proudly announced: 'Oh, my wife is a doctor - a gynaecologist - by the way.' 'Is she a food specialist or nutritional biochemist as well?' I quickly retorted. 'Um, ah, well, no, but she is a doctor,' he offered."

Charming. But flax seeds contain oestrogenic compounds, and fibre, so they're not very "assimilable" unless you crush them, in which case they taste foul, and they're sold as a laxative in doses of 15g. And you will need a lot of them. When you account for the poor conversion in the body from plant-form omega oils to the animal forms that are most beneficial (called DHA and EPA) then flax seeds and fish contain roughly the same amounts.

But in the real world, rather than the raw figures, it's very easy to eat 100g of mackerel, whereas it's tricky to get a tablespoon of flax seed into you. (Similarly, parsley is a rich source of vitamin C, but you're not going to eat an orange sized lump of it.) As for "properly balanced", I don't know if she means spiritually or biologically, but fish is much higher in omega-3, which most people would say is better.