It could be said that Gillian McKeith lives off the fat of the land. The television food guru has done very nicely from berating the obese and inspecting their bodily wastes. Now that her spin-off diet book You Are What You Eat has soared through the 1m sales barrier, she is in organic clover. The cherry on her muesli is that her cookbook derivative has taken the No 1 manuals slot in The Sunday Times.

The 45-year-old Scots nutritionist is proof of the Newtonian law that for every action there is an equal and opposite reaction. Her mung beans represent a backlash to the protein-rich Atkins diet. And whereas the voluptuous Nigella Lawson urges us to cram down another piece of cheesecake, the petite and almost emaciated McKeith gives us a tongue lashing if we so much as look at Fray Bentos corned beef. In fact she has been accused of having a weakness for porkies, of which more later.

McKeith has prospered by joining television’s ranks of bossy women, alongside Trinny and Susannah, Kim and Aggie, Penney Poyzer and Jo Frost who police our sartorial, hygienic, dietary, environmental and parenting inadequacies.

She has become a cult success by transforming the overweight into healthy, energetic people such as Michelle McManus, the 22-stone winner of Pop Idol 2 who shed six stone and a penchant for junk food and alcohol. “Gillian didn’t mince her words,” she said later.

Reducing fat miscreants to jelly is more her style on Channel 4’s You Are What You Eat. The ritual humiliation begins when she heaps a family’s typical weekly fare on to their kitchen tables — a pullulating pile of pizzas, burger buns, sausages, sugar, chocolate and drinks — and then launches into a harangue that sends their blood sugar levels soaring.

“If you carry on like this you’re gonna die,” she rasps in an accent midway between Perth and Beverly Hills. She told one couple who fed crisps to their baby that they should be “arrested and locked up for child abuse”.

This ferocious image is at odds with the flighty figure observed by a journalist who passed through the electric gates of McKeith’s home on Hampstead Heath, northwest London, where she lives with her husband, Howard, and daughters, Skylon and Afton: “She was quite giggly and chatty. She was small and couldn’t stand properly because of a childhood disability — something that is well disguised on television.”

Broadly speaking, McKeith advocates a common-sense approach of exercising more and reducing processed and high-calorie foods in favour of organic fruit and vegetables, notably mung, black and aduki beans. Eschewing fad diets, she lays down elementary rules that cure fat people of their worst ailment — ignorance of their bodies.

What raises eyebrows among nutritionists and dietitians is her zeal for colonic irrigation and closely scrutinising what she calls “poo”. This has been a long-standing obsession, she admits. “Even as a child I would always look,” she has said. Po-faced experts have taken issue with her statement in You Are What You Eat that “floating stools that will not flush show a liver imbalance”. Such a diagnosis is impossible, dietitians intone.

But criticism is like water off a client’s back. McKeith claims she gets results and is totally honest. “People like that. I’m real.” Yet it is a matter of conjecture just how real McKeith and her credentials are.

She originally claimed to have a PhD from the American College of Nutrition, an institution based in Florida. Last year an ITN documentary revealed her “diploma” was gained from a distance-learning programme from Clayton College of Natural Health in Birmingham, Alabama, whose degrees are not recognised by the US secretary of education. Her PhD was from Edinburgh University, where she studied languages and international business.

On this basis she is singled out on Quackwatch, an American internet site run by a doctor, which notes: “From 2002 to 2004, one of her websites described her as ‘the world’s top nutritionist’,” and stated that she had “spent several years” studying at the college. “Its credentials are a reliable sign of someone not to consult for advice,” Quackwatch reported.

Despite being given several opportunities to put the record straight, McKeith has prevaricated. “My education is stellar,” she said recently. “I never stop learning. I’ve been on so many courses.”

But she was unable to name one from which she had benefited.

She has also been denounced as “a charlatan” by John Garrow, professor emeritus in human nutrition at London University, who said her approach was unscientific. In 2000, when she published Living Food for Health in which she claimed to have developed a “living food powder” to help patients inject the enzymes contained in raw foods, Garrow offered to pay her £1,000 if she would subject one of her treatments to a clinical test. He is still waiting.

Then there were her celebrity contacts. She boasted of having been the “healthy living expert” for The Joan Rivers Show, but a spokesman said “she was certainly not a regular contributor”, although she had appeared on a less high-profile show hosted by Rivers.

This casts doubts on McKeith’s claim that during her 15-year stay in America she gave advice to Michelle Pfeiffer, Demi Moore and Sylvester Stallone. According to her, Michael Jackson, Debbie Reynolds and Madonna subjected themselves to monthly colonic irrigation sessions on her advice.

So who is the real McKeith? In interviews she makes much play of growing up in a working-class family on a council estate, but even this has been queried. She was born in Perth in 1959, the daughter of a civil servant and an office worker. At the age of 12, she says, she had a vision of being on stage and talking to hundreds of people.

After Edinburgh University her account grows vague. She went to Spain and then America, where she married the student she had met at Edinburgh University. Together they hosted a Philadelphia radio show called Healthline Across America.

She had suffered from glandular fever, migraine and scoliosis (curvature of the spine), and thought she had a brain tumour. Her medical epiphany came on the show when a guest who could “see through the body” told her she merely had a vitamin deficiency. “She was like an angel. I remember saying to her: ‘Help me die nicely’.” Instead, she found mung beans and her fortune.

Moving to Britain and feeling messianic about nutrition, she was desperate to get on television, finally landing the Feel Fabulous Forever health slot on Richard and Judy’s This Morning.

She also briefly became a presenter of Anne Diamond and Nick Owen’s show Good Morning when Diamond fell sick. But her insistence that she become her permanent stand-in saw her frozen out. She recalled protesting: “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 set up a nutritional clinic near her home (either in 1992 or 1995, she guessed recently), which she admits she has neglected. According to her website she also launched McKeith Research Ltd in 1998 “for the purpose of launching organic living food supplements into the retail sector”. McKeith’s Living Food Love Bar, which claims to “nourish libido energy” and feed love organs for only £1.69, has been dismissed as “a myth” by Catherine Collins, chief dietitian at St George’s hospital in London.

The nutritionist remains unabashed. “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 for that. But, at the end of the day, my results are phenomenal.”

The irony is that she is doing more than anyone in this country to raise awareness about healthy eating. But the bottom line is that as long as questions persist, she will remain caught between two stools.