McKeith was born in Perth, Scotland, in 1959 and grew up on a council estate. Her father was a civil servant and her mother an office worker. She has said that she was raised eating the junk food she now advises against; her father also smoked for many years and died of lung cancer at the age of 74.

She has scoliosis (curved spine) causing a visible hunching of her back.

According to the Mail on Sunday, "McKeith met her American husband, lawyer Howard Magaziner, in Edinburgh where he was spending a year studying. At the time he ran an extremely successful chain of health food shops in the United States with which she was to become involved. The couple now live in London and have two daughters, Skylar (born 1994) and Afton (born 2000). According to McKeith's Channel 4 biography, she was "celebrity health reporter" for the Joan Rivers Show in the U.S., but this claim is disputed by the Mail on Sunday. Her mission, according to her website, is to empower people to "improve their lives through information, food and lifestyle".

McKeith obtained a degree in linguistics from the University of Edinburgh in 1981, later moving to the United States, where she worked in marketing and international business. In 1984, she received a master's in international relations from the University of Pennsylvania. In 1994, she obtained a master's degree and, in 1997, a PhD, both in holistic nutrition via a distance-learning programme from the non-accredited American Holistic College of Nutrition, now the Clayton College of Natural Health in Birmingham, Alabama. McKeith is a member of the American Association of Nutritional Consultants. Her website lists post-graduate membership of The Centre for Nutrition Education and certificates from the London School of Acupuncture and the Kailash Centre of Oriental Medicine among her qualifications. Her website states that she "After two years of study and exams, recently achieved a Master Herbalist Diploma (Honors) from The American College of Healthcare Sciences (USA), successfully completing all requirements and examinations."

Physician Ben Goldacre, writing in the The Guardian's "Bad Science" column, speculated on rumours that parts of McKeith's PhD thesis may have been published as a 48-page pamphlet entitled "Miracle Superfood: Wild Blue-Green Algae" and ridiculed the pamphlet as Cargo Cult science full of "anecdote, but no data." Goldacre applied on-line to the American Association of Nutritional Consultants in the name of his dead cat and succeeded in getting the same "certified professional" membership as that possessed by McKeith. The price was $60. In 2004, professor emeritus of nutrition, John Garrow, questioned McKeith's credentials and her earlier claims to be "...a scientist doing research and studies."

When questioned by the Glasgow Herald about her doctorate, McKeith said: "I have nothing to be ashamed of. My qualifications are second to none. People out there would love to have my qualifications and expertise." Responding to criticisms of her degrees from Clayton College, she said: "I could have gone anywhere I wanted but I chose Clayton. There was cutting-edge research being put forward by people who were pioneers at the time." Her PhD thesis remains "unavailable", unlike PhD theses produced at accredited universities.

On 12 February 2007 it was reported that McKeith agreed to cease using the academic title "Dr" in advertisements. A spokesman for British Advertising Standards Authority (ASA), the UK's advertising industry's self-regulatory body, said: "The complainant was challenging whether Gillian McKeith was a qualified, accredited doctor. We put the complaint to the advertiser McKeith Research and they agreed to remove it." The Guardian reported that ASA had concerns that her use of the title "Doctor" was "likely to mislead the public." McKeith told The Guardian she understood the offending ad was a leaflet without the usual disclaimer she was not a medical doctor. She said she understood the honorific had to go from leaflets, but not from all adverts. Max Clifford, McKeith's PR representative, said that she had not misled the public: "This was one complaint in relation to one leaflet from one trade show, and it was withdrawn. I hardly think that's misleading."

is a popular author; her book You Are What You Eat reportedly sold just under one million copies up to August 2005, and was the most borrowed non-fiction library book in the UK between July 2005 and June 2006 according to the British Public Lending Right organisation. At her website she sells books, advice, club membership, food (e.g. Goji berries, hemp seeds, "Living Food Energy Powder", "Immune Defence" pills, weight loss pills, "Raw and unprocessed wild blue green algae", etc.), and accessory equipment (blender, juicers, sprouters, and a mini-trampoline).

In November 2006 McKeith was censured by the Medicines and Healthcare products Regulatory Agency (MHRA) for selling unproven herbal sex aids. The products, "Fast Formula Wild Pink Yam Complex" and "Fast Formula Horny Goat Weed Complex" were both advertised as having been shown to promote sexual satisfaction in a controlled "study". The MHRA found that McKeith was guilty of "selling goods without legal authorisation whilst making medicinal claims about their efficacy" i.e. advertising and selling unlicensed medicines. The products have since been withdrawn.

McKeith's website suggested the sex aids had been withdrawn "due to the new EU licensing laws regarding herbal products". According to McKeith, "the EU bureaucrats are clearly concerned that people in the UK are having too much good sex." The MHRA disagreed, according to Ben Goldacre: "The MHRA press office were very helpful and told me: 'This has nothing to do with new EU regulations.' And just to be absolutely clear: 'They were never legal for sale in the UK.' They also point out that there's no excuse for not knowing about the regulations, and that  the MHRA’s Medicines Borderline Section offers free advice on the phone." The MHRA said that "As Dr sic McKeith’s organisation had already been made aware of the requirements of medicines legislation in previous years there was no reason for all the products not to be compliant with the law."

McKeith was the presenter of You Are What You Eat, a Celador-produced television programme that was broadcast on Channel 4 until it was cancelled in 2007 in which she attempted to motivate the people featured in the programme to lose weight and change their lifestyle. Ian Marber, a nutritionist, describes her as very fervent in her beliefs and thinks of her as a sort of health televangelist. In each episode of the fourth series, called Gillian Moves In: You Are What You Eat, two people are chosen to stay with McKeith at a house in London "with no escape". She first shows each of the subjects a display of their typical week's food consumption. The food is laid out on a table in a cold, congealed and unpleasant state. McKeith often makes comments at this point about this high-fat diet putting the subjects at risk of an early death. The subjects are often videoed emptying the display into refuse sacks.

According to Jan Moir in The Daily Telegraph, she is seen "shouting at sobbing, fat women while forcing them to eat quinoa and undergo frequent sessions of colonic irrigation enthusiastically administered by her good self." She then imparts advice on diet and exercise and forbids alcoholic beverages. Once trained, the participants can return home and are expected to stick to their new regime of diet, exercise and abstinence for eight weeks. If they fail to stick to the diet, McKeith moves in with them to ensure they follow her advice. The participants are shown at the end of the eight weeks to have lost body mass, and say that they feel healthier.

She often attributes some of the featured clients' health problems to a vitamin or mineral deficiency. There are certain foods she considers to be particularly nutritious, and these are often mentioned in her programmes. These can be unusual foods, some of which are available only from healthfood shops or from McKeith's own range of products.

Asked about McKeith's advice, Amanda Wynne, senior dietician with the British Dietetic Association, said: "We are appalled. I think it is obvious she has not a clue about nutrition. In fact her advice, if followed to the limit, could be dangerous. Her TV programme takes obese people and puts them on a crash diet that is very hazardous to health." A spokesperson for Celador, the production company behind McKeith's television series, said that McKeith had never claimed to be a medical doctor, and that the criticism of her is reflective of her rejection of traditionalist approaches to nutrition: "You have to realise that when someone takes a holistic approach, there is always going to be an old school of traditionalists who are going to be sceptical and besmirch that. That's what's going on."

The Daily Mail reports that Lorna Slater took part in McKeith's programme in 2005, lost twenty-one pounds, but regained it afterwards. Slater said, "she demoralised me totally ... she gave me a balloon - put two drinking straws underneath it and said: "That's what you look like" — I ended up in tears. Then she gave me a completely unrealistic eating plan which involved very little meat or fish and lots of food that disagreed with my system like avocado — which makes me sick — and cucumber. I had to boil mung beans all day long, which took hours, made the flat smell horrible and tasted more like the gravel at the bottom of a fish tank than food. Although I tried hard and did lose weight, it was totally unrealistic."

In 2007, McKeith presented Three Fat Brides, One Thin Dress for Channel 4. The series is a competitive version of the You Are What You Eat format in which three women compete for a designer wedding dress. In addition to presenting her own TV shows, McKeith occasionally appears in other programmes: she competed in Battle of the Stars, singing her rendition of "The Shoop Shoop Song". She was the second act to be eliminated.

McKeith appeared in the first series of a health show transmitted on E4 called Supersize vs Superskinny.

In 2009 a programme ran on W Network in Canada entitled Eat Yourself Sexy, where McKeith employs the same practices as You Are What You Eat, except the (Canadian) participants are claiming a diminished sense of sex appeal or sex drive.

In her book You Are What You Eat, McKeith advocates a variety of diagnostic techniques not regarded as valid among those with medical qualifications. Among these are examination of the tongue, the mapping of pimples, and detailed scrutiny of faecal matter and urine. Many exterior parts of the body she says provide insight into illness: "I always think of the tongue as being like a window to the organs. The extreme tip correlates to the heart, the bit slightly behind is the lungs. The right side shows what the gallbladder is up to and the left side the liver. The middle indicates the condition of your stomach and spleen, the back the kidneys, intestines and womb."

McKeith also assesses people's nutritional needs based on the appearance of their nails, hair, lips, or skin. The presence of depression or PMS she also attributes to mineral deficiencies. McKeith maintains that the locations of pimples provide a means of locating the source of health problems. John Garrow says of McKeith's diagnostic abilities: "One of the programmes showed her prodding at the abdomen of a very large lady saying she could feel her intestines were inflamed. That is impossible. There is a large layer of fat between you and any intestines ... it would be like trying to guess what's under a mattress."

McKeith argues that the appearance, smell and consistency of faeces can give clues to bodily misfunction. She frequently engages in this activity during her television shows. The technique has led to Ben Goldacre dubbing her "the awful poo lady (TAPL)". Her management company, NCI, boasts that "in You Are What You Eat, Gillian literally gets to the bottom of some of the country's worst eaters."

McKeith's advice is based on both standard and non-standard medicine, including common sense tips, such as avoiding shopping when hungry, eating fruit and vegetables instead of cakes or buns, and preferring fresh fish over fish-fingers. She recommends a detox diet in which the "top 12 toxic terrors to avoid" are: smoking; caffeine; alcohol; chocolate and sweet snacks; pub snacks: crisps, nuts, pork scratchings; processed meat; the white stuff: white bread, white pasta, white rice; added sugar; lazy food: takeaways and ready meals; table salt; bad fats ... saturated fats; and fizzy drinks.

Dr Ben Goldacre writes in a Media Watch column in the British Medical Journal that he finds it offensive that the British media is "filled with people who adopt a cloak of scientific authority while apparently misunderstanding the most basic aspects of biology." As an example, he discusses McKeith's recommendation of eating darker leaves because they are rich in chlorophyll, pointing out her claim that it will "really oxygenate your blood" as erroneous.

Criticizing statements made by McKeith in 2007, independent charity Sense About Science said they 'had low expectations about some of the self-styled nutritionists but they appear to be straying more into medical advice Gillian McKeith has unfortunately extended her poor performance on nutritional science into the area of obesity'.[1]

McKeith's advice in her book Miracle Superfood: Wild Blue-Green Algae is disputed. Jan Krokowski of the Scottish Environment Protection Agency wrote a letter to New Scientist as a concerned expert, warning readers that "blue-green algae — properly called cyanobacteria — are able to produce a range of very powerful toxins, which pose health hazards to humans and animals and can result in illness and death."

In response, McKeith argues:

I am on a crusade to change the nation and fortunately, or unfortunately, that is going to put me in the limelight. But you cannot have change without a bit of resistance. They can try to attach stigma [sic] to me, but it will bounce off, back on to them. I refute [sic] anyone who is trying to bring me down. I'm proof that if you're trying to forge a new way ahead, you're going to ruffle a few feathers.

Many claims she presents, however, are disputed as lacking scientific evidence.

McKeith commenced legal action against The Sun over comments made about her in 2004. She has also threatened legal action against:

• Nutrition professor John Garrow, who questioned her nutrition research in a medical newsletter;
• Google, for linking to a critical web site;
• Dr. Dorothy King for writing on a blog called PhDiva commenting on claims by nutritionists and mentioning McKeith;
• Eclectech, a website, for hosting a humorous animation of her.

Discussing such cases in The Guardian, physician and journalist Dr Ben Goldacre criticised McKeith for using such legal threats to silence critics. His Book Bad Science dedicates a chapter to an analysis of McKeith's scientific credibility.

2010 Twitter row

In July 2010, the official Gillian McKeith Twitter account, @gillianmckeith, described Bad Science as "lies". After a response from Ben Goldacre and much 'tweeting' between various parties, @gillianmckeith said "do you really believe this is real Twitter site for the GM?" The Twitter account had previously been written in the first person, as if by McKeith herself, and in June 2010, carried a message stating it was McKeith's official Twitter account. Various individuals pointed out that the account is linked to from the official McKeith website. That link was then deleted, but it remained in the source code for the page. Other official parts of McKeith's web presence, including her YouTube page, also link to the Twitter account.

Goldacre has described the "lies" accusation as "actionable", i.e. that he could sue for libel, and requested a correction through the Twitter account.

In May 2005, McKeith was given the Best Organic Businesses 2005 Consumer Education Award by the Soil Association, a British charity promoting organic food and standards, in recognition of her work in "tackling obesity, championing healthier eating and promoting the contribution that organic fruit, vegetables and other products can make to sound nutrition."