Nutritional therapists are giving "dangerous" advice to patients, including those with cancer, according to a Which? investigation.
The consumer group said therapists are providing "expensive dietary advice that could seriously harm patients' health".
It sent undercover investigators posing as patients to 15 consultations with nutritional therapists. All the nutritionists charged between £50 and £80 per visit.
The "patients" included a 46-year-old woman and another aged 40 recently diagnosed with ductal carcinoma in situ (DCIS), the most common type of non-invasive breast cancer. And a 56-year-old male and a 52-year-old woman complained of suffering serious fatigue for the last three months. The final patient, a female aged 31, told nutritionists she had been trying to conceive for more than a year.
From the 15 visits, six were rated as "dangerous fails" by a Which? expert panel, which includes a GP. A further eight were rated as "fails" and only one was deemed a "borderline pass".
One of the breast cancer sufferers was told by the nutritional therapist to delay the radiotherapy treatment recommended by her oncologist, saying they could rid the body of cancer through diet.
The therapist advised her to follow a no-sugar diet for three to six months, saying "cancer feeds off sugar". By cutting out sugar we have a better chance of the cancer going away". The expert panel concluded this was "highly irresponsible" and incorrect advice.
In another consultation, the woman trying for a baby was subjected to an examination of the iris of her eye and was told she had "a bit of bowel toxicity" and a "leathery bowel". The expert panel said these are both meaningless terms.
Several of the therapists also used tests with no evidence base to diagnose a range of symptoms, the investigation found. Some therapists recommended unnecessary high-dose supplements costing up to £70 a month, which the panel said could have side effects, such as stomach pain and diarrhoea.
Which? executive director, Richard Lloyd, said: "Our research shows that not only were they a waste of money, but some of their recommendations could seriously harm people's health. This is largely a self-regulated industry where anyone can set up and practice as a nutritional therapist, meaning there is no real protection for consumers."