They are also criticised for failing to refer people to their GP when they complain of medical conditions.
A team of undercover researchers posing as patients went to 15 nutritional therapists complaining of the symptoms of irritable bowel syndrome (IBS).
Anyone can set themselves up as a nutritional therapist, also known as diet therapists, nutrition counsellors or nutritionists, and they are often confused with state-registered dieticians who are fully trained and regulated.
The advice the mock "patients" were given by the nutritional therapists was based on general healthy eating advice and was not tailored to the needs of people with IBS, reports Health Which? magazine.
Some dietary advice was simply wrong, such as recommending carrot juice as a good source of calcium.
And products were recommended or sold at 12 of the consultations.
These included vitamin and mineral supplements, digestive enzymes, linseed oil, blue green algae and psyllium husks, most of which are not considered useful for IBS.
In one case, a "patient" was sold "Aerobic Oxygen" at £16.95, which experts said would contain no more oxygen than a glass of standard water.
'Inadequate questioning'
The report also criticises inadequate questioning and note-taking, and unproven tests, including iridology, which involves examining the markings and colours of the eyes to indicate health.
Sue Freeman, assistant editor of Health Which?, said: "Be wary of therapists who don't encourage you to see your GP if you have medical problems, or don't attempt to tailor the advice to suit your needs."
Dr Wendy Doyle, spokeswoman for the British Dietetic Association, said she was not surprised by the findings of the report.
She said: "We have been aware of this problem for some time and we get calls from people who are worried about the advice they are given.
"The general public should be made aware of the difference between people who have done a full degree course and who are externally regulated and those who are not.
"They should always look for a state-registered dietician."
Code of conduct
State-registered dieticians would not give therapy advice without a referral from a GP and they would report back to a doctor afterwards, she added.
A spokeswoman for the British Association of Nutritional Therapists said its members were subject to a strict code of conduct.
They should refer patients with a medical complaint to a GP and they must not be affiliated to any firm selling supplements or other products.
She added: "It is obviously difficult to police each consultation but members are under these guidelines.
"If the patient felt he or she had an issue for redress, they could contact BANT."