Ssuperdiets, which call for drinking large amounts of grapefruit juice or eating only raw fruit and vegetables, are based on just food myths, an expert has said.
Professor Chris Hawkey, president of the British Society of Gastroenterology (BSG), said some people developed a "quasi-religious" attitude towards what was the best thing to eat, based on little or no scientific evidence.
He highlighted more than a dozen famous diets including rawism, which argues that cooking food makes it less nutritious.
The grapefruit diet, based on the idea that an enzyme in the juice breaks down fat, and the alkaline diet, which seeks to maintain the slightly alkaline nature of the blood by eating certain foods.
"Food has been shrouded in myths and fairy tales since time immemorial," the Telegraph quoted Professor Hawkey as saying at the Gastro 2009 conference, which is being held in London until November 25.
"But what's important is to recognise that, despite the popularity of fad diets, we are losing a grip on the fight with obesity," he said.
He said the grapefruit diet, which Kylie Minogue has reportedly used, was unlikely to have an effect because the enzyme would probably be broken down in the gut before being able to get at body fat.
Professor Hawkey also flagged up the lack of evidence for the 'chewing movement', which dates back to the 19th century and counsels chewing 32 times to aid digestion.
"[Former Prime Minister] Gladstone was apparently very eccentrically in favour of this diet. The idea is that salivary enzymes start digestion," he said, adding that it was based more on "theory than evidence".
However he had mixed feelings about the controversial Atkins diet, which says people should avoid carbohydrate and eat protein.
"It is not terribly healthy in the sense that you are going to have a lot of fat, but if you lose weight then it is a good thing," Professor Hawkey said.
"The theory is that it resets the metabolic rate and there is some science to back that up," he added.