Some swear by chewing 32 times to aid digestion, others stick to raw vegetables and fruit, many opt for high-protein diets in the form of fish, chicken and beef and a few proclaim the powers of grapefruit juice.

Whichever diet you follow, there is a good chance that it will be challenged tomorrow, when a leading doctor exposes the myths and fairytales surrounding some of the world's best-known food fads.

Professor Chris Hawkey, president of the British Society of Gastroenterology (BSG), will list more than a dozen famous diets when he addresses Gastro 2009, a major conference for doctors in London. They include "rawism", the grapefruit diet and the alkaline diet.

The chewing movement emerged in the 19th century with the claim that chewing each mouthful 32 times helped digestion.

"Gladstone was apparently very eccentrically in favour of this diet," said Professor Hawkey of the British prime minister, who died in 1898. "The idea is that salivary enzymes start digestion."

But like many other diets, it was based more on "theory than evidence".

As for the Hollywood grapefruit diet, which is based on the belief that the fruit contains an enzyme that breaks down fat and which Kylie Minogue is reported to have used, Professor Hawkey argued that the chemical is unlikely to even make it through the gut and into the body where it is meant to do its work.

"Food has been shrouded in myths and fairytales since time immemorial," he said, arguing that some people become "quasi-religious" about what they eat.

"But what's important is to recognise that, despite the popularity of fad diets, we are losing a grip on the fight with obesity."

His comments come as a survey by the BSG shows that one in five Londoners would turn to weight-loss pills to slim down. As for the Atkins Nutritional Approach, the famous diet that is low in carbohydrates and high in protein, one in five women would try it, but only 2 per cent believe it is healthy.

For Professor Hawkey, the diet is one of the few that carries at least a small amount of evidence. "The theory is that it resets the metabolic rate, and there is some science to back that up."

He argues that there is no harm in any diet that retains some nutritional balance and makes people lose weight.

Among the more balanced diets he will mention is one promoted by the nutritionist Esther Blum, who advocates eating full-fat foods in moderation to help metabolise cholesterol and to improve sex drive. Its fans include Sarah Jessica Parker and Teri Hatcher.

"I'm all for informed scientists and practitioners actually debunking some of the mythology around diets," said Andrew Hill, professor of medical psychology at Leeds University. "People are looking for quick-fix repairs, but in fact they are very rare, particularly in relation to being overweight.

"The idea that some new discovery or new way of combining food will give you an instant fix to your weight or health problem is nearly always misinformed.

"Health isn't immediately reparable; weight isn't immediately modifiable."