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An allergy can produce symptoms including swollen lips, an itchy throat, nausea and anaphylaxis, and can prove fatal. Food intolerances are more common, but less easily diagnosed. They are not life-threatening — symptoms may include headaches, bloating and nausea — and often require no form of dietary manipulation.

Whereas the cause of a food allergy is an abnormal immune response, the exact mechanism for most food intolerances remains unknown, says Jonathan Brostoff, professor of allergy and environmental health at University College London.

“Intolerances have many causes, none of which involves the immune system,” he says. “They can be the result of the lack of a required enzyme for digestion, as in lactose intolerance, the sensitivity of an individual or bad (toxic) foods.”

That said, a food intolerance can have nasty consequences. “For some people, food intolerances can be a nightmare,” says Muriel Simmonds, chief executive of Allergy UK. “But mostly they produce only mild symptoms such as bloating that other people might take for granted.

“While they are not all in the mind, the power of suggestion and persuasion certainly convinces some people that they have an intolerance when they don’t.” Simmonds cites the rise in high-street food intolerance tests as contributing to the boom in perceived food problems.

“Some, like Vega tests (available in some health food shops) and hair analysis, are not scientific and many prescribe highly restrictive diets that mean people cut out entire food groups,” she says. “Only the York Nutritional Laboratory test has so far been clinically proven to diagnose an intolerance related to irritable bowel syndrome; the others are a waste of time and money.”

Sarah Stanner, a BNF scientist, agrees: “We would agree completely with the American researchers that some people mistakenly believe they should cut foods from their diet.

“Allergies and intolerances have become quite fashionable, but the only accurate way to confirm an allergy is via free medical blood tests at a hospital on referral from your GP.”

Even if an intolerance is diagnosed, it may not require the removal of a food from the diet. In fact, says Catherine Collins, chief dietitian at St George’s Hospital, South London, the best way to deal with many cases of food intolerance is to build up a resistance by gradually eating more, not less, of the offending item.

“Research has proven that constant exposure to small amounts of a food helps to lessen the intolerance risk,” Collins says.

Stanner says cutting food groups from the diet can cause problems in itself. “Some people have seriously restricted diets because they cut out entire food groups. They can become deficient in vital nutrients like calcium.”

She adds that many of the symptoms often associated with an intolerance can usually be attributed to something else. “There is a huge psychological aspect to the rise in reported food allergies,” she says.

“People now assume, or are told in inappropriate tests, that something as common as a headache is linked to what they have eaten and they come to believe it. But it’s usually not the case.”

Collins adds: “People are generally very food-phobic these days, which has allowed their neuroses and the market for intolerance tests to blossom.”

So what of the persistent and often convincing claims that avoiding wheat relieves tiredness or cutting out on dairy results in less bloating and a flat stomach? “What often happens is that people ascribe their lack of certain, commonplace symptoms like fatigue, headaches or water retention to them having cut out a particular food,” Collins says.

“There is a placebo effect at play sometimes. They think it will change them for the better, so it does up to a point. Most people in the West eat too much wheat and enough dairy foods anyway, so cutting down will make them feel less bloated and may help them lose weight. But to say they have an intolerance to these foods is ridiculous.”

Similarly, avoiding foods to which people develop an aversion, through binge eating or food poisoning, will often lead to a sense of wellbeing “simply because they have learned to dislike them”.

According to Loftus, convincing people that their minds might be playing tricks when it comes to them having a food intolerance will not be easy. There is a misconception, she says, that our brains record everything in life with objective clarity. “We don’t like the idea that our memories are filled with bits of fiction,” she says. “But it’s true for everyone and we ought to accept it.”

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