It’s hard to imagine a high-street fashion chain selling something that jokily referenced irritable bowel syndrome or a nut allergy. Yet last year, Zara brought out a T-shirt emblazoned with the slogan: “Are you gluten-free?” Understandably, coeliacs – those people who cannot process gluten – were furious over what they saw as a trivialising of their condition, and the chain was forced to apologise and pull the item. The implication was clear: gluten-free diets were a trend – something for the fashion conscious to buy into, rather than a medical necessity.

The many celebrities who avoid gluten for non-medical reasons don’t help: Victoria Beckham is said to follow a gluten-free diet because it helps her keep her weight down, while Gwyneth Paltrow writes in her book It’s All Good that “every single nutritionist, doctor and health-conscious person I have ever come across … seems to concur that [gluten] is tough on the system and many of us are at best intolerant of it and at worst allergic to it”. Miley Cyrus, who once labelled gluten “crappp”, is evangelical about the benefits of giving it up: “Everyone should try no gluten for a week. The change in your skin, physical and mental health is amazing.”

The wellness brigade is, of course, no big fan of gluten either – the Hemsley sisters describe it as a common “gut irritant”. Even the world of professional sport has forsaken the traditional carb-loading pasta parties. Most pro-cycling teams are now on “gluten-moderate” diets, and Novak Djokovic credits his rise to superstardom to a Serbian doctor who diagnosed a gluten sensitivity just by watching the tennis player on television, and then persuaded him to cut it out of his diet. He immediately felt better, Djokovic says: “I was lighter, quicker, clearer in mind and spirit … I could tell, the moment I woke up each morning, that I was different than I had been, maybe since childhood. I sprang out of bed, ready to tear into the day ahead.”

Facebook Twitter Pinterest Jasmine and Melissa Hemsley: no fans of gluten. Photograph: S Meddle/ITV/Rex/Shutterstock

In the foreword to Serve to Win, the gluten-free diet book Djokovic subsequently released to inspire the rest of us to follow suit, cardiologist Dr William Davis writes that modern varieties of wheat, quite apart from their propensity to contribute to everything from ulcerative colitis to schizophrenia, have “the potential to cripple performance, cloud mental focus and bring a champion to his knees”.

With press like that it’s little wonder that 8% of us report avoiding gluten as part of a healthy lifestyle, compared to just 5% of the British population that does so because of an allergy or intolerance. According to Mintel, 12% of new food products launched in the UK in 2015 were gluten free – Sainsbury’s almost doubled the number of products in its Freefrom aisle last autumn – while global sales of gluten-free foods jumped 12.6% in 2016 to $3.5bn. That is an awful lot of rice crackers.

But what is this bogeyman, this “modern poison”, as Dr David Perlmutter, author of the hugely successful, if widely derided, book Grain Brain, terms it? For all its terrifying reputation, gluten is nothing more than a couple of proteins found in wheat and other cereals that help give bread, pasta and so on their characteristic structure and texture. When dough is kneaded, the protein strands stretch out and interlink to form a strong, but elastic network that traps air and helps bread to rise. The reason that bread made from wheat flour is so much lighter and taller than, say, an all-rye loaf is that the gluten in wheat is particularly suited to the task.

About 1% of the UK population is thought to suffer from coeliac disease, a serious auto-immune condition (rather than the 5% with an intolerance or allergy) in which the body mistakes substances in gluten for threats, and attacks them, damaging the surface of the intestines and thus the ability to absorb nutrients. This is why, as well as abdominal pain and bloating, symptoms often include fatigue and unexpected weight loss. The disease appears to be becoming more common, although no one is sure why. If you are worried you might be affected, then consult your doctor as soon as possible, before it can do any more damage.

Facebook Twitter Pinterest Supermarkets have been quick to cater to the growing demand for ‘free-from’ products. Photograph: Alamy

Non-coeliac gluten sensitivity is more complicated, because although sufferers report many of the same symptoms as coeliacs, there are no specific diagnostic tests available. Anecdotal evidence suggests that many people feel better after giving up gluten, but there is a school of thought that points the finger of blame at other components that are removed from the diet at the same time: the catchily named fermentable oligo-, di- and monosaccharides and polyols, generally abbreviated to Fodmaps. These are also found in a variety of fruit, vegetables, beans and dairy products, and take some time for the gut to break down and process. In the meantime, they ferment, producing gas that can lead to bloating, flatulence and other unpleasant digestive complaints. Again, if you suspect this may be your problem, talk to a doctor before cutting anything out.

Then there are the rest of us – the 8% who avoid gluten because they believe it to be healthier for them, rather than because they have a specific intolerance. This grouping has helped to increase the availability of gluten-free options for the 5% of allergy or intolerance sufferers almost exponentially – while also, some believe, undermining the public perception of the seriousness of their condition. Although most of us could do with cutting back on our processed carbohydrate consumption, not only is there no evidence that removing gluten from the diet has any benefits for non-coeliacs – apart from its possible replacement by more vegetables and pulses – but some studies have shown that it may actually be a bad thing.

For a start, although the Hemsleys may advocate replacing grains with indisputably good things such as kelp pot noodles and black bean brownies, most of us reach for the kind of products that make up for any deficiencies in the gluten department with an extra helping of sugar, fat and salt. A cake is a cake, whether it’s made from wheat or not, and “gluten-free” is certainly not synonymous with “healthy”, as a recent study by the European Society for Paediatric Gastroenterology Hepatology and Nutrition discovered. After comparing 654 products to similar items containing gluten, it found that the gluten-free versions had a significantly higher fat content and were often less nutritious than their ordinary equivalents.

In fact, according to American research published in the British Medical Journal in May, avoiding gluten may actually lead to an increased risk of heart disease for non-coeliacs if it involves cutting out whole grains from the diet. Researchers concluded that “promotion of gluten-free diets among people without [coeliac] disease should not be encouraged”. Another, smaller, study published in the journal Epidemiology found that arsenic levels were almost twice as high among those on a gluten-free diet as those eating normally, while mercury levels were 70% higher in the gluten-free group. The authors speculated this could be because rice, a crop notorious for soaking up metals from its environment, is often used as a wheat substitute, but this is just a theory.

So, if you don’t have a medical issue around gluten, I implore you – on behalf of the thousands of people who would love to, but really can’t: please, shut up and have a crumpet.