My doctor thinks I have coeliac disease. So why am I being told to eat gluten?

Coeliac disease (spelled celiac in the US) may show up first as troublesome gut symptoms, but its reach can be far wider. It sets off an unhelpful immune reaction that can affect the skin, the nerves, mood and energy, and over many years it raises the risk of some serious conditions. That is why staying gluten free matters so much for people who genuinely have it: this is far more than a food intolerance.

How it shows up varies a lot from person to person: gut signs such as reflux, loose stools, weight loss, stomach pains, bloating and constipation; slowed growth in children; fatigue; mouth ulcers; low vitamin levels; weaker bones; low mood and anxiety; nerve problems; anaemia; reduced fertility; and an intensely itchy skin condition called dermatitis herpetiformis.

Very often, by the time someone sees a doctor or a gut specialist, they have already solved it themselves: they have gone completely gluten free, and they feel better. So if this condition is serious and they feel well, why on earth would a doctor ask them to start eating gluten again before testing?

Because on a gluten-free diet, people who truly have coeliac disease can still test negative. Testing usually starts with a blood test called tTG-IgA, which looks for tissue transglutaminase antibodies. The laboratory should measure your total IgA at the same time, because some people are low in IgA altogether, and that alone can make the main test look negative when it is not. The blood test is often followed by an endoscopy, a camera test in which tiny samples of the small intestine's lining are examined under a microscope. Both depend on gluten being in the diet. Without it, a negative result is false reassurance, not an answer.

Guidance is specific about what the test needs: gluten in more than one meal, every day, for at least six weeks before testing, and kept up until all the tests are done, not just the blood test. American practice works the same way: a gluten challenge before testing, so the result means something.

Certain genes are strongly linked to coeliac disease, particularly the group known as HLA (human leukocyte antigen). The link is real, but current evidence says a genetic test alone cannot settle the question. Many people carry the genes and never develop the condition.

Fortunately, the serious consequences of unrecognised coeliac disease build up over long periods, so a short, deliberate spell of gluten before testing is what the guidance says the test needs. Do it alongside your doctor rather than on your own, and tell them first if you have ever had a severe reaction to gluten. The rough patch during the challenge, commonly loose stools, headaches and stomach pain, usually settles within a few days of testing. Two things can make it easier: keeping the rest of your diet lower in FODMAPs, and building the gluten up gradually day by day rather than starting at full tilt.

One important caveat: this applies to adults. Children are looked after along a different pathway, and some can be diagnosed without an endoscopy at all, so if the question is about your child, speak to their doctor or paediatric team before changing their diet in either direction.

If you are wondering about coeliac disease, do not quietly cut gluten first. Talk to your doctor, and get tested while the test can still tell you the truth.