If you have been told to avoid gluten, or you suspect that gluten is causing you problems, this article is for you. "Gluten-free" has become a fixture on menus. But behind those words sits a range of different conditions, and one of them — coeliac disease — is neither a preference nor a simple sensitivity. It is a chronic autoimmune condition that causes real damage to your gut every time you eat gluten. Understanding what is happening in your body, and why it matters, is what this post is about.
What coeliac disease actually is
Coeliac disease is an autoimmune condition. That means your immune system, instead of attacking a foreign invader, attacks your own tissue by mistake.
The trigger is gluten — the protein found in wheat, barley, and rye. When you eat gluten and you have coeliac disease, your immune system reacts to fragments of that protein and launches a defensive response. But instead of clearing a real threat, that response damages the lining of your small intestine.
Your small intestine is lined with tiny finger-like projections called villi. Their job is to absorb nutrients from food — vitamins, minerals, proteins, fats — and pass them into your bloodstream. When the immune attack flattens these villi, that absorption is impaired. The result is malnutrition: not because you are eating badly, but because your gut can no longer extract what it needs from what you eat [1].
This is different from both IgE-mediated food allergy and enzyme-based food intolerance, covered in Series 1. Coeliac disease involves specialised immune cells called T-cells, which are activated by gluten fragments and trigger the damaging response. It also has a genetic component: almost all people with coeliac disease carry gene variants called HLA-DQ2 or HLA-DQ8. Around 30–40% of the general population carry these variants, but only about 1% develop coeliac disease — so the genes are necessary but not enough on their own. Other factors, including gut microbiome composition and early-life exposures, are thought to determine whether the disease develops in people who are genetically susceptible [1].
How common is it — and how often is it missed?
Coeliac disease affects about 1% of the population worldwide [1]. In the UK, the estimated rate is similar, but more than 75% of people with the condition have never been formally diagnosed [2,3]. That is a large number of people with a condition causing damage to their gut who do not know they have it.
The reason so many cases are missed is that coeliac disease does not always look like what you might expect. The classic picture — diarrhoea, big weight loss, obvious malnutrition — is now comparatively rare. Most people with coeliac disease have symptoms that are subtle or easily put down to something else: persistent tiredness, anaemia, joint pain, or no gut symptoms at all [1]. Some people feel perfectly well day-to-day, right up until a significant gluten exposure causes a more obvious flare.
If you have been experiencing symptoms like these and have not been tested for coeliac disease, it is worth asking your GP. A blood test for tissue transglutaminase (tTG) antibodies is the usual first step, followed by an intestinal biopsy if the blood test is positive. It is important to keep eating gluten during this process — if you have already cut it out, the tests may come back negative even if you have the condition.
Why even tiny amounts matter
In IgE-mediated food allergy, the amount of allergen needed to trigger a reaction varies from person to person. In coeliac disease, the threshold for gut damage is consistently very low. A clinical trial found that 50mg of gluten per day caused significant measurable damage to the intestinal lining in coeliac patients over three months, while 10mg per day did not [4]. That makes 10mg the current lower bound of the established daily harm threshold — roughly the equivalent of a large breadcrumb.
This has practical consequences for how you eat. Coeliac disease is not just about leaving wheat out of the dish. Cross-contact matters at every stage: shared cooking water, shared utensils, shared preparation surfaces, shared frying oil, and even airborne flour in bakery environments can introduce enough gluten to cause damage. A gluten-free pasta dish cooked in water that was used for regular pasta is not actually gluten-free. A salad with a dressing containing a wheat-derived thickener is not gluten-free, even if the salad itself has no gluten ingredients [1].
What this means for eating out
When you eat at a restaurant and you have coeliac disease, you need a different level of care from someone who avoids gluten by choice or who has non-coeliac gluten sensitivity.
You need to know not just that a dish does not contain gluten ingredients, but that it has not come into contact with gluten during preparation. You need specific information about hidden sources of gluten — soy sauce, malt vinegar, modified starch, barley-based ingredients — that may not be obvious from a dish name or description.
The most useful thing a restaurant can give you is a menu that clearly states what is in every dish, broken down by component, with cross-contact information included. That is more useful than a "gluten-free" label on its own, because it lets you apply what you know about your own condition to make a real decision.
When you are checking a restaurant's allergen information, look for this level of detail. If the menu just says "GF" next to a dish name with no further information, that may not tell you enough. If it breaks down the ingredients and says how the dish is prepared, that is a better signal. And if you are not sure, asking to speak to the chef — specifically about cross-contact, not just ingredients — is always reasonable. You are not being difficult. You are protecting your gut from invisible damage.
The spectrum of gluten-related conditions
Not everyone who avoids gluten has coeliac disease. Understanding where you sit on this spectrum helps you know what level of care you actually need.
Coeliac disease requires strict, permanent gluten avoidance with careful attention to cross-contact. Even trace exposures cause intestinal damage, often without obvious symptoms. This is a medical diagnosis requiring monitoring.
Non-coeliac gluten sensitivity produces real symptoms in response to gluten — bloating, fatigue, brain fog — without the autoimmune intestinal damage seen in coeliac disease. People with this condition typically have a higher tolerance for trace amounts and can often rely on their own experience to gauge what they can handle.
Wheat allergy is an IgE-mediated immune response to wheat proteins specifically, with the potential for anaphylaxis. The concern is the presence of wheat protein, not gluten as such.
Gluten avoidance by choice involves no clinical condition and no medical threshold.
Knowing which of these applies to you changes everything about what you need from a restaurant: the questions you ask, the detail you look for, and how much trace exposure actually matters. If you have not been formally diagnosed but suspect you may have coeliac disease, getting tested is the single most useful thing you can do — because the answer shapes your entire approach to food.
Coming in Part 4: oral tolerance — how your immune system decides what is safe to eat, what happens when that decision goes wrong, and what emerging treatments might mean for your future.
References
Catassi C, Verdu EF, Bai JC, Lionetti E. Coeliac disease. Lancet. 2022;399(10344):2413–2426. DOI: 10.1016/S0140-6736(22)00794-2
Kurien M, Sanders DS. Determining whether coeliac disease case-finding in primary care is better than random testing: a retrospective study. BJGP Open. 2019;3(2):bjgpopen19X101648. DOI: 10.3399/bjgpopen19X101648
Nartey Y, Crooks C, West J, Card T, Tata L. The incidence and prevalence of coeliac disease in the United Kingdom. Ann Fam Med. 2023;21(Suppl 3):5051. DOI: 10.1370/afm.22.s1.5051
Catassi C, Fabiani E, Iacono G, D'Agate C, Francavilla R, Biagi F, et al. A prospective, double-blind, placebo-controlled trial to establish a safe gluten threshold for patients with celiac disease. Am J Clin Nutr. 2007;85(1):160–166. DOI: 10.1093/ajcn/85.1.160
