Some people can’t eat gluten because their immune system treats it as a threat. The specific reason varies: it may be an autoimmune disease that damages the gut, an allergic reaction to wheat proteins, or a less understood sensitivity that triggers digestive symptoms. About 1% of the population in Western countries has celiac disease, the most well-defined of these conditions, and the majority of cases remain undiagnosed.
Celiac Disease: An Autoimmune Attack on the Gut
Celiac disease is the most serious and best-understood reason someone can’t eat gluten. It’s an autoimmune disorder, meaning the immune system mistakenly attacks the body’s own tissue. When someone with celiac disease eats gluten, a protein found in wheat, barley, and rye, their immune system launches an inflammatory response in the lining of the small intestine.
Here’s what happens at a cellular level: gluten is partially broken down in the gut, and some of those fragments interact with an enzyme in the intestinal wall. That enzyme chemically modifies the gluten fragments, making them appear far more threatening to the immune system. White blood cells then attack, releasing inflammatory signals that gradually destroy the tiny finger-like projections (called villi) lining the small intestine. These villi are responsible for absorbing nutrients from food. As they flatten and erode, the gut becomes less and less able to do its job.
This damage isn’t just a stomachache. Malabsorption of iron, folate, and vitamin B12 leads to anemia and chronic fatigue. Poor calcium absorption contributes to weakened bones and a higher risk of fractures. Children with undiagnosed celiac disease may experience stunted growth. Over years, the ongoing immune activation raises the risk of certain cancers, including a rare type of intestinal lymphoma. Long-term adherence to a gluten-free diet lowers the risk of all these complications.
Genetics Play a Major Role
Not everyone can develop celiac disease. It requires a specific genetic predisposition: nearly all celiac patients (about 98%) carry one or both of two immune system gene variants called HLA-DQ2 and HLA-DQ8. These genes code for molecules on immune cells that present gluten fragments to the rest of the immune system, essentially sounding a false alarm.
But having the genes doesn’t mean you’ll get the disease. Around 20% to 40% of the general population carries these same gene variants, and most of them eat gluten without any problem. Something else, likely an environmental trigger such as an infection, stress, or changes in gut bacteria, tips genetically susceptible people into active disease. In a mass screening of over 4,400 children, about 40% had the genetic predisposition, but only a fraction had celiac disease. Of those who did, 60% had never been diagnosed.
Wheat Allergy: A Different Immune Pathway
Wheat allergy is distinct from celiac disease, though both involve the immune system. In a wheat allergy, the body produces allergy-specific antibodies (IgE) against certain wheat proteins. The most allergenic of these are components of gluten itself, along with another protein called lipid transfer protein. When someone with a wheat allergy eats wheat, or in some cases exercises after eating it, these antibodies trigger a rapid allergic response that can include hives, swelling, breathing difficulty, or in severe cases, anaphylaxis.
Wheat allergy is more common in children and is often outgrown. It’s diagnosed through standard allergy testing, including skin prick tests and blood tests for specific antibodies. People with wheat allergy need to avoid wheat specifically but can often tolerate other gluten-containing grains like barley and rye, since the allergenic proteins differ between grains.
Non-Celiac Gluten Sensitivity
A third group of people feels genuinely unwell after eating gluten but tests negative for both celiac disease and wheat allergy. This condition, called non-celiac gluten sensitivity, remains poorly understood. There are no blood markers or tissue changes that identify it. The antibodies that flag celiac disease are, by definition, absent. Diagnosis currently relies on a cumbersome process: documenting that symptoms improve on a gluten-free diet, then return when gluten is reintroduced, ideally through a blinded challenge where neither the patient nor the clinician knows whether the test food contains gluten or a placebo.
Symptoms overlap heavily with irritable bowel syndrome: bloating, abdominal pain, diarrhea, fatigue, and brain fog. A positive diagnosis typically requires at least a 30% worsening of symptoms during the gluten challenge compared to the placebo period. Because this testing is complex and not widely available, true prevalence is unknown.
It Might Not Be Gluten at All
One of the most important findings in this area is that some people who believe they’re sensitive to gluten may actually be reacting to something else in wheat. A double-blind crossover trial of 59 people who had put themselves on gluten-free diets (and who did not have celiac disease) tested gluten, fructans, and placebo separately. Fructans are a type of fermentable carbohydrate found in wheat, onions, garlic, and other foods. The results were striking: fructans produced significantly worse symptoms than placebo, while gluten did not. There was no measurable difference between the gluten and placebo groups.
This suggests that for at least some people, the real culprit is a carbohydrate in wheat rather than the gluten protein. A low-FODMAP diet, which reduces fermentable carbohydrates including fructans, may be more effective than a gluten-free diet for these individuals. This distinction matters because a gluten-free diet is restrictive, expensive, and unnecessary if fructans are the actual trigger.
Gluten’s Effects Beyond the Gut
For people with celiac disease, the damage isn’t confined to the digestive system. Dermatitis herpetiformis is a skin condition directly caused by gluten. The same immune response that attacks the gut also produces antibodies that deposit in the skin, triggering intensely itchy, blistering rashes on the elbows, knees, and buttocks. The itching is often so severe that scratching destroys the blisters before they’re even noticed, leaving raw patches instead. Like intestinal celiac disease, dermatitis herpetiformis resolves on a strict gluten-free diet.
Gluten can also affect the nervous system in some celiac patients. Gluten ataxia causes problems with coordination and balance due to immune-mediated damage to the cerebellum. Joint pain, muscle aches, and neurological symptoms like numbness or tingling have all been documented in untreated celiac disease.
How Gluten Problems Are Diagnosed
If you suspect gluten is causing your symptoms, the most important thing to know is that you should not start a gluten-free diet before getting tested. Celiac disease testing requires that you’re actively eating gluten. Removing it first can cause false negatives and make diagnosis impossible without reintroducing gluten for weeks.
The standard first step is a blood test measuring antibodies against tissue transglutaminase (tTG-IgA), along with a check of total IgA levels since some people are naturally deficient and would show a false negative. If blood work is positive, the next step is typically a biopsy of the small intestine taken during an upper endoscopy, which looks for the characteristic villous damage. No single test is definitive on its own. Diagnosis relies on the combination of symptoms, blood markers, and biopsy findings.
For non-celiac gluten sensitivity, the process is essentially one of exclusion: celiac disease and wheat allergy must be ruled out first. Only then does a supervised elimination and rechallenge make sense.
Living Gluten-Free
For people with celiac disease, a strict gluten-free diet is the only treatment. “Gluten-free” on a food label means the product contains fewer than 20 parts per million of gluten, a threshold set by regulators to protect most people with celiac disease. In practical terms, that’s a trace amount, but even traces matter. Research has suggested that daily gluten intake should stay well below 10 milligrams to prevent ongoing intestinal damage, which is why cross-contamination from shared cooking surfaces, toasters, and fryers is a real concern.
The intestinal lining does heal on a gluten-free diet, though it can take months to years depending on the severity of damage. Most people notice symptom improvement within weeks. Bone density can improve over time, and the elevated cancer risk gradually decreases with sustained adherence. For people with non-celiac sensitivity, the diet doesn’t need to be as strict, and periodic reassessment makes sense since the condition may not be lifelong.