For most people, gluten is simply a protein that passes through the digestive system without causing harm. But for roughly 1 in 70 people worldwide, gluten triggers an immune response that can damage the gut lining and eventually affect the bones, skin, brain, and other organs. Understanding what gluten actually does once you swallow it helps explain why it’s harmless for some and genuinely dangerous for others.
Why Gluten Is Hard to Digest
Gluten is a family of proteins found in wheat, barley, and rye. It’s made up of two main components: gliadin and glutenin. Together, they give bread dough its stretchy, elastic texture. But that same structure makes gluten unusually resistant to digestion.
Your stomach and small intestine break down most proteins into tiny fragments that can be absorbed safely. Gluten resists this process because it contains an exceptionally high percentage of two amino acids, proline and glutamine, and your digestive enzymes lack the ability to cut through proline-rich chains effectively. The result is that large, partially digested gluten fragments survive intact in the small intestine far longer than fragments of other proteins. In a healthy gut, these fragments pass through without consequence. In a genetically susceptible person, they set off a chain reaction.
The Immune Cascade in Celiac Disease
Celiac disease is an autoimmune condition affecting about 1.4% of the global population based on blood tests. It occurs in people who carry specific immune system genes known as HLA-DQ2 or HLA-DQ8, which are present in roughly 30 to 40% of the general population. Carrying these genes is necessary but not sufficient on its own. Something else, likely a combination of gut bacteria, infections, or environmental triggers, flips the switch.
Here’s what happens once the switch is on. Those large, undigested gluten fragments interact with an enzyme in the gut wall called tissue transglutaminase. This enzyme chemically modifies the fragments, converting certain amino acids from neutral to negatively charged. That change is critical: it makes the gluten fragments fit snugly into the HLA-DQ2 or DQ8 molecules on the surface of immune cells. These molecules essentially present the modified gluten to T cells like a wanted poster, activating an aggressive immune response directed at the lining of the small intestine.
Over time, this attack flattens the tiny finger-like projections (villi) that line the small intestine and are responsible for absorbing nutrients. As villi are destroyed, the surface area available for absorption shrinks dramatically.
How Gluten Affects Gut Permeability
Gluten also influences how tightly the cells of the intestinal lining are sealed together. A protein called zonulin acts as a gatekeeper for these seals, known as tight junctions. Gliadin, the component of gluten that drives immune reactions, binds to a receptor on intestinal cells and triggers zonulin release. When zonulin levels rise, the tight junctions loosen, allowing larger molecules to slip through the gut wall and into the bloodstream.
This increased permeability has been documented in people with celiac disease and, to a lesser degree, in people without it. In celiac disease, the leakier gut wall lets more gluten fragments reach immune cells beneath the surface, amplifying the inflammatory cycle. Whether this process causes meaningful problems in people without celiac disease is still being studied, but it’s one reason gluten gets attention beyond the celiac community.
Nutrient Deficiencies From Intestinal Damage
Because celiac disease destroys the absorptive surface of the small intestine, people with untreated or undiagnosed disease commonly develop nutritional deficiencies. The most frequent include iron, calcium, magnesium, zinc, folate, vitamin B12, vitamin D, niacin, and riboflavin. Deficiencies in copper and vitamin B6 also occur, though less commonly. Protein and calorie absorption can drop as well.
These deficiencies explain many of the symptoms people experience before diagnosis: fatigue (from low iron and B12), bone thinning (from poor calcium and vitamin D absorption), and neurological symptoms (from B vitamin shortfalls). Some of these deficiencies develop even when someone eats a nutritionally complete diet, because the problem isn’t intake, it’s absorption. Many people with celiac disease need supplementation even after starting a gluten-free diet, until the intestinal lining has had time to heal.
Long-Term Damage When Celiac Goes Untreated
Left untreated, the chronic inflammation from celiac disease reaches well beyond the gut. According to Mayo Clinic, long-term complications include skin rashes, lactose intolerance, infertility, bone weakness, and nerve damage. These problems often develop even in people who have no obvious digestive symptoms, which is one reason celiac disease is underdiagnosed. Some of the most advanced complications, particularly severe bone loss and infertility, may not be fully reversible even after adopting a strict gluten-free diet.
Gluten and the Nervous System
One of the lesser-known effects of gluten involves the brain and spinal cord. A condition called gluten ataxia occurs when antibodies produced in response to gluten cross-react with tissues in the cerebellum, the brain region that controls balance and coordination. Research published in The Lancet showed that this immune attack causes lymphocyte infiltration of the cerebellum, damage to the posterior columns of the spinal cord, and inflammation in peripheral nerves.
People with gluten ataxia typically experience progressive difficulty with balance, unsteady walking, and problems with fine motor control. The condition can occur with or without intestinal symptoms, which means someone could have significant neurological damage from gluten without ever experiencing digestive complaints.
Skin Reactions: Dermatitis Herpetiformis
Gluten can also cause a specific, intensely itchy blistering rash called dermatitis herpetiformis. This happens when the immune response to gluten produces antibodies (IgA) that deposit at the junction between the outer and inner layers of skin. These antibody deposits attract white blood cells called neutrophils, which release enzymes that break apart the structural proteins holding the skin layers together. The result is small, fluid-filled blisters that typically appear on the elbows, knees, buttocks, and scalp.
Dermatitis herpetiformis is considered the skin manifestation of celiac disease. Nearly everyone who has it also has some degree of intestinal damage, even if they feel no gut symptoms. The rash resolves on a strict gluten-free diet, though it can take months to fully clear.
Non-Celiac Wheat Sensitivity
Some people experience bloating, abdominal pain, fatigue, or brain fog after eating wheat but test negative for celiac disease and wheat allergy. This condition is often called non-celiac gluten sensitivity, but the name is somewhat misleading. Research now suggests the symptoms are frequently triggered not by gluten itself but by other components of wheat.
One major culprit is a group of proteins called amylase-trypsin inhibitors (ATIs), which plants produce as a defense against pests. ATIs activate a specific immune receptor on cells in the gut lining, triggering mild inflammation, disrupting tight junctions, and potentially contributing to symptoms beyond the gut, including allergies and metabolic problems. Another trigger is fructans, a type of carbohydrate in wheat that ferments in the large intestine and can cause gas, bloating, and discomfort in people sensitive to fermentable sugars.
Interestingly, research has shown that sourdough fermentation breaks down ATIs and reduces their inflammatory activity. This may partly explain why some people who react to conventional bread tolerate traditional sourdough products better, though this hasn’t been tested rigorously enough to serve as dietary advice for people with confirmed celiac disease.
Getting Tested Before Going Gluten-Free
If you suspect gluten is causing problems, getting tested before removing it from your diet is important. Celiac diagnosis relies on two main steps: a blood test that looks for elevated antibodies indicating an immune reaction to gluten, and often a follow-up endoscopy where a small tissue sample from the small intestine is examined for damage to the villi. Genetic testing for HLA-DQ2 and DQ8 can help rule celiac disease out, since the condition doesn’t develop without these gene variants.
The critical point is that these tests only work when you’re actively eating gluten. Cutting it out before testing can normalize antibody levels and allow the intestinal lining to begin healing, producing a falsely negative result. If you’ve already gone gluten-free and want accurate testing, you’ll typically need to reintroduce gluten for several weeks beforehand.
What Gluten Does in a Healthy Body
For the majority of people who don’t have celiac disease, wheat allergy, or wheat sensitivity, gluten passes through the digestive tract without triggering meaningful inflammation. The undigested fragments are either broken down further by gut bacteria or simply excreted. Whole grains containing gluten provide fiber, B vitamins, and minerals, and large population studies have consistently linked whole grain intake with lower rates of heart disease and type 2 diabetes. Removing gluten without a medical reason can lead to lower fiber intake and missed nutrients if gluten-containing grains aren’t replaced thoughtfully with other whole grains like oats, rice, or quinoa.