When someone with celiac disease eats gluten, their immune system attacks the lining of the small intestine. This isn’t a food sensitivity or an upset stomach that passes quickly. It’s an autoimmune reaction that damages the finger-like projections (called villi) responsible for absorbing nutrients from food. Symptoms typically appear 48 to 72 hours after eating gluten, and the internal damage can persist long after those symptoms fade.
What Happens Inside Your Body
Gluten is a protein found in wheat, barley, and rye. In people with celiac disease, a specific enzyme in the gut modifies fragments of gluten in a way that makes them highly visible to the immune system. These modified fragments bind tightly to certain genetic markers (carried by virtually all people with celiac disease) and trigger specialized immune cells into action.
Those immune cells release inflammatory signals that damage the intestinal lining. At the same time, the immune system produces antibodies against both gluten and the enzyme that modified it. This is why blood tests for celiac disease look for those specific antibodies: their presence confirms the immune system is actively reacting to gluten. The entire process is self-reinforcing. As long as gluten keeps arriving in the gut, the immune attack continues.
Symptoms You Might Feel
Because celiac disease is a delayed immune reaction rather than an immediate allergic one, symptoms usually show up two to three days after exposure. The most common include diarrhea, bloating, abdominal pain, fatigue, and weakness. Some people experience constipation instead of diarrhea, which can make the condition harder to recognize.
The severity varies enormously from person to person. Some people feel violently ill after a small crumb of bread. Others have mild or even no obvious digestive symptoms, which doesn’t mean the damage isn’t happening. So-called “silent” celiac disease still causes intestinal injury and nutrient loss even when you feel fine. Mouth sores are another underrecognized symptom, often linked to low levels of iron, folic acid, or B12.
Damage to the Small Intestine
The small intestine is lined with millions of tiny, finger-like projections called villi that increase its surface area for absorbing nutrients. In celiac disease, repeated gluten exposure erodes and flattens these villi. Doctors grade this damage on a scale from 0 (healthy villi) to 4 (completely flat, atrophied villi). At the severe end, the intestinal lining looks cracked under a microscope, with patches of destroyed villi visible across the surface.
This flattening substantially reduces your body’s ability to absorb food as it passes through. Even a single exposure won’t just cause symptoms; it restarts the inflammatory process and can set back healing that was already underway. On a strict gluten-free diet, the small intestine typically heals in 3 to 6 months, though some people take longer. The villi do grow back and resume normal function, but only if gluten stays out of the picture.
Nutrient Deficiencies From Malabsorption
Damaged villi can’t do their job, so nutrients pass through the gut without being absorbed. Iron deficiency is the most common result, affecting anywhere from 12 to 69% of people with untreated celiac disease, and it frequently causes anemia. Vitamin D and calcium malabsorption weaken bones over time. Folic acid deficiency shows up in 20 to 30% of newly diagnosed adults, and B12 deficiency affects 8 to 41%.
The list goes further. In studies of untreated celiac disease, 88% of patients had low vitamin E levels, 71% had low vitamin B1, and about one in five had low vitamin K. Vitamin A deficiency, found in roughly 7.5% of newly diagnosed patients, can cause night blindness and other eye problems. Deficiencies in B-complex vitamins, copper, and selenium contribute to nerve damage, which helps explain many of the neurological symptoms associated with the disease. In rare cases, severe deficiencies in niacin or vitamin C have caused pellagra or scurvy.
Neurological Effects
Celiac disease doesn’t only affect the gut. Gluten exposure can trigger autoimmune damage to the nervous system, and this can happen even in people with minimal intestinal symptoms.
The most common neurological complication is gluten ataxia, a condition where the immune response damages the cerebellum, spinal cord, and peripheral nerves. This leads to problems with balance, coordination, and walking. Up to 70% of people with gluten ataxia also develop slurred speech, involuntary eye movements, and difficulty controlling their limbs. It mostly affects women, with a mean age of onset around 48.
The second most common neurological issue is peripheral neuropathy, which causes pain, numbness, tingling, and burning, usually starting in the hands and feet. The median age of onset is 55. Cognitive impairment, headaches, and epilepsy have also been linked to celiac disease, and these symptoms are often slow to develop, making them easy to miss as gluten-related.
Long-Term Risks of Continued Exposure
If someone with celiac disease keeps eating gluten, whether intentionally or through repeated accidental exposure, the consequences extend well beyond digestive discomfort. Ongoing intestinal damage leads to progressive bone loss that can become irreversible. Infertility is another recognized complication, and in some cases, it cannot be reversed even after starting a gluten-free diet.
The most serious long-term risk is lymphoma. People with celiac disease have roughly 2.8 times the general population’s risk of developing this type of cancer. The distinction that matters most is whether the intestine has healed. Patients with persistent villous atrophy (ongoing intestinal damage) face an annual lymphoma risk of about 102 per 100,000, compared to 31.5 per 100,000 in those whose intestines have healed. Among patients with healed intestines, the increased risk is small enough that it doesn’t reach statistical significance. In other words, sticking to a gluten-free diet brings the cancer risk close to normal.
How Much Gluten Causes Damage
It takes remarkably little. Research shows that as little as 50 milligrams of gluten per day, roughly the amount in a small crouton, is enough to cause measurable intestinal damage. Doses of 200 milligrams or more clearly induce gut abnormalities. In some studies, even 1.5 milligrams per day triggered symptoms in sensitive individuals, though doses around 34 to 36 milligrams per day did not cause detectable changes in most people.
This is why food labeling matters. In the U.S., the FDA defines “gluten-free” as containing less than 20 parts per million of gluten, a threshold used internationally because it’s the lowest level that can be reliably measured with validated testing methods. Most people with celiac disease tolerate foods at or below this level without harm. But gluten hides in unexpected places: soy sauce, beer, communion wafers, some medications, and foods processed on shared equipment. Accumulated small exposures throughout the day can add up past the threshold that triggers damage.
Accidental Exposure vs. Ongoing Exposure
A single accidental exposure, like eating something at a restaurant that contained hidden gluten, will likely make you feel terrible for a few days. It restarts the inflammatory process and may cause a flare of symptoms. But one incident generally won’t undo months of healing, especially if you return immediately to a strict gluten-free diet.
Regular or repeated exposure is a different story. Even if individual doses are small, consistent gluten intake prevents the villi from recovering, maintains the cycle of inflammation and nutrient malabsorption, and raises the risk of the serious complications described above. The intestine can’t heal while the immune attack is ongoing. This is why people who are “mostly” gluten-free but allow occasional cheating often have persistent villous atrophy on follow-up biopsies, even if their day-to-day symptoms feel manageable.