Is It Gluten, or Is It Something Else?

You gave up bread for two weeks and the bloating eased. Pasta went next, then the cereal, and by the end of the month you were telling people at lunch that gluten is a problem for you. Nobody asked how you knew, because nobody ever asks.

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The experiment felt clean, since you changed one thing and something happened. What you changed was four or five things at once, and one of them was the only item on the list that a doctor can actually test you for.

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What leaves the plate when the bread does

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Gluten is a protein found in wheat, barley, and rye. Those same grains carry fructans, a kind of carbohydrate that your small intestine does not absorb well, and fructans belong to a group of sugars called FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols). Monash University in Australia, which did the original work on them, lists the first letter of that acronym this way: "Oligosaccharides / Fructans & GOS - found in foods such as wheat, rye, onions, garlic and legumes/pulses."

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Monash describes what happens next in three steps: "When FODMAPs reach the small intestine, they move slowly, attracting water. When they pass into the large intestine, FODMAPs are fermented by gut bacteria, producing gas as a result. The extra gas and water cause the intestinal wall to stretch and expand." Those three steps happen in everybody. The difference in someone with a sensitive gut is how much the stretching hurts.

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Most of the fructans in an American diet come from one place. A USDA survey took two days of diet recall from more than 15,000 people and reported that "Major food sources of naturally occurring inulin and oligofructose in American diets were wheat, which provided about 70% of these components, and onions, which provided about 25% of these components." Teenage boys and adult men ate the most, around 3.5 grams a day.

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So the sandwich was carrying at least two suspects, and cutting it removed both of them on the same afternoon.

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The one with a blood test

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NIDDK, the federal institute that handles digestive disease, defines celiac disease as "a chronic digestive and immune disorder that damages the small intestine," set off by eating gluten. The immune system attacks the gut lining, the lining stops absorbing nutrients properly, and the damage keeps going as long as gluten keeps arriving. The American College of Gastroenterology puts it at "nearly 1% of residents of the United States."

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In someone your age it does not always look like a stomach problem. NIDDK's list of what celiac does to a still-growing body includes "damage to the permanent teeth's enamel," "delayed puberty," "slowed growth and short height," and "mood changes or feeling annoyed or impatient." Outside of the digestive tract it shows up as fatigue, joint or bone pain, canker sores, and an itchy blistering rash called dermatitis herpetiformis. Plenty of people get diagnosed after a blood test for anemia (low iron) comes back odd, with no gut complaint at all.

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Diagnosis runs in two steps: a blood test for antibodies your immune system makes when it is attacking the gut, then a biopsy, where a doctor passes a camera down and takes a few tissue samples from the small intestine. There is also a genetic test, which is mostly useful backwards. About 30 percent of people carry the gene variants called DQ2 or DQ8, and only about 3 percent of those people ever develop celiac disease, so a positive result means very little. A negative one closes the case: "If you do not have these gene variants, you are very unlikely to have celiac disease."

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Left alone, the damage accumulates. NIDDK lists the long-term complications as "accelerated osteoporosis or bone softening," anemia, malnutrition, nervous system problems, and reproductive problems. Most people carrying it have no idea: the Celiac Disease Foundation says celiac "is estimated to affect 1 in 100 people worldwide, but only about 30% are properly diagnosed."

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The blood test stops working when you stop eating gluten

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The antibodies the test looks for only show up while your immune system is reacting to something. Take gluten away and they fade, the gut lining starts to heal, and the test comes back clean whether or not you have the disease.

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NIDDK puts the question on its own page and answers it in one word: "Should I start a gluten-free diet before I talk with my doctor? No. If you think you might have celiac disease, you should talk with your doctor about testing to diagnose celiac disease before you begin a gluten-free diet. If you avoid gluten before you have testing, the test results may not be accurate." The American College of Gastroenterology makes it a formal recommendation at its highest grade, using "serologic" to mean blood testing: "All diagnostic serologic testing should be done with patients on a gluten-containing diet. (Strong recommendation, high level of evidence)" The UK's NICE guidance gives an amount, telling patients "to eat some gluten in more than 1 meal every day for at least 6 weeks before testing."

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Most people do it in the opposite order. Researchers surveyed 555 high school students at two schools in Sicily, average age 17. Seven of them had diagnosed celiac disease, 12.2% said that wheat caused them problems, and of those 68 students, 15 had mentioned it to a doctor and nine had ever had the celiac blood test. Nine out of 68.

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If you already quit, getting an answer means going back. The process is called a gluten challenge, and the ACG describes the standard version as "a diet containing at least 10 g of gluten per day for 6 to 8 weeks," while admitting in the same sentence that "there are few data to indicate the diagnostic efficacy of this approach or the optimum dose or duration of challenge." A shorter version may work: the guideline notes a study where patients who could only tolerate 3 grams a day showed diagnostic changes "after as little as 2 weeks of gluten ingestion." Coeliac UK tells patients to "include at least 3g but preferably 6g of gluten every day for at least six weeks before testing and throughout the diagnosis process." The short version costs you accuracy: by the ACG's reading of the same study, "a 2 week gluten challenge may yield false negative results in 10% of patients."

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Weeks of deliberately eating the thing you removed is a miserable assignment, and the guidelines say so. The ACG notes that "patients who develop severe symptoms following gluten ingestion are not suitable candidates for gluten challenge." NICE tells doctors that when someone cannot face the reintroduction, refer them anyway, and "explain that it may be difficult to confirm their diagnosis by intestinal biopsy."

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The diagnosis you get by ruling things out

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Suppose the celiac test is negative and the symptoms are still there. The label for that is non-celiac gluten sensitivity, and it is the only one of the three that has no test of its own.

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A 2025 review in The Lancet puts the two numbers side by side: "Although approximately 10% of adults worldwide self-report gluten or wheat sensitivity, meta-analyses suggest that, during controlled challenge studies, 16-30% of these individuals have symptoms specifically triggered by gluten." Its verdict on diagnosis: "Until causative agents are identified and diagnostic tests developed, NCGS remains a diagnosis of exclusion, requiring careful systematic evaluation."

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Allergists say the same thing in blunter language. The American College of Allergy, Asthma & Immunology: "A gluten intolerance is not an allergy, and there are currently no tests for accurate diagnosis." Same page, on a phrase you have heard a hundred times: "There is no such thing as a gluten allergy, but there is a condition called Celiac Disease."

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There is a research protocol for confirming it, written by a group of experts who met in Salerno, Italy. You eat gluten-free for at least six weeks to establish that it helps, go strictly gluten-free for four more, then take 8 grams a day of gluten for a week, then a week of nothing, then a week of a fake version, with you and the researchers both blind to which is which. A 30% difference in your symptoms between the real week and the fake week counts as positive. The authors say plainly that "the threshold of 30% increment in symptoms is somewhat arbitrary and needs scientific validation." Almost nobody who says they have gluten sensitivity has been through that protocol.

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What happens when nobody knows which week is which

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Two researchers pooled ten double-blind trials covering 1,312 adults and found that "only 38 of 231 NCGS patients (16%) showed gluten-specific symptoms. Furthermore, 40% of these subjects had a nocebo response." A nocebo response is a placebo in reverse, where you expect something to hurt you and then it does.

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An Australian group went back and checked its own work. Its 2011 trial had found a gluten effect, so in 2013 the same team ran the study again with 37 people, first dropping everyone's FODMAP intake for two weeks and then adding back gluten, whey protein, or neither: "Gluten-specific effects were observed in only 8% of participants," and symptoms got worse to about the same degree on gluten and on whey.

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The muesli bars in an Oslo trial contained gluten, fructans, or nothing, a week at a time in random order, and the 59 people eating them were all on gluten-free diets they had put themselves on. Fructans beat gluten on overall symptoms and on bloating, and the count of who reacted to what went like this: "Thirteen participants had the highest overall GSRS-IBS score after consuming gluten, 24 had the highest score after consuming fructan, and 22 had the highest score after consuming placebo." More people had their worst week on the fake bar than on the real gluten.

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In 2024 a European team split the question in half, randomly assigning 84 people to be told they were eating gluten or not, and separately to actually eat it or not. Being told gluten was in the bread and eating it scored 16.6 mm on a symptom scale. Eating real gluten while believing it was gluten-free scored 6.9, and eating nothing while believing it was nothing scored 7.4, so gluten nobody had been warned about was indistinguishable from no gluten at all. The authors are careful: "The combination of expectancy and actual gluten intake had the largest effect on gastrointestinal symptoms, reflecting a nocebo effect, although an additional effect of gluten cannot be ruled out."

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Both sides of the argument have money in it. That expectancy trial was funded largely by baking and wheat-industry groups, and the Salerno meeting in the previous section was funded by Dr. Schär, a gluten-free food company.

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The case for gluten that hasn't gone away

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An Italian team gave 61 adults gluten inside of gelatin capsules, 4.375 grams a day for a week, against rice starch. Capsules carry no fructans, so the confusion is removed by design. Gluten came out significantly worse than placebo overall, and specifically on bloating, abdominal pain, foggy mind, depression, and mouth ulcers. Every one of those p-values sits between 0.019 and 0.047, which is barely inside the usual cutoff for calling a result unlikely to be chance.

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Tissue samples complicate it further. When 78 people who had already reacted to wheat on a blinded challenge were biopsied and compared against 55 people who had not reacted, the reactors had more immune cells in both the small intestine and the rectum, including eosinophils, a white blood cell associated with allergic reactions. The authors suggested eosinophils "could serve as a biomarker." Seven years later, the 2025 Lancet review still says there is no biomarker, so nobody has been able to build a test out of it.

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There is also a candidate that is neither gluten nor fructan. Wheat contains amylase-trypsin inhibitors, natural pest-resistance proteins, which a 2012 paper found could switch on immune responses in human cells and in mice. It has not been tested in people with gluten sensitivity, and the paper's own verb is cautious: ATIs "may fuel inflammation." Whatever they do, they leave your kitchen at the same moment that gluten and fructans do.

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Gluten-free bread has fructans in it too

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Researchers at King's College London measured the fructan content of ordinary and gluten-free breads sold in the UK. Regular breads ran 0.61 to 1.94 grams per 100 grams, rye being the highest. Then: "Surprisingly, gluten-free bread contained similar quantities of fructan (1.00 g/100 g) as other breads," with individual gluten-free brands ranging from 0.36 all the way to 1.79. Manufacturers add chicory root and inulin to gluten-free products for texture and fiber, and both are fructans, so swapping the loaf does not reliably swap out the thing the Norwegian trial pinned the symptoms on. Someone who goes gluten-free and still bloats has a straightforward explanation available.

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That outcome is common enough to show up in the search box. If you type "gluten free but still" into Google, the completions it offers are "gluten free but still bloated," "still having symptoms," "still have diarrhea," "still gassy," and "still constipated," in that order.

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The British Society of Gastroenterology is blunt about it in its guideline on irritable bowel syndrome: "A gluten-free diet is not recommended in IBS (recommendation: weak, quality of evidence very low)." The same guideline states the fructan hypothesis outright, saying the benefit of a gluten-free diet is "in the main, not due to the removal of gluten per se, but rather a reduction in dietary fructan content, which is a FODMAP, resulting from wheat exclusion." The American College of Gastroenterology says it in its irritable bowel syndrome guideline too: gluten sensitivity in IBS patients "is believed to be a nonimmunologically mediated event and possibly even an adverse reaction to the nondigestible, nonabsorbable carbohydrate, fructan."

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One more suspect gets named a lot and does not answer the question in the title. A USDA chemist went looking for evidence that wheat breeding raised the gluten content of American wheat and reported that "the results do not support the likelihood that wheat breeding has increased the protein content (proportional to gluten content) of wheat in the United States." He did find that consumption of vital wheat gluten, the concentrated form that manufacturers add to bread, appears to have tripled since 1977, and said it is hard to know what to make of it. Neither finding tells you what is happening in your own gut.

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The organized way to test fructans is the low-FODMAP diet, which runs two to six weeks of restriction and then adds foods back one group at a time to find out which ones actually bother you. It is covered in How an Elimination Diet Works, including why the restriction phase is temporary. The ACG recommends "a limited trial of a low FODMAP diet in patients with IBS" and grades its own recommendation "conditional recommendation; very low quality of evidence." And Monash, which built the diet, gates it: "If a medical doctor has not diagnosed your gastrointestinal symptoms, you should not be following this diet. There are many conditions with symptoms that are similar to IBS, such as coeliac disease, inflammatory bowel disease, endometriosis and bowel cancer. You should not self-diagnose yourself with IBS."

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Wheat allergy runs on a different clock

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A wheat allergy is your immune system treating wheat protein as a threat, the same machinery behind peanut and shellfish allergies, and it can land within minutes of a bite. The list runs hives, an itchy rash, swelling of the mouth or throat, a stuffy nose, trouble breathing, cramps, vomiting, and at worst anaphylaxis, which the ACAAI describes as something that "can occur within seconds or minutes, can worsen quickly and can be deadly."

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A survey of more than 40,000 US adults estimated that about 2.0 million of them have a wheat allergy. Most people who have it as children lose it, and the ACAAI puts that at "about 65 percent of children with a wheat allergy will outgrow it by the time they are 12."

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If food consistently makes your lips swell, your throat tighten, or your skin break out in hives, that is not a digestion question and it does not belong in a two-week diet experiment. See a doctor quickly.

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What to do, in order

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Keep eating gluten until you have been tested. Every source in this article agrees on that one, for the plain reason that the test measures a reaction which stops when the exposure stops.

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Ask for the celiac blood test by name. It is a tissue transglutaminase IgA test, usually written tTG-IgA, along with a total IgA level to make sure your body makes enough of that antibody for the first test to mean anything. For teenagers with diarrhea-type symptoms, the request is routine. The ACG guideline for irritable bowel syndrome carries it as a strong recommendation: "We recommend that serologic testing be performed to rule out celiac disease (CD) in patients with IBS and diarrhea symptoms."

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Expect IBS to be diagnosed on purpose rather than by elimination. The same guideline recommends "a positive diagnostic strategy as compared to a diagnostic strategy of exclusion," which means a careful history and a small number of tests instead of a year of scans ruling things out one at a time.

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If celiac comes back negative and you still react, test the fructans properly. Run a structured low-FODMAP trial with a dietitian, keep it time-limited, and do the reintroduction phase, because the reintroduction is the part that tells you which foods actually matter. Wandering into permanent restriction on your own gets you a smaller diet and no information.

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Know what a lifetime gluten-free diet costs if you do not need one. A Columbia University team priced a basket of gluten-free products against their regular versions across four kinds of store and five US regions and found them "183% more expensive than their wheat-based counterparts." An Austrian analysis of 63 gluten-free products found "the protein content of GF products is >2 fold lower across 57% of all food categories." NIDDK, citing a 2015 review in the Journal of the American Academy of Physician Assistants, says there is "no evidence that a gluten-free diet promotes better health or weight loss for the general population."

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Watch how the restricting feels. Monash's own dietitians warn that diets like this one can lead people to "develop black-and-white thinking about certain foods" and to "avoid eating out and social occasions around food for fear of eating trigger foods." A four-week trial measured lower levels of bifidobacteria, one of the bacterial groups fed by the fibers you would be cutting, and a review of the same diet reports "a decrease in the proportion of patients meeting the recommended intake for calcium," which matters more at 16 than it will at 40. If the rules are growing and the symptom they were solving is not, stop and tell somebody.

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When to get checked sooner

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NHS guidance says to be tested for celiac disease if you have any one of these: gut symptoms that persist and have no explanation, faltering growth, fatigue that will not lift, weight loss you did not intend, severe or persistent mouth ulcers, unexplained iron, B12, or folate deficiency anemia, type 1 diabetes, or autoimmune thyroid disease. Testing is also recommended if a parent, sibling, or child has celiac disease.

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Add the ones that apply to any gut complaint: blood in your stool, black tarry stool, pain bad enough to wake you up, vomiting you cannot stop. Those belong to a doctor this week, not to a diet experiment. The longer warning-sign list is in What's Wrong With My Gut?.

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Keep the bread until somebody takes blood

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Most people who feel better without wheat did have something in that food bothering them. The label they reached for was a guess with four candidates standing behind it.

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One of those candidates damages your intestine for as long as you keep eating it, has a blood test that costs almost nothing, and becomes much harder to find the longer you have been avoiding bread. Getting that one ruled in or out first is worth two more weeks of sandwiches, and it is the only step here you cannot take later.

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