FODMAP Intolerance vs. Gluten Sensitivity: How to Tell Which One You Have

Gluten sensitivity is the common label people reach for when bread and pasta make them feel bad. They cut wheat, feel better, and the diagnosis sticks. A large share of those people aren't reacting to gluten at all. They're reacting to fructans, a carbohydrate that happens to live in wheat alongside gluten. When you cut wheat, you cut both, and the gluten gets credit the research keeps pointing toward fructans.

This post walks through the difference, what the studies show, and how to test it for yourself. One thing up front: celiac disease is a real autoimmune condition, and that part of the story is not something to self-diagnose.

Three conditions that look the same from the outside

"Bread makes me feel bad" can mean three distinct things.

Celiac disease. An autoimmune disorder in which the immune system responds to gluten by attacking the lining of the small intestine. Johns Hopkins Medicine describes it as an autoimmune disease in genetically predisposed people, where gluten exposure damages the villi (the tiny absorptive bumps in the small intestine). Long-term, untreated celiac increases risk of nutrient deficiencies, bone disease, and other autoimmune conditions. Diagnosis involves blood antibody testing followed by an endoscopic biopsy of the small intestine, and it has to be done while gluten is still in the diet. Going gluten-free first makes the tests unreliable.

Non-celiac gluten sensitivity (NCGS). A diagnosis of exclusion. Celiac and wheat allergy have both been ruled out, but the person still reports feeling better without gluten-containing foods. The Celiac Disease Foundation describes the symptom picture: bloating, abdominal pain, diarrhea, brain fog, headaches, fatigue. The mechanism is unclear. Researchers have proposed several candidates, and one of them is that the culprit isn't gluten at all.

FODMAP intolerance. The gut reacts to fermentable carbohydrates like fructans, lactose, and polyols. Wheat is one of the biggest sources of fructans in the typical Western diet. Onion and garlic are among the most fructan-dense common ingredients. When fructans are the trigger, wheat-based meals and onion-or-garlic-heavy meals tend to be what expose it, especially at larger portions. Gluten has nothing to do with it.

The first one needs a doctor. The other two are where things get interesting.

What the research shows

Two studies changed how researchers think about "gluten sensitivity."

Biesiekierski et al., 2013, Gastroenterology. Patients who self-identified as non-celiac gluten sensitive were first put on a low-FODMAP diet for two weeks. Their symptoms improved significantly. They were then re-challenged with gluten, low-dose gluten, or placebo in a blinded trial. No specific or dose-dependent gluten effect showed up. The symptom improvement tracked with FODMAP reduction, not gluten removal.

Skodje et al., 2018, Gastroenterology. A double-blind crossover trial gave self-reported gluten-sensitive patients either fructans, gluten, or placebo hidden in muesli bars. Fructans produced significantly more GI symptoms than gluten did. Gluten, in this study, wasn't meaningfully different from placebo.

Monash University (the team that originally built the low-FODMAP diet) summarizes the practical takeaway on their blog: for people with IBS who think gluten is the trigger, the evidence suggests fructans are often doing the actual work.

None of this means gluten can never cause symptoms outside celiac. NCGS is still an active research area, and some people do seem to react to gluten or to other wheat components like amylase-trypsin inhibitors. It does mean the default assumption ("I feel bad after bread, so I must be gluten sensitive") skips past a more likely explanation.

Celiac disease is not in this conversation

Before going further, a line in the sand.

Ongoing GI symptoms, especially with weight loss, anemia, chronic diarrhea, or a family history of celiac, warrant a doctor visit and a celiac test before any diet change. Celiac testing requires gluten in the diet. A gluten-free or low-FODMAP trial done first will make the blood tests and biopsy unreliable.

Untreated celiac is not a lifestyle problem. It's bowel damage, malabsorption, and increased risk of osteoporosis, infertility, and certain cancers. The treatment is a strict, lifelong gluten-free diet. Low-FODMAP is not a substitute.

Nothing in this post is a reason to skip that workup. When celiac is on the table, rule it out properly first, then come back.

How to tell for yourself (once celiac is ruled out)

After celiac and wheat allergy have been cleared, a few practical tests can point toward fructans rather than gluten.

The onion-and-garlic test. Someone "gluten-free" who still eats onion, garlic, or large amounts of other high-fructan foods (artichokes, leeks, some beans) and still has flare-ups is probably not reacting to gluten. Fructans are in all of those, at higher concentrations than in most bread.

The sourdough test. Traditional, slow-fermented sourdough contains gluten but can be lower in fructans, because the fermentation process breaks some fructans down. Monash has tested specific sourdough breads as low FODMAP at specific serving sizes. Outcomes depend on the recipe, the starter, and the fermentation time, so sourdough is not automatically low FODMAP (and spelt, which is a wheat variety, isn't automatically low FODMAP either). When regular bread wrecks you but a slice of real sourdough sits fine, that pattern points at fructans, not gluten. Note: "sourdough" at most US grocery stores is flavored bread made with commercial yeast and no real long fermentation. The real thing, in a tested serve size, is what matters.

The gluten-free junk food test. A lot of gluten-free packaged foods (cookies, crackers, breads) still use inulin, chicory root fiber, or other high-fructan ingredients as texture replacements. This is exactly what reading food labels on low FODMAP is designed to catch. When gluten-free products still cause bloating, the culprit is probably fructans hiding in the "healthy fiber" ingredient.

The low-FODMAP trial. The most decisive test. A structured elimination + reintroduction protocol isolates each FODMAP group, including fructans, and reveals which ones cause symptoms. This is the approach Biesiekierski and Skodje used in their studies, and it's what Monash recommends for people whose gluten story doesn't quite add up.

Why this matters

Going gluten-free "just in case" sounds harmless, but it has real downsides.

Gluten-free packaged foods are often lower in fiber and higher in refined starches and added sugars. Long-term gluten-free eating without a medical reason has been linked to lower whole-grain intake, which matters for cardiovascular and metabolic health. And going gluten-free without testing for celiac first closes off the ability to get an accurate diagnosis without a months-long gluten challenge.

A low-FODMAP trial, by contrast, is time-limited. The elimination phase runs 2 to 6 weeks. The reintroduction phase tests FODMAP groups one at a time and ends with a personalized list of real triggers, instead of a blanket restriction. Many people who assumed they were gluten sensitive find they can eat sourdough bread, regular pasta in moderate portions, or wheat outside of garlic-and-onion-heavy meals. The real trigger was somewhere else the whole time.

What to do next

When celiac has never been ruled out, that comes first. Ask a doctor for the celiac antibody panel (tTG-IgA and total IgA at minimum), done while gluten is still in the daily diet.

After celiac is ruled out and GI symptoms continue, a low-FODMAP trial is a more evidence-based starting point than an open-ended gluten-free diet. The elimination phase will show quickly whether fermentable carbs are involved. The reintroduction phase will show which specific ones.

For a practical starting point on fructans, our post on garlic walks through why it's one of the highest-fructan foods in a normal diet and what to use instead. Garlic is where most people notice the fructan pattern first.

For recipe ideas that skip high-fructan ingredients without going gluten-free, see our low-FODMAP recipes. Most of them use wheat in forms that are tolerable at the portion sizes Monash has tested.

When wheat makes you feel bad, gluten is not the only candidate. A structured low-FODMAP elimination and reintroduction protocol will show which carbohydrate group is driving the symptoms. FODMAP Tracker handles the food logging and symptom timestamps so you can see whether fructans, lactose, or another group is actually the driver.

Track your symptoms and discover patterns with FODMAP Tracker. Includes a database of 1,000+ foods with FODMAP ratings.