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

Michael Eisenwasser 7 min read

"I think I'm gluten sensitive" is one of the most common things you'll hear from someone with unexplained gut symptoms. Bread makes them bloat. Pasta knocks them out. They try going gluten-free, they feel better, and the diagnosis sticks.

Here's the twist. A lot of those people aren't reacting to gluten. They're reacting to fructans, a carbohydrate that happens to live in wheat alongside gluten. When you cut wheat, you cut both. The gluten gets the credit, but the research keeps pointing at fructans.

This post walks through the difference, what the studies actually show, and how to test it on 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 different things that all look the same from the outside

From the outside, "bread makes me feel bad" can mean three very different things.

Celiac disease. An autoimmune disorder. Your immune system responds to gluten by attacking the lining of your 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 you're still eating gluten. If you go gluten-free first, the tests stop working.

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. Your 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. If fructans are your 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 actually 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. Then they were 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 in anyone 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.

If you have ongoing GI symptoms, especially with weight loss, anemia, chronic diarrhea, or a family history of celiac, see a doctor and get tested for celiac before changing your diet. Celiac testing requires you to be eating gluten. 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. If celiac is on the table, rule it out properly first, then come back.

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

If you've been cleared of celiac and wheat allergy, there are a few practical tests that can tell you whether you're actually reacting to fructans rather than gluten.

The onion-and-garlic test. If you're "gluten-free" but still eat onion, garlic, or large amounts of other high-fructan foods (artichokes, leeks, some beans), and you still have flare-ups, it's probably not 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). If regular bread wrecks you but a slice of a 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. You need the real thing, in a tested serve size.

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. If gluten-free products still cause bloating, your 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 lets you see which ones actually 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 if you go gluten-free without testing for celiac first, you've closed off your 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 so you end up with a personalized list of what actually triggers you, instead of a blanket restriction. Many people who thought 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

If you've never had celiac ruled out, do that first. Ask your doctor for the celiac antibody panel (tTG-IgA and total IgA at minimum), done while you're still eating gluten daily.

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

If you want a practical starting point on fructans specifically, 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.

The short version: if you feel better off wheat, don't assume gluten is the reason. Test it. The answer often isn't what you think.

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