The confusion is understandable. Wheat, rye and barley contain both gluten and fructans, and many gluten-free staples, such as rice, corn, potato and quinoa, are also low in FODMAPs.[1] So when people feel better after cutting out bread and pasta, they often credit going gluten-free. Blinded trials suggest the fructans are usually responsible.[2]
What's the difference between gluten and FODMAPs?
Gluten is a protein found in wheat, rye, barley and some oats. FODMAPs (fermentable oligo-, di-, monosaccharides and polyols) are carbohydrates, and the main one in these grains is fructan.[1] They cause trouble in different ways. In celiac disease, gluten triggers an autoimmune reaction that inflames the small intestine.[1] FODMAPs draw water into the small bowel or are fermented into gas in the colon,[3] which causes symptoms in a sensitive gut, with no evidence that it damages the bowel.[4]
The trial evidence points at fructans. In 59 people who believed they were sensitive to gluten, with celiac disease ruled out, fructans caused more symptoms than gluten, and gluten caused no more symptoms than placebo.[2] In an earlier trial of people with self-reported gluten sensitivity, once their FODMAP intake was reduced, only 8% reacted specifically to gluten.[5]
Are gluten-free foods low FODMAP?
Not always, and the reverse isn't true either. Monash University, where the low-FODMAP diet was developed, gives these examples:[1]
| Food | Contains gluten? | FODMAP level |
|---|---|---|
| Regular wheat or rye bread | Yes | High (fructans) |
| Gluten-free products with added honey, pear or onion | No | Can be high, so check the label |
| Rice, corn, potato and quinoa | No | Low |
| Oats | Some oats do | Relatively low in a normal serving |
| Traditional spelt sourdough | Yes | Low to moderate |
| Soy sauce, wheat starch, wheat thickeners, barley malt | Yes | Not high in fructans; fine on a low-FODMAP diet |
| "Wheat-free" products made with rye or barley | Yes | Can be high |
So a low-FODMAP diet isn't gluten-free, and people with celiac disease can't treat it as one. Going strictly gluten-free when you only need a low-FODMAP diet means restricting more than you have to.[1] Read the ingredients: onion and garlic can turn up in marinades, sauces and gravies, gluten-free or not.[1] Tools such as IBS Clue estimate the fructan content of a meal from a photo, including hidden garlic and onion, but they're estimates, so check the ingredient list when you can.
Should I eat gluten-free if I have IBS?
Usually not. The British Society of Gastroenterology doesn't recommend a gluten-free diet for IBS,[6] and Monash reserves a strict, lifelong gluten-free diet for people diagnosed with celiac disease.[1] What you should do first is get tested for celiac disease. UK guidance is to offer celiac blood tests to adults with IBS,[7] and the American College of Gastroenterology suggests them for people with IBS and diarrhea.[8] The tests are accurate only while you're eating gluten. NICE advises eating some gluten in more than one meal every day for at least 6 weeks before testing, and not going gluten-free until a specialist has confirmed the diagnosis.[7] See IBS vs celiac disease.
Is the low FODMAP diet low in fiber?
It can be. Wheat, rye, barley and legumes are major fiber sources, so cutting them back risks lowering your fiber intake.[9] Some studies show fiber falls during the strict phase and others don't, and intake often improves once foods are reintroduced.[4] Many people with IBS start low anyway: at one London hospital, only 5% met the 30-gram daily fiber target before changing their diet.[10] To keep fiber up, lean on oats, quinoa, brown rice, potatoes with their skins, carrots, oranges, kiwifruit, nuts, chia seeds and small servings of canned chickpeas or lentils.[11][12] See what to eat with IBS-C for more.
Is the low FODMAP diet healthy?
As a short, supervised experiment, yes; as a permanent diet, it isn't meant to be. In a dietitian-led study, four weeks of low-FODMAP eating didn't change most nutrient intakes, but overall diet-quality scores were lower than in people eating their usual diet.[10] Restriction also lowers bifidobacteria, a group of gut bacteria considered beneficial.[13] Reintroduction seems to undo much of this. After reintroduction, patients' diets were nutritionally adequate,[14] and after a year on a personalized diet, bifidobacteria were back to their original levels, although some products of bacterial fermentation were lower.[15] That's why Monash recommends the strict phase for only 2–6 weeks.[1]
It isn't a diet to start alone if you have, or have had, an eating disorder, or for children, teenagers, or people who are pregnant or breastfeeding. These groups may need a gentler, dietitian-guided approach.[16]
Is the low FODMAP diet anti-inflammatory?
Not in any proven sense. IBS is a disorder of gut-brain interaction,[4] and the diet is designed to ease symptoms in a sensitive gut, not to treat inflammation. The clearest test comes from inflammatory bowel disease (IBD). In people with IBD in remission who still had gut symptoms, four weeks of a low-FODMAP diet improved symptoms but made no significant difference to markers of inflammation. It also lowered some gut bacteria thought to help regulate the immune system,[17] another reason not to stay strict longer than you need to. For how the two conditions differ, see IBS vs IBD.