Wheat and onions are the two biggest sources of fructans in most Americans’ diets, but these fermentable carbohydrates show up in a surprisingly wide range of foods, from rye bread and garlic to watermelon and cashews. Fructans are chains of fructose molecules that your body can’t break down in the small intestine. Instead, they travel intact to the colon, where gut bacteria ferment them. For most people that’s fine and even beneficial, but for those with irritable bowel syndrome or fructan sensitivity, this fermentation causes bloating, gas, and abdominal pain.
Why Your Body Can’t Digest Fructans
Fructans are carbohydrate chains made of fructose units linked together with a glucose molecule at the end. The key detail is the type of chemical bond holding them together. Human digestive enzymes simply cannot break these bonds. While a small amount of breakdown happens in the acidic environment of the stomach, the vast majority of fructans pass through the small intestine untouched.
Once they reach the colon, resident bacteria ferment them and produce short-chain fatty acids, which feed the cells lining your gut. This is why fructans are classified as prebiotics and why food manufacturers add them to products. But when someone is sensitive to this fermentation process, the rapid gas production and water movement into the colon trigger uncomfortable digestive symptoms.
Grains and Bread
Wheat is the single largest grain source of fructans for most people, simply because it appears in so many staple foods: bread, pasta, crackers, cereal, and baked goods. Wheat-based breads typically contain around 0.4 to 0.8 grams of fructans per 100 grams of dry weight, which adds up quickly across a day of sandwiches, toast, and snacks.
Rye contains significantly more. Pure rye breads from bakeries have been measured at 1.6 to 2.2 grams of fructans per 100 grams of dry weight, roughly two to three times higher than standard wheat bread. Barley also contains fructans, though typically at lower concentrations than rye. Spelt, another ancient wheat relative, carries fructans as well.
One practical finding: sourdough fermentation substantially reduces fructan levels. A conventional sourdough process lasting around 12 hours breaks down 65 to 70 percent of the fructans in wheat dough. Standard yeast-risen bread, by comparison, reduces less than 50 percent. So if you tolerate some fructans but not large amounts, a long-fermented sourdough may be easier on your gut than regular bread made from the same flour.
Vegetables With the Highest Levels
Onions, garlic, and their relatives are among the most concentrated vegetable sources of fructans. This family includes shallots, leeks, and the green (spring) onion. Garlic is particularly potent relative to its weight, which is why even small amounts can trigger symptoms in sensitive people.
Beyond the onion family, notable high-fructan vegetables include:
- Artichokes (especially Jerusalem artichokes, which are extremely high)
- Asparagus
- Beets
- Brussels sprouts
- Savoy cabbage
- Fennel
- Snow peas
Cooking does not eliminate fructans in the way sourdough fermentation does for bread. Because fructans are water-soluble, boiling vegetables and discarding the water may reduce levels somewhat, but sautéing or roasting keeps the fructans intact.
Fruits That Contain Fructans
Fruits are generally lower in fructans than grains or vegetables, but several contain enough to matter if you’re sensitive. Watermelon, grapefruit, nectarines, persimmons, plums, pomegranates, and ripe bananas all carry measurable fructan levels. The distinction between fructans and fructose matters here: many fruits are high in fructose (the single sugar) but low in fructans (the chain). Both can cause digestive trouble, but through different mechanisms, so knowing which one affects you helps narrow down problem foods.
Nuts and Legumes
Pistachios stand out among nuts as a food to watch. Even small amounts can cause symptoms in fructan-sensitive individuals. Cashews also contain fructans, though in moderate amounts. Portions of about 10 cashews (30 grams) are generally tolerated by most people managing their intake, while larger handfuls may push past the threshold.
Chickpeas and lentils contain fructans as well, but at levels that many sensitive individuals can manage in portions of about half a cup cooked. This makes them a useful protein source even on a reduced-fructan diet, as long as you watch the serving size. Other legumes like kidney beans and black beans contain related fermentable carbohydrates (galactans) but tend to be lower in fructans specifically.
Hidden Fructans in Processed Foods
This is where things get tricky. Food manufacturers widely use chicory root fiber, also labeled as inulin, as an added ingredient. Inulin is a fructan. It’s extracted primarily from chicory roots and added to products for three purposes: as a sugar replacement (shorter-chain inulin tastes mildly sweet), as a fat replacement (longer-chain inulin mimics creamy texture), and as a way to boost the fiber content listed on the nutrition label.
You’ll find inulin or chicory root fiber in protein bars, high-fiber cereals, yogurts, ice cream alternatives, fiber supplements, and “keto” or “low-sugar” snack foods. The FDA classifies chicory inulin as generally recognized as safe, and for most people it supports healthy digestion. But if you’re fructan-sensitive, a protein bar with 10 grams of added chicory root fiber can hit harder than a slice of wheat bread.
Check ingredient lists for: inulin, chicory root fiber, chicory root extract, oligofructose, and fructooligosaccharides (FOS). These are all fructans by different names.
Fructans, Not Gluten, May Be the Real Trigger
A notable 2018 double-blind crossover trial tested 59 people who had put themselves on a gluten-free diet because of digestive symptoms, after celiac disease had been ruled out. Over separate seven-day periods, participants ate bars containing either gluten, fructans, or a placebo, without knowing which was which. The results were striking: fructans produced significantly worse symptoms than gluten, particularly bloating. Twenty-four participants had their worst symptoms during the fructan challenge, compared to only 13 during the gluten challenge. There was no significant difference between gluten and placebo.
This matters because wheat contains both gluten and fructans. When people feel better on a gluten-free diet, they often credit the removal of gluten, but they’ve also removed one of the largest fructan sources in the typical diet. For some, it may be the fructans that were causing trouble all along. This distinction is relevant because gluten-free grains like rice, oats, quinoa, and corn are also naturally low in fructans, which explains why they feel safe.
Lower-Fructan Swaps
If you’re trying to reduce fructans without overhauling your entire diet, targeted substitutions work well. For grains, rice, oats, quinoa, corn, and potatoes are all low in fructans and can replace wheat in most meals. For the onion and garlic flavor that’s hard to give up, the green tops of spring onions (not the white bulb) are low in fructans, and garlic-infused oil provides the flavor without the carbohydrate, since fructans are water-soluble and don’t transfer into fat.
For vegetables, carrots, zucchini, bell peppers, tomatoes, cucumbers, spinach, and lettuce are all low-fructan options. Among fruits, strawberries, blueberries, oranges, and grapes are generally well tolerated. And for nuts, walnuts, pecans, and macadamias are safer choices than pistachios or large portions of cashews.
Portion size matters as much as food choice. Many moderate-fructan foods are fine in small servings but cause symptoms when you eat a large amount or stack multiple fructan sources in a single meal. Half a cup of cooked lentils with rice and low-fructan vegetables, for example, is a very different fructan load than lentil soup made with onions, garlic, and a side of rye bread.