Fructan — Intolerance, Food Sources, Health Benefits, & More
Introduction
Fructans are soluble fibers found in many plant-based foods; they are non-digestible oligosaccharides or polysaccharides. As oligosaccharides, they are part of the FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols) and are similar to fructose and lactose.
They consist of heterogeneous fructose polymers (fructose units) and a glucose molecule and serve as storage carbohydrates in nearly 15% of flowering plants.
These compounds exhibit a range of health-promoting properties but cause gastrointestinal symptoms such as gas and diarrhea in intolerant individuals.
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Fructan Types & Food Sources
Based on their structural bonds, fructans are classified into several groups (1, 2):
- Inulin: Inulin-type fructans are the most studied type and are primarily found in chicory root, Jerusalem artichoke, dahlia, garlic, agave, and dandelion.
- Levan: They are found in grasses such as ryegrass and bluegrass.
- Graminan (mixed levan): They are commonly found in wheat, rye, barley, and oats.
- Inulin neoseries: They are found in plants of the Liliaceae family.
- Levan neoseries: They are considered rare and likely present only in oats.
Fructans are also classified into fructooligosaccharides (FOS) and oligofructans (OF) according to their degree of polymerization (2).
Fructans are mainly found in several plant families: the daisy family (e.g., daisy and chicory), the amaryllis family (e.g., amaryllis, garlic, onions, chives, and leeks), the asparagus family (e.g., asparagus and agave), grasses (e.g., corn, oats, wheat, spelt, barley, and rye), the lily family (e.g., aloe vera), and the Musaceae family (e.g., bananas) (3).
The table below shows the calculated values of fructans of various foods per 100g fresh weight (4).
| Food | Fructan levels per 100g |
| Kidney beans, boiled | 0.54g |
| Haricot beans, boiled | 0.26g |
| Lima beans, boiled | 0.29g |
| Lentils, boiled | 0.14g |
| Split peas, boiled | 0.73g |
| Textured soy protein (TSP) | 1.1g |
| Soy yogurt, plain | 0.14g |
| Wheat germ | 2.5g |
| Jerusalem artichoke | 16-20g |
| Garlic | 9.8-16g |
| Onion | 1.1-7.5g |
| Leek | 0.5-7.1g |
| Asparagus | 2-3g |
| Banana | 0.7g |
| Rye bread (sourdough) | 1.1g |
| Whole wheat bread | 0.68g |
| Button mushrooms | 0.27g |
| Kimchi | 3.9-4.1g |
| Wakame flakes | 0.04g |
Another study calculated fructan values in foods per 100g fresh weight (5).
| Vegetables | Fructans per 100g | Legumes | Fructans per 100g |
| Onion | 1.97g | Canned peas | 0.33g |
| Carrot | 0.11g | Green beans | 0.03g |
| Tomato | 0.01g | Canned beans | 0.24g |
| Lettuce | 0.01g | Fruits | Fructans per 100g |
| Bell pepper | 0.02g | Banana | 0.11g |
| Cauliflower | 0.09g | Apple | 0.09g |
| Broccoli | 0.02g | Orange | 0.21g |
| Cabbage | 0.03g | Grape | 0.06g |
| Leek | 0.24g | Pear | 0.13g |
| Zucchini | 0.01g | Tangerine | 0.11g |
| Grain products | Fructans per 100g | Nuts | Fructans per 100g |
| White bread | 0.14g | Almond | 0.60g |
| Wheat flour | 0.75g | Hazelnut | 0.73g |
| Rolled oats | 0.32g | Walnut | 0.37g |
Fructans are also present in burdock, raisin, Indian shot, palm, purple yam, potato, couscous, arrowroot, beet, Brussels sprouts, and aloe vera (6, 7).
Absorption & Metabolism
Human digestive enzymes cannot hydrolyze or break down fructans; instead, they are fermented by the colonic microbiota to produce short-chain fatty acids (SCFAs), lactic acid, and hydrogen (H2), lowering the pH of the colonic environment and making it more acidic.
SCFAs (acetate, propionate, and butyrate) are the primary metabolic by-products produced by bacterial fermentation of dietary fiber. They have anti-inflammatory and antioxidant effects, beneficially affect the immune system and metabolism, and regulate appetite and energy intake (1, 2, 8).
Additionally, it was found that fructans with a higher degree of polymerization (DP) are more likely to be fermented distally in the large intestine (1).
Fructan Intolerance
As mentioned above, fructans are fermented by the colonic microbiota and produce hydrogen and SCFAs. Additionally, they draw more water into the intestinal lumen. Only about 5-15% of fructans may be absorbed in the small intestine.
Individuals developing symptoms after fructan ingestion are considered fructan intolerant. The produced gas and bulking effects of fructans can result in bloating, abdominal distension, abdominal cramps/discomfort, flatulence (gas), and diarrhea in such individuals (11).
Fructan intolerance is more common in individuals with functional gastrointestinal (GI) disorders, such as irritable bowel syndrome (IBS); these individuals commonly benefit from low-FODMAP diets, including diets low in fructans. According to data, around 24% of IBS patients may develop GI symptoms after fructan intake. IBS patients with unexplained symptoms and negative for fructose and lactose intolerances and SIBO, a common finding during IBS, may test positive for fructan intolerance (11).
Fructan Intolerance & Fructose Intolerance
Fructan and fructose are both considered high FODMAP and lead to similar symptoms; however, they are different types of carbohydrates: fructose is a monosaccharide, whereas fructans are oligosaccharides.
High-fructose foods include the majority of fruits, especially fruit juices, canned and dried fruits (e.g., prunes, raisins, and dates), artichoke, asparagus, broccoli, chutney, leeks, mushrooms, okra, onions, peas, red pepper, shallots, most tomato products, wheat products, many marinated or processed meat products, and flavored milk and other yogurts (11).
Fructan Intolerance or Other GI Disorders?
Many gastrointestinal disorders, such as small intestinal bacterial overgrowth (SIBO), lactose intolerance, celiac disease, and non-celiac gluten sensitivity, may lead to similar symptoms.
Therefore, a healthcare professional is essential for a proper diagnosis and dietary restriction.
Health Benefits of Fructans
Prebiotic, Antioxidant & Immunomodulatory Effects
One of the main health benefits of fructans is linked to their prebiotic effects; fructans stimulate the growth of intestinal beneficial or probiotic bacteria, such as Lactobacillus and Bifidobacterium species, and decrease the number of pathogenic bacteria, such as Clostridium, thus improving the composition of the intestinal microbiota (1, 2, 7).
Moreover, fructans with higher DP values and in greater amounts are likely to have stronger probiotic effects (1).
Maintaining the normal composition of gut bacteria is essential for maintaining homeostasis in a healthy person. Gut bacteria provide the host with nutrients, metabolize indigestible compounds, help defend against harmful pathogens, help develop intestinal architecture, and stimulate the local immune system. The host controls gut microbiota composition by secreting antimicrobial peptides and immunoglobulins.
Prebiotic consumption is linked to modulated local and systemic immune parameters: got-associated lymphoid tissue or GALT, secondary lymphoid tissues, and peripheral circulation. GALT distinguishes between harmful and harmless agents, protects against infections, and avoids generating hypersensitivity reactions to gut bacteria and harmless antigens (2).
Another mechanism by which fructans promote the immune system is linked to the production of SCFAs. SCFAs reduce the growth of some pathogenic bacteria and increase mucin production. They also lead to the activation of G protein-coupled receptors (GPR), recruitment of white blood cells (WBC) to inflammatory sites, and suppression of the production of proinflammatory agents. Butyrate, a SCFA, decreases the requirement of glutamine for epithelial cells and beneficially alters gene expression (2).
Interestingly, recent animal studies proposed the role of fructans as ROS (reactive oxygen species) scavengers, suggesting that inulin may act as a barrier and protect the colonic mucosa against oxidative stress. Additionally, SCFAs have been reported to induce antioxidant enzyme levels and reduce NADPH oxidase in the colon (2, 7).
The Effects of Fructans on Health & Disease
- Anti-diabetic effects.
Consumption of fructans may significantly reduce mean fasting blood glucose levels, improve insulin tolerance, and suppress insulin response to foods (2, 7, 9, 10).
- Improved lipid metabolism and parameters
Fructan consumption may lead to decreased triglyceride and cholesterol levels and decreased liver, epididymal, inguinal, and subcutaneous fat tissue. It is also linked to decreased weight gain (2, 7, 10). Moreover, inulin and oligofructose may inhibit triglyceride synthesis, leading to decreased cholesterol levels (10).
- Constipation relief
Fructans, such as inulin, are soluble dietary fibers that draw water into the intestines and act as a bulking agent, softening stool and relieving functional constipation symptoms in some individuals. The effective daily dose of inulin has been studied to be 15-20g (2, 7).
- Improved mineral uptake
Fructooligosaccharides (FOS) are studied to stimulate fractional calcium and iron absorption (2, 9).
- Regulated gut peptides and satiation
Fructans may increase glucagon-like peptide-1 and glucagon-like peptide-2 concentrations, reduce ghrelin levels, and induce satiety (2, 7, 9).
- Body weight and energy intake reduction by the induction of satiety (2).
- Decreased risk of tumors and cancers
Fructans are linked to a decreased risk of cancers, predominantly gastrointestinal, due to prebiotic effects, SCFAs, and enhanced responses for IFN-γ on CD8+T cells. Additionally, inulin may enhance the anti-tumor efficacy of immunotherapy with PD-1 inhibitors (7, 9, 10).
- Decreased risk of bad breath
According to a study, inulin decreases the tongue pH and stimulates oral acidic microbiota, thus reducing the chances of halitosis or bad breath (7).
Applications of Fructans
Fructans, mostly inulin and inulin-type, are widely used in several industries. They are used as fat and sugar substitutes and in tequila production.
Fructans and galactooligosaccharides are major components in forming prebiotic foods on the industrial scale.
Inulin enhances the sensory formulation of foods by increasing their size and texture. It also protects foods from browning during heating, prevents food drying, and lowers the freezing temperature of foods.
Fructans can be used to produce high fructose syrup instead of starch or sucrose (sugar). DFAs, which are synthesized from inulin, may be used as a low-calorie sugar alternative.
Interestingly, inulin-containing plant waste may be used in biofuel and co-product production (7).
Summary
Fructans are soluble fibers found in many plants. The primary sources of fructans are chicory, Jerusalem artichoke, garlic, dahlia, agave, dandelion, wheat, rye, barley, oats, ryegrass, and bluegrass.
Human digestive enzymes cannot break down fructans; instead, they are fermented by the colonic microbiota to produce SCFAs, lactic acid, and hydrogen (H2). Fructans stimulate the growth of probiotic bacteria and decrease the number of pathogenic bacteria, thus improving the normal composition of the intestinal microbiota and showing immunomodulatory effects.
Fructans exhibit a range of health-promoting properties, including prebiotic, antioxidant, immunomodulatory, anti-inflammatory, anticancer, and antidiabetic properties; they also improve constipation symptoms and lipid metabolism, regulate gut peptide levels, and aid in weight loss.
Individuals developing symptoms after fructan ingestion are considered fructan intolerant. The produced gas and bulking effects of fructans can result in bloating, flatulence (gas), abdominal cramps or discomfort, and diarrhea in such individuals.
Sources
- https://www.mdpi.com/1420-3049/28/4/1613
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4417592/
- https://www.sciencedirect.com/science/article/abs/pii/S0889157521000843
- https://www.sciencedirect.com/science/article/pii/S0924224419309252
- https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2024.1446771/full
- https://www.researchgate.net/publication/281403282
- https://www.sciencedirect.com/science/article/abs/pii/S0889157521000843
- https://www.frontiersin.org/journals/endocrinology/articles/10.3389/fendo.2020.00025/full
- https://www.tandfonline.com/doi/full/10.1080/10942912.2012.716475
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10517269/
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3934501/
