Prebiotic Fiber, Explained: Which Fibers Count, Foods That Have It, and How Much to Take

Last updated October 11, 2026 · By the Fiber Society team

Prebiotic fiber is fiber that your gut microbes selectively use, with a documented health benefit for you. That is the 2017 definition from the International Scientific Association for Probiotics and Prebiotics (ISAPP): a fiber is not prebiotic just because it is fiber, and probiotics (live microbes) are not fiber at all. The prebiotic fibers with the most human trials are inulin and fructooligosaccharides (FOS), galactooligosaccharides (GOS), resistant starch, acacia (gum arabic), resistant maltodextrin and partially hydrolyzed guar gum, studied at roughly 5 to 25 g a day. Food sources include onions, garlic, leeks, chicory root, oats, barley, green bananas and legumes.

Key takeaways

  • ISAPP defines a prebiotic as "a substrate that is selectively utilized by host microorganisms conferring a health benefit." Selectivity and a documented benefit are both required.
  • Prebiotics are food for microbes; probiotics are "live microorganisms which when administered in adequate amounts confer a health benefit on the host."
  • In human trials, inulin raised bifidobacteria at 5 to 8 g a day, GOS at 5 g or more, acacia at 10 g, PHGG at 5 g, resistant maltodextrin at 25 g and resistant starch at about 33 g.

What is prebiotic fiber? The ISAPP definition

ISAPP convened an expert panel in December 2016, and its 2017 consensus in Nature Reviews Gastroenterology & Hepatology defined a prebiotic as "a substrate that is selectively utilized by host microorganisms conferring a health benefit." Three words do the work:

  • Substrate. Something microbes consume; in practice, specific fermentable fibers and oligosaccharides.
  • Selectively utilized. The fiber must feed particular microbes (in most trials, a rise in bifidobacteria), not be fermented indiscriminately.
  • Health benefit. "Beneficial health effects must be documented for a substance to be considered a prebiotic." A bigger bifidobacteria count is a marker, not a benefit in itself.

ISAPP's plain-language summary adds the point most shoppers miss: not all dietary fibers are prebiotics. A fiber that bacteria barely touch can still add stool bulk, but it is not feeding anyone; see psyllium vs. inulin for that contrast. The reverse also holds: FDA counts inulin, galactooligosaccharide, resistant starch 2, resistant maltodextrin, guar gum and acacia as "dietary fiber" on labels, but 10 g of fiber on a label says nothing about whether any of it is prebiotic.

Prebiotic vs. probiotic

A probiotic is a living organism. ISAPP's 2014 consensus reaffirmed the FAO/WHO definition: "live microorganisms which when administered in adequate amounts confer a health benefit on the host." Prebiotic fiber is not alive; its job is to be eaten by microbes you already have.

Which fibers count as prebiotic fiber? The ones with human trials

Each of these has randomized human trials; the doses below both produced the effect and are where tolerance was measured.

Inulin and fructooligosaccharides (FOS)

Fructose chains, usually from chicory root. In a double-blind crossover of 30 healthy adults, 5 g and 8 g a day of inulin for two weeks both raised fecal bifidobacteria; in a 200-person study, 10 g a day of short-chain FOS was bifidogenic within a week. Tolerance is the catch: in a 26-person breakfast challenge, 10 g of native inulin was well tolerated but 10 g of oligofructose "substantially increased" symptoms, mainly flatulence and bloating.

Galactooligosaccharides (GOS)

Made from lactose. In an 18-person study taking 0, 2.5, 5 and 10 g a day for three weeks each, bifidobacteria rose at 5 g and 10 g but not at 2.5 g, and only 9 of 18 people were clear responders even at the high dose.

Resistant starch

Starch that escapes digestion in the small intestine, found in legumes, whole grains, green banana, raw potato and cooked-then-cooled starches. In a crossover of 10 adults eating crackers with about 33 g of RS2 a day for three weeks, Ruminococcus bromii and Eubacterium rectale rose, and the starch was well tolerated with flatulence rising moderately.

Acacia (gum arabic)

Dried acacia-tree sap. In a four-week study of healthy volunteers taking 5, 10, 20 or 40 g a day, bifidobacteria and lactobacilli were significantly higher than with water, the optimal dose was 10 g, and all subjects tolerated it well even at 40 g a day. Several authors worked for the ingredient's maker. More in acacia fiber benefits.

Resistant maltodextrin (Fibersol-2)

Corn starch processed so most of it resists digestion. At 15 g a day in 38 adults, bifidobacteria tended to rise (not statistically significant) with "very minor effects" on tolerance; in a 51-person crossover, only 25 g a day significantly increased bifidobacteria and stool weight. See resistant maltodextrin vs. maltodextrin.

Partially hydrolyzed guar gum (PHGG)

Guar gum cut into shorter chains so it no longer thickens. In a 44-person randomized trial, 5 g a day for 12 weeks increased Bifidobacterium and moved stool form closer to the middle of the Bristol scale, with no dropouts.

Prebiotic fibers: trial doses and tolerance at a glance

Fiber Dose used in trials What changed Tolerance in those trials
Inulin (native) 5 to 8 g/day, 2 weeks Bifidobacteria up Up to 10 g/day well tolerated
Oligofructose / short-chain FOS 10 g/day, 7 days Bifidobacteria up 5 g well tolerated; 10 g substantially increased gas and bloating
GOS 5 to 10 g/day, 3 weeks per dose Bifidobacteria up at 5 g and above; half of subjects responded Not reported in the dose-study abstract
Resistant starch (RS2) About 33 g/day, 3 weeks R. bromii and E. rectale up Well tolerated; flatulence rose moderately
Acacia (gum arabic) 10 g/day optimal (5 to 40 g tested), 4 weeks Bifidobacteria and lactobacilli up Well tolerated even at 40 g/day
Resistant maltodextrin 15 to 25 g/day, 3 weeks Bifidobacteria up at 25 g; stool weight up Very minor effects at 15 g; no diarrhea up to 1.0 g/kg single dose (men)
Partially hydrolyzed guar gum 5 g/day, 12 weeks Bifidobacterium up; stool form closer to type 4 No dropouts; events judged unrelated

Prebiotic fiber foods, with grams

USDA FoodData Central reports total fiber, not inulin or resistant starch, because standard fiber assays miss inulin and oligofructose, which a 1995 review noted "are often not mentioned in food tables." So the table gives USDA total fiber plus the prebiotic component each food is known for. A USDA survey analysis estimated American diets provide 2.6 g of inulin and 2.5 g of oligofructose a day, about 70% from wheat and 25% from onions.

Food (USDA description) Serving Total fiber Prebiotic component
Onions, raw 1 medium (110 g) 1.9 g Inulin and FOS; second-largest source in US diets
Garlic, raw 3 cloves (9 g) 0.2 g Inulin and FOS (concentrated, but portions are tiny)
Leeks, raw 1 cup (89 g) 1.6 g Inulin and FOS
Oats, dry 1/3 cup (27 g) 2.7 g Beta-glucan; also listed among natural inulin sources
Barley, pearled, cooked 1 cup (157 g) 6.0 g Beta-glucan; listed among natural prebiotic sources
Bananas, raw 1 small (101 g) 2.6 g Small amounts of inulin and FOS; resistant starch (RS2) while green
Lentils, cooked 1 cup (198 g) 15.6 g Resistant starch (about 3.4 g per 100 g) plus other fermentable fiber

Chicory root and Jerusalem artichokes are the concentrated inulin sources, but few Americans eat them regularly. For resistant starch, a 2013 review lists lentils at 3.4 g, chickpeas at 2.6 g and kidney beans at 2.0 g per 100 g, versus brown rice at 1.7 g and white potatoes at 1.3 g, and notes that cooked legumes and cooked-then-cooled starchy foods are high in resistant starch (5 to 15% on a dry-matter basis). Americans average about 4.9 g a day.

How much prebiotic fiber per day, and how to ramp up

No US authority sets a prebiotic target; the official recommendation of 14 g per 1,000 calories is for total fiber. The trials give a working range: 5 g of inulin, GOS or PHGG, or 10 g of acacia, shifted bifidobacteria, while resistant maltodextrin needed 25 g and resistant starch about 33 g. Fermentation makes gas, so how you add prebiotic fiber matters as much as which one you pick.

  1. Start at 2 to 3 g a day, about half the smallest effective dose, and hold for three or four days before adding another 2 to 3 g.
  2. Split the dose. Some inulin trials used 5 g three times a day rather than 15 g at once.
  3. Pick by tolerance if you are gas-prone. Acacia (40 g), resistant maltodextrin (15 g) and PHGG (5 g) had the gentlest profiles above; oligofructose hit a clear threshold at 10 g. See why fiber makes you bloated.
  4. Expect individual results. Only 9 of 18 people responded in the GOS study, and a low starting bifidobacteria count predicted who benefited most in the 200-person study.

Where Fiber Society fits

Fiber Society's Daily Fiber Blend (Strawberry Lemonade) is built mostly from two fibers in the table. Each 14.2 g scoop delivers 10 g of dietary fiber (36% of the Daily Value): 7.3 g of organic acacia (gum arabic), 2.5 g of psyllium husk, 1.95 g of Fibersol-2 resistant maltodextrin and 300 mg of BeniCaros carrot-derived RG-I fiber. It contains no inulin. One scoop is not a replica of the studies above, which used 10 g of acacia and 15 to 25 g of resistant maltodextrin; it is a daily fiber blend with 10 g of fiber per glass. Because most of the fiber is acacia and psyllium is kept to 2.5 g, it is designed to drink thin in 350 to 400 ml of cold water.

It has about 20 calories and 0 g sugar per scoop, is sweetened with stevia (Reb M) with no sucralose, aspartame or synthetic dyes, and is vegan and gluten-free. It is gentle by design: start with half a scoop for the first few days, do not exceed two scoops a day, take it at least two hours apart from oral medications, and always use at least a full glass of liquid. A pouch holds 28 servings and costs 20% off with Subscribe & Save every 4 weeks, or one-time at the regular price.

Frequently asked questions

What is the difference between prebiotics and probiotics?

Probiotics are live microorganisms that confer a health benefit in adequate amounts. Prebiotics are substrates, mostly fermentable fibers, that host microbes selectively use, with a documented benefit. One is the organism; the other is its food.

Is inulin a prebiotic fiber?

Yes. Chicory inulin raised bifidobacteria at 5 g and 8 g a day in a two-week crossover trial. Gas is the trade-off at higher doses, especially with shorter-chain oligofructose.

What foods are highest in prebiotic fiber?

Chicory root, Jerusalem artichokes, garlic, leeks and onions for inulin (though wheat and onions supply about 95% of what Americans actually eat); lentils, chickpeas, kidney beans, green bananas and cooked-and-cooled potatoes, rice and pasta for resistant starch.

How much prebiotic fiber should I take a day?

There is no official target. Effective trial doses ran from 5 g (inulin, GOS, PHGG) to 10 g (acacia), 25 g (resistant maltodextrin) and about 33 g (resistant starch). Start at 2 to 3 g and work up.

Sources

  1. Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The ISAPP consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491-502
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  3. ISAPP: Prebiotics resource page
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  8. Bouhnik Y, Raskine L, Simoneau G, et al. The capacity of nondigestible carbohydrates to stimulate fecal bifidobacteria in healthy humans. Am J Clin Nutr. 2004;80(6):1658-1664
  9. Bonnema AL, Kolberg LW, Thomas W, Slavin JL. Gastrointestinal tolerance of chicory inulin products. J Am Diet Assoc. 2010;110(6):865-868
  10. Davis LM, Martinez I, Walter J, Hutkins R. A dose dependent impact of prebiotic galactooligosaccharides on the intestinal microbiota of healthy adults. Int J Food Microbiol. 2010;144(2):285-292
  11. Martinez I, Kim J, Duffy PR, Schlegel VL, Walter J. Resistant starches types 2 and 4 have differential effects on the composition of the fecal microbiota in human subjects. PLoS One. 2010;5(11):e15046
  12. Birt DF, Boylston T, Hendrich S, et al. Resistant starch: promise for improving human health. Adv Nutr. 2013;4(6):587-601
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  14. Calame W, Weseler AR, Viebke C, Flynn C, Siemensma AD. Gum arabic establishes prebiotic functionality in healthy human volunteers in a dose-dependent manner. Br J Nutr. 2008;100(6):1269-1275
  15. Fastinger ND, Karr-Lilienthal LK, Spears JK, et al. A novel resistant maltodextrin alters gastrointestinal tolerance factors, fecal characteristics, and fecal microbiota in healthy adult humans. J Am Coll Nutr. 2008;27(2):356-366
  16. Burns AM, Solch RJ, Dennis-Wall JC, et al. In healthy adults, resistant maltodextrin produces a greater change in fecal bifidobacteria counts and increases stool wet weight. Nutr Res. 2018;60:33-42
  17. Kishimoto Y, Kanahori S, Sakano K, Ebihara S. The maximum single dose of resistant maltodextrin that does not cause diarrhea in humans. J Nutr Sci Vitaminol. 2013;59(4):352-357
  18. Yasukawa Z, Inoue R, Ozeki M, et al. Effect of repeated consumption of partially hydrolyzed guar gum on fecal characteristics and gut microbiota. Nutrients. 2019;11(9):2170
  19. Reider SJ, Moosmang S, Tragust J, et al. Prebiotic effects of partially hydrolyzed guar gum on the composition and function of the human microbiota: results from the PAGODA trial. Nutrients. 2020;12(5):1257
  20. USDA FoodData Central, SR Legacy (FDC IDs 170000, 169230, 169246, 173904, 170285, 173944, 172421)
  21. FDA: Questions and Answers on Dietary Fiber

This article is for general education and isn't medical advice. Talk to your healthcare provider about your own needs, especially if you take medication or have a digestive condition.

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