Prebiotic Foods: Inulin, Onions, Garlic, Chicory and More
What prebiotic means, which foods are rich in inulin-type fructans, what trials found (mostly for added inulin), and why these foods can cause gas in some people.
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Prebiotic foods get a lot of attention, but the word has a specific meaning. In 2017 the International Scientific Association for Probiotics and Prebiotics (ISAPP) defined a prebiotic as "a substrate that is selectively utilized by host microorganisms conferring a health benefit," and said beneficial health effects must be documented before a substance is called a prebiotic. [1] A food that contains a fermentable fiber is therefore not automatically a documented prebiotic. This page covers foods that are rich in inulin-type fructans. Isolated fibers sold as supplements are on prebiotic fibers.
Who should be careful first. Fructans are not digested in the small intestine, and they can cause gastrointestinal symptoms in some people. [2] The American College of Gastroenterology recommends a limited trial of a low-FODMAP diet for people with irritable bowel syndrome, and fructans are one of the FODMAP groups. [3] [4] A clinician or dietitian should guide that, not a food list.
Foods with the most fructans
A laboratory study measured fructans in 60 vegetables and 43 fruits. The vegetables highest in fructans were garlic, artichoke, shallots, leek bulb and onions, from 1.2 to 17.4 g per 100 g fresh weight. [2] Trials have also added inulin from chicory and Jerusalem artichoke to snack bars and to fruit-and-vegetable shots. [5] [6] Wheat also contains fructans, which a 2022 review suggests may explain some bloating blamed on gluten. [7]The foods above are the ones the sources we read named. Garlic, onion and chicory are also sold as supplements or extracts, which carry different risks: see garlic and chicory root. For the foods themselves see onions, garlic and allium vegetables as food.
What research shows
| Question | What the source found | Strength of evidence |
|---|---|---|
| Inulin-type fructans (ingredient or supplement) and gut bacteria | A systematic review of trials in healthy adults found they promoted Bifidobacterium, Lactobacillus and Faecalibacterium prausnitzii. [8] | Promising but limited |
| Inulin and gut microbiome | A review of nine human studies (5 to 20 g a day) found the most consistent change was more Bifidobacterium, but short-chain fatty acids did not rise. [9] | Promising but limited |
| Inulin and constipation | A meta-analysis of five trials (252 people) found better stool frequency and consistency, but no improvement in pain or bloating. [10] | Promising but limited (small, older) |
| Inulin added to foods | In 45 volunteers, snack bars with chicory or Jerusalem artichoke inulin (7.7 g fructan a day, then double) raised bifidobacteria, with mild flatulence in a few. [5] | Promising but limited |
| Fruit-and-vegetable shots with Jerusalem artichoke inulin | In 66 healthy adults, 5 g a day of inulin in shots raised bifidobacteria; flatulence increased slightly but stayed mild. [6] | Promising but limited |
| Eating whole onions, garlic or leeks | Proper evidence not found. We searched PubMed for human trials of these foods and gut bacteria and found none we could read. | Not established |
Note the difference: nearly all trial evidence is for purified inulin or inulin added to products. A trial of purified inulin does not show what a serving of onion does.
Tolerance
In a trial of 84 healthy volunteers, inulin-type fructans at 5 to 20 g a day tended to raise digestive symptoms mildly, and the change was significant for one product at 20 g a day only. [11] A review of 103 trials of non-digestible carbohydrates concluded overconsumption can cause undesirable gastrointestinal side effects. [12] Advice from MedlinePlus is to add fiber slowly, since increasing it too quickly can lead to gas, bloating and cramps. [13]Frequently asked questions
What does prebiotic mean?
ISAPP's 2017 definition: a substrate selectively utilized by host microorganisms conferring a health benefit. [1]
Which foods are highest in inulin-type fructans?
In one laboratory survey, garlic, artichoke, shallots, leek bulb and onions. [2]
Is inulin the same as eating onions or chicory?
No. The trials we read used purified inulin or inulin added to bars and shots, not whole vegetables. [8] [5] [6]
Can prebiotic foods cause gas?
Yes, in some people. Trials of inulin reported mild flatulence or bloating. [6] [11]
Should I avoid them if I have IBS?
Ask your clinician or dietitian. Fructans are one group within the low-FODMAP approach, which the American College of Gastroenterology guideline endorses as a limited trial. [3] [4]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety and individual considerations
- IBS or functional bowel symptoms: see the note at the top.
- Allergy, pregnancy, children, medicines: not described in the sources we read for this page. For general fiber safety see fiber amounts and side effects.
- Probiotics are different: prebiotics feed microbes; probiotics are live microbes. See probiotics, prebiotics and fermented foods.
References
- Gibson GR et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nature reviews. Gastroenterology & hepatology 2017. PMID 28611480. Read 2026-10-23.
- Muir JG et al. Fructan and free fructose content of common Australian vegetables and fruit. Journal of agricultural and food chemistry 2007. PMID 17625872. Read 2026-10-23.
- Lacy BE et al. ACG Clinical Guideline: Management of Irritable Bowel Syndrome. The American journal of gastroenterology 2021. PMID 33315591. Read 2026-10-23.
- Varney J et al. FODMAPs: food composition, defining cutoff values and international application. Journal of gastroenterology and hepatology 2017. PMID 28244665. Read 2026-10-23.
- Kleessen B et al. Jerusalem artichoke and chicory inulin in bakery products affect faecal microbiota of healthy volunteers. The British journal of nutrition 2007. PMID 17445348. Read 2026-10-23.
- Ramnani P et al. Prebiotic effect of fruit and vegetable shots containing Jerusalem artichoke inulin: a human intervention study. The British journal of nutrition 2010. PMID 20187995. Read 2026-10-23.
- Brouns F et al. Do ancient wheats contain less gluten than modern bread wheat, in favour of better health?. Nutrition bulletin 2022. PMID 35915783. Read 2026-10-23.
- Hughes RL et al. The Prebiotic Potential of Inulin-Type Fructans: A Systematic Review. Advances in nutrition (Bethesda, Md.) 2022. PMID 34555168. Read 2026-10-23.
- Le Bastard Q et al. The effects of inulin on gut microbial composition: a systematic review of evidence from human studies. European journal of clinical microbiology & infectious diseases : official publication of the European Society of Clinical Microbiology 2020. PMID 31707507. Read 2026-10-23.
- Collado Yurrita L et al. Effectiveness of inulin intake on indicators of chronic constipation; a meta-analysis of controlled randomized clinical trials. Nutricion hospitalaria 2014. PMID 25208775. Read 2026-10-23.
- Bruhwyler J et al. Digestive tolerance of inulin-type fructans: a double-blind, placebo-controlled, cross-over, dose-ranging, randomized study in healthy volunteers. International journal of food sciences and nutrition 2009. PMID 18608562. Read 2026-10-23.
- Mysonhimer AR et al. Gastrointestinal Effects and Tolerance of Nondigestible Carbohydrate Consumption. Advances in nutrition (Bethesda, Md.) 2022. PMID 36041173. Read 2026-10-23.
- MedlinePlus (US National Library of Medicine). Dietary Fiber. Read 2026-10-23.