What Are Prebiotics? Definition, Trials and Prebiotics vs Probiotics

A prebiotic is "a substrate that is selectively utilized by host microorganisms conferring a health benefit": something you eat that feeds the microbes already living in you, that particular microbes use rather than the whole community, and that a controlled trial has shown to do something useful in the people who take it.¹ The definition comes from a panel convened in December 2016 by the International Scientific Association for Probiotics and Prebiotics, the body whose other panels defined probiotics and postbiotics. A probiotic is the live organism, a prebiotic is the food for organisms already there, and a postbiotic is the organism after it has been deliberately killed.^{1, 2, 3} The most researched prebiotics are the inulin-type fructans (inulin and oligofructose, sold as chicory root fibre) and galacto-oligosaccharides.¹ Trials of inulin-type fructans have measured a rise in bifidobacteria at 3 to 20 g a day across 50 trials in 2,525 people, more frequent stools at 12 g a day in constipated adults, and symptom scores that rise with the dose; what live cultures have been measured doing is a separate record, set out use by use on the probiotics guide.^{4, 5, 7} Great Britain authorises one prebiotic-fibre wording, for chicory inulin at 12 g a day, and lists every wording that uses the word "prebiotic" as non-authorised.^{14, 15}

Evidence at a glance

• The definition: a consensus statement in Nature Reviews Gastroenterology and Hepatology, 2017; several panel members declare industry-sponsored projects or consultancies.¹
• Trials: a pooling of 50 trials in 2,525 people for bifidobacteria; two supplier-funded crossover trials of 12 g chicory inulin in 44 and 39 constipated adults; four tolerance trials at 5 to 20 g a day in 84, 35, 36 and 98 adults; a pooling of 11 trials in 729 adults with IBS; one trial of a galacto-oligosaccharide against a low-FODMAP diet in 40 adults (literature read to 3 November 2026).^{4, 5, 6, 7, 8, 9, 10, 11, 12}
• The law: one authorised wording, chicory inulin at 12 g a day; 30 non-authorised entries across 22 foods carry the word "prebiotic" (register last updated 19 May 2026).^{14, 15}

Sources checked on 3 November 2026.

What are prebiotics?

A prebiotic is food for your microbes, not the microbes themselves, and three parts of the definition do the work.¹

Substrate. The thing is eaten by microbes, so it has to reach them: "dietary prebiotics must not be degraded by the target host enzymes", meaning it passes the small intestine undigested and is fermented in the colon. Things that change the microbes without feeding them are excluded by name: "antibiotics, minerals, vitamins and bacteriophages, which are not growth substrates", are not prebiotics.¹

Selectively utilised. The panel kept the requirement that a prebiotic feeds particular microbes rather than everyone: "prebiotics differ from most dietary fibres such as pectins, cellulose and xylans, which encourage growth of a wide variety of gut microorganisms". So a prebiotic is usually a fibre, but most fibre is not a prebiotic. The established prebiotics are carbohydrates: the fructans (inulin and fructo-oligosaccharides) and the galactans (galacto-oligosaccharides), which the panel calls "the most researched prebiotics". Candidates with less evidence include human milk oligosaccharides and, with "far more studies in the target host" still needed, plant polyphenols, with some fatty acids a further possibility.¹ What is in a food is on the inulin page, with the grams.

Health benefit. "Demonstration of health benefits in well-controlled studies in the target host is required": a blinded placebo-controlled trial in which a marker or a symptom changes after a specific change in the microbes. A rise in bifidobacteria on its own is a prebiotic effect, not yet a health benefit. The panel also writes the dose into its criteria: "An appropriate dose must be sufficient to generate a prebiotic effect, but not too high to induce unwanted or adverse effects such as excessive gas formation". The site need not be the gut; the vagina and the skin are named as other colonised sites a prebiotic could be applied to.¹

Prebiotics vs probiotics vs postbiotics

Each of the three words has been defined or confirmed by an expert panel of the same association, and each is defined by a trial of the thing itself.^{1, 2, 3}

A trial result belongs to what was tested. A prebiotic trial measures the microbes you already have and what follows from feeding them; a live-culture trial measures a named strain given in a counted dose; a postbiotic trial measures a described, killed preparation. None of the three results transfers to the others.^{1, 3} What postbiotics are takes the third word further.

Is it better to take prebiotics or probiotics?

Neither is better; they are different things with separate trials, and the useful question is what each has been measured doing.^{1, 2} Inulin-type fructans have been measured raising bifidobacteria, by a standardised mean difference of 0.83 across 50 trials in 2,525 people aged 0 to 83, and raising weekly stools from a median of three to four at 12 g a day in 44 constipated adults.^{4, 5} Live cultures have been measured use by use: during an antibiotic course, for instance, diarrhoea fell from 19% to 8% across 33 children's trials in 6,352 children, with moderate certainty.¹⁸ The two records answer different questions, so the choice follows the question: a bowel habit or a count of bifidobacteria you want higher points one way, an antibiotic course the other. A product with both is sold as a synbiotic. In the one pooling that separates them, 30 live-culture trials in adults with chronic constipation raised stool frequency against control, while the four synbiotic trials did not differ from control on stool output or symptom scores; four trials are too few to say whether adding a fibre changes a culture's effect.¹⁹

What have trials of prebiotics found?

The seven records in the table are the trials and poolings this page cites with their dose, duration, result and funding: a pooling, two bowel-habit trials, two tolerance trials, a pooling in IBS and a galacto-oligosaccharide trial; two further tolerance trials are summarised beneath it. Who paid is in the last column, because the inulin trials are mostly supplier-funded.

Read the doses with the symptoms. In the two tolerance trials the inulin page sets out, symptom scores rose only mildly at 5 to 20 g a day and 5 g a day was indistinguishable from placebo; the rise reached significance for one product at 20 g, and at 16 g of oligofructose the score did not fall over eight weeks.^{7, 8, 10} The panel's own line is that the right dose is "not too high to induce unwanted or adverse effects such as excessive gas formation", which is why the 2017 bowel-habit trial gave its 12 g as 4 g three times a day, and why why fibre makes you bloated is the next page for anyone who feels it.^{1, 5}

What does the GB register let a label say about prebiotics?

One wording, for chicory inulin at 12 g a day; no wording that uses the word "prebiotic" is authorised.^{14, 15} "Chicory inulin contributes to normal bowel function by increasing stool frequency" may be used only for a food that provides at least 12 g a day of native chicory inulin, with the consumer told that "the beneficial effect is obtained with a daily intake of 12 g chicory inulin".¹⁵ On the non-authorised list, the word appears in 30 entries across 22 foods and combinations: inulin and fructo-oligosaccharides, galacto-oligosaccharides, acacia gum, polydextrose, partially hydrolysed guar gum, resistant starch, lactulose, xylo-oligosaccharides, wheat dextrin and a generic "Prebiotics" entry among them, each recorded as not complying with the regulation because the claimed effect was not substantiated, the food was not sufficiently characterised, the effect was not sufficiently defined or it was not a beneficial physiological effect.¹⁴ The panel that wrote the definition noted the same position in the European Union, whose rule Great Britain kept: to its date, "only one prebiotic, chicory inulin, has received an EU health claim".¹

Advertising follows the register. The Advertising Standards Authority's guidance says "prebiotic" claims are likely to be health claims, and in a 2023 ruling it treated "Prebiotic goodness to fuel your gut" on a breakfast cereal as a general health claim, allowed only beside an authorised claim whose conditions are met; the cereal gave 5.4 g of chicory root fibre a serving against the 12 g condition, so the claim was not acceptable as used, and "helping support digestive health by increasing levels of important bacteria living in your gut" was a specific claim with no authorisation.¹⁶ What a label can state is what is in the pack: the fibre by name and its grams per serving, and for a live culture the species and the count. The 12 g is the condition for the wording, not a line below which the fibre does nothing: the bifidobacteria pooling ran from 3 g a day, and a gummy, sachet or powder states its grams so you can set them against the trial doses above.^{4, 15} The Wellgard fibre guide sets those grams against the daily target.

Who should be careful with prebiotics?

People with IBS, first: in the 11-trial pooling, prebiotics did not differ from placebo for symptoms or quality of life, and inulin-type fructans worsened wind.¹¹ A low-FODMAP diet excludes inulin by design, because fructans are its "O"; the one trial that set a prebiotic against that diet used 1.37 g of a galacto-oligosaccharide, a dose far below the inulin trials, and still left wind and rumbling as the symptoms that did not fall.¹² If you are pregnant, breastfeeding or on medication, ask your GP first. For live cultures the NHS's line is that anyone with an existing health condition or a weakened immune system should talk to a doctor before taking a supplement.²⁰ See a GP if a bowel habit has changed and stays changed; a fibre is not a reason to wait.

What we don't know

• What a rise in bifidobacteria does for a healthy person beyond the bowel-habit effects the pooling reports in healthy subjects; the pooling measured the count, not an outcome that follows from it.⁴
• Whether the wind settles. The one eight-week trial at 16 g of oligofructose found no change in symptom score over time; no inulin trial has followed symptoms for longer.¹⁰
• Whether chain length matters: across 47 publications the short-chain fructans (fewer than 10 units) raised stool frequency by 0.36 stools a day and the long-chain ones did not as a group, while native chicory inulin, a mixture with a mean of nine units or more, raised weekly stools at 12 g in the two crossover trials above.^{5, 6, 13, 15}
• Whether a fibre added to a live culture changes what the culture does: four synbiotic trials in constipation, against 30 of live cultures alone (literature read to 3 November 2026).¹⁹

Frequently asked questions

Is yoghurt a prebiotic or a probiotic?

A yoghurt with live cultures is on the probiotic side: it carries live organisms, which is what the 2014 definition counts.² It is not a prebiotic, because yoghurt contains no fibre for resident microbes to feed on; a yoghurt with inulin or oligofructose added carries both, and the label states the grams.²¹

Is a banana a prebiotic?

Under the definition a substance is a prebiotic, not a food, and bananas hold only a little inulin and oligofructose against chicory root, Jerusalem artichoke, garlic, leek and onion. Food tables rarely list these fructans because standard fibre methods do not measure them; the inulin page gives the grams for each food.¹⁷ Across American diets, wheat supplies about 70% of the inulin and oligofructose eaten and onions about 25%, so a few grams a day come from ordinary food, and a supplement's label states what it adds on top.²²

Do prebiotics make you poop?

Chicory inulin at 12 g a day did, in the two crossover trials in the table, and 16 g of oligofructose a day raised stools in healthy adults from about one a day to nearly two.^{5, 6, 10} Chain length may matter: across 47 publications the short-chain fructans raised stool frequency and long-chain ones did not as a group, while native chicory inulin, the mixture the authorised 12 g wording covers, did so in both crossover trials.^{5, 6, 13, 15}

What are the signs you need prebiotics?

There are none that a test can show: no prebiotic deficiency exists, and no trial has picked out people who need one. The measurable thing is how much fibre you eat, which the fibre guide sets against the UK target. A bowel habit that has changed and not changed back is a reason to see a GP first.

Sources

1. 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 and Hepatology, 2017. Consensus statement from a panel convened in December 2016; several authors declare industry-sponsored research projects or consultancies for prebiotic and probiotic companies. PubMed 28611480.
2. Hill C, et al. Expert consensus document. The International Scientific Association for Probiotics and Prebiotics consensus statement on the scope and appropriate use of the term probiotic. Nature Reviews Gastroenterology and Hepatology, 2014. Consensus statement. PubMed 24912386.
3. Salminen S, et al. The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics. Nature Reviews Gastroenterology and Hepatology, 2021. Consensus statement from a panel convened in 2019; several panel members declare industry fees; two errata published, no retraction. PubMed 33948025.
4. Nagy DU, et al. Effect of chicory-derived inulin-type fructans on abundance of Bifidobacterium and on bowel function: a systematic review with meta-analyses. Critical Reviews in Food Science and Nutrition, 2023. Systematic review and meta-analysis, 50 randomised trials, 2,525 people, 3 to 20 g a day for at least seven days; two authors at an inulin supplier's institute. PubMed 35833477.
5. Micka A, et al. Effect of consumption of chicory inulin on bowel function in healthy subjects with constipation: a randomized, double-blind, placebo-controlled trial. International Journal of Food Sciences and Nutrition, 2017. Randomised placebo-controlled crossover trial, 44 adults, 4 weeks per arm, 4 g three times a day; supplier-funded (the registered sponsor is an inulin supplier, NCT02548247), two authors at its institute. PubMed 27492975.
6. Puhlmann ML, et al. Inulin-induced improvements on bowel habit and gut microbiota in adults with functional constipation: findings of a randomized, double-blind, placebo-controlled study. BMC Gastroenterology, 2025. Randomised placebo-controlled crossover trial, 39 adults, 4 weeks per arm, 12 g a day; two authors current and one a former employee of an inulin supplier. PubMed 41233756.
7. 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. Randomised placebo-controlled crossover trial, 84 healthy adults, 5, 10 and 20 g a day of three supplier products for 2 weeks each; funding not stated on the record, first author at a consultancy. PubMed 18608562.
8. Ripoll C, et al. Gastrointestinal tolerance to an inulin-rich soluble roasted chicory extract after consumption in healthy subjects. Nutrition, 2010. Two randomised double-blind studies, 18 adults for 6 days at 5.0 and 7.8 g of inulin and 35 adults for 4 weeks at 5 g a day; non-government research support, first author at a contract research company. PubMed 19931416.
9. Azpiroz F, et al. Effect of chicory-derived inulin on abdominal sensations and bowel motor function. Journal of Clinical Gastroenterology, 2017. Randomised double-blind placebo-controlled parallel trial, 36 adults with gas-related complaints, 8 g a day for 4 weeks; supported by a grant from an inulin supplier's institute and by the Spanish government, two authors employees of the institute. PubMed 27680592.
10. Dahl WJ, et al. Consuming foods with added oligofructose improves stool frequency: a randomised trial in healthy young adults. Journal of Nutritional Science, 2014. Randomised double-blind parallel trial, 98 healthy adults, 16 g of oligofructose a day in yoghurt and snack bars for 8 weeks; funded by a food company and an inulin supplier, which employ two authors, with a third a past employee. PubMed 25191615.
11. Wilson B, et al. Prebiotics in irritable bowel syndrome and other functional bowel disorders in adults: a systematic review and meta-analysis of randomized controlled trials. American Journal of Clinical Nutrition, 2019. Systematic review and meta-analysis, 11 randomised trials, 729 adults; Medical Research Council funding, authors at a London university. PubMed 30949662.
12. Huaman JW, et al. Effects of prebiotics vs a diet low in FODMAPs in patients with functional gut disorders. Gastroenterology, 2018. Randomised double-blind parallel trial, 40 adults, 4 weeks plus 2 weeks of follow-up, 1.37 g of a galacto-oligosaccharide a day against a low-FODMAP diet; two authors at a research company linked to the supplement's maker, one author a member of the 2017 definition panel. PubMed 29964041.
13. de Vries J, et al. Effects of β-fructans fiber on bowel function: a systematic review and meta-analysis. Nutrients, 2019. Meta-analysis of 47 publications; funded by a fructan supplier that employed two authors; the authors nonetheless declare no conflict of interest. PubMed 30621208.
14. Department of Health and Social Care. Great Britain nutrition and health claims (NHC) register. Published 31 December 2020, last updated 19 May 2026; the authorised list searched for inulin and prebiotic entries (one authorised, native chicory inulin) and the non-authorised list for every entry carrying the word "prebiotic" (30 entries, 22 foods and combinations). gov.uk.
15. Commission Regulation (EU) 2015/2314 as retained in GB law, authorising the health claim for native chicory inulin and its conditions of use (12 g a day, mean degree of polymerisation at least 9). legislation.gov.uk.
16. Advertising Standards Authority and Committee of Advertising Practice. Food: probiotic claims. Advice online, including the "What about prebiotic?" section; the ruling of 2 August 2023 on a breakfast cereal read in full. asa.org.uk.
17. van Loo J, et al. On the presence of inulin and oligofructose as natural ingredients in the western diet. Critical Reviews in Food Science and Nutrition, 1995. Narrative review with food analyses; authors employed by an inulin producer. PubMed 8777017.
18. Guo Q, et al. Probiotics for the prevention of pediatric antibiotic-associated diarrhea. Cochrane Database of Systematic Reviews, 2019. Cochrane review, 33 randomised trials, 6,352 children; no conflicts declared. PubMed 31039287.
19. van der Schoot A, et al. Probiotics and synbiotics in chronic constipation in adults: a systematic review and meta-analysis of randomized controlled trials. Clinical Nutrition, 2022. Pooled analysis, 30 live-culture and 4 synbiotic trials; authors declare research grants from food-industry bodies. PubMed 36372047.
20. NHS. Probiotics. Page last reviewed 24 November 2022 (review overdue). nhs.uk.
21. Public Health England. McCance and Widdowson's Composition of Foods Integrated Dataset (CoFID), 2021. Food composition dataset. gov.uk.
22. Moshfegh AJ, et al. Presence of inulin and oligofructose in the diets of Americans. Journal of Nutrition, 1999. Analysis of a US government dietary survey, over 15,000 people; US Department of Agriculture authors. PubMed 10395608.

This article is general information, not medical advice. Anyone with an existing health condition or a weakened immune system should talk to a doctor before taking a live-culture supplement, and anyone pregnant, breastfeeding or taking medication should ask a GP before adding a fibre supplement. Report a reaction to a prescribed medicine through the Yellow Card scheme, and a reaction to a food or food supplement to your local authority's food safety team, which works with the Food Standards Agency.

Spotted something out of date? Email support@wellgard.co.uk with the page address and the sentence.