Probiotics, prebiotics and postbiotics: what each term means

Probiotic, prebiotic, postbiotic: three words that differ by one prefix and a lot of science, usually sold from the same shelf. If you have ever wondered whether you need all three, or any, this is the one-page version of what each word means, what has actually been tested, and how to read the pack.

Here is the quick sort: they are different things, not a three-step programme. A probiotic is a live microorganism with a demonstrated benefit, a prebiotic is a food for gut microbes with a demonstrated benefit, and a postbiotic is a preparation of inactivated microbes or their parts with a demonstrated benefit. In every case the benefit belongs to a specific strain or ingredient for a specific purpose. None is a general "gut fix", and for most people the first move is fibre and ordinary food.

Three words, one shelf, four definitions

The definitions come from consensus statements by ISAPP, the International Scientific Association for Probiotics and Prebiotics, whose panels set out what each term should mean.

• Term: Probiotic; Definition: Live microorganisms that, given in adequate amounts, confer a health benefit (Hill et al., 2014); What the word doesn't tell you: That every strain helps with every problem
• Term: Prebiotic; Definition: A substrate selectively used by host microorganisms, conferring a health benefit; the benefit has to be documented (Gibson et al., 2017); What the word doesn't tell you: That all fibre qualifies
• Term: Postbiotic; Definition: A preparation of inanimate microorganisms and/or their components that confers a health benefit; it must contain inactivated cells or cell components, with or without metabolites (Salminen et al., 2021); What the word doesn't tell you: That anything "made by bacteria" counts
• Term: Fermented food; Definition: Foods made through desired microbial growth and enzymatic conversion of food components (Marco et al., 2021); What the word doesn't tell you: That it contains live microbes, or is a probiotic

All four sources are in Nature Reviews Gastroenterology & Hepatology. The pattern to notice is that each definition carries a benefit requirement. A label can borrow the word without anyone having shown the benefit for that particular preparation.

Probiotics: the strain and the purpose do the work

NCCIH, the US National Center for Complementary and Integrative Health, puts the central problem plainly: if one kind of Lactobacillus helps prevent an illness, that does not mean another kind of Lactobacillus, or any Bifidobacterium, would do the same. Its overview also says that for most uses we still do not know which probiotics help, how much to take or who benefits most (NCCIH, Probiotics: Usefulness and Safety).

Where evidence exists, it is tied to a use. NCCIH's page (last updated August 2019) summarises reviews from 2017 and 2018, so treat these as a snapshot rather than the latest word:

• Antibiotic-associated diarrhoea. A 2017 review of 17 studies (3,631 people not in hospital) linked probiotics taken alongside antibiotics with roughly half the likelihood of diarrhoea, though NCCIH calls the conclusion tentative because study quality was only moderate.
• C. difficile diarrhoea. A 2017 analysis of 31 studies (8,672 people, mostly hospital patients) concluded it was moderately certain that probiotics reduce risk in adults and children receiving antibiotics, while the best types, durations and doses remain uncertain.
• Irritable bowel syndrome. A 2018 review of 53 studies (5,545 people) found probiotics may help global symptoms and abdominal pain, but could not say which species, strains or combinations are most likely to help.

The American Gastroenterological Association's 2020 guideline reads the same way: it suggests particular named products for a short list of uses (preventing C. difficile infection in adults and children on antibiotics, and pouchitis), recommends probiotics for C. difficile infection itself, Crohn's disease, ulcerative colitis and irritable bowel syndrome only in the context of a clinical trial, and suggests against them for children with acute infectious gastroenteritis. It also suggests certain named combinations for preventing necrotising enterocolitis in preterm, low-birth-weight infants; for very premature babies that is a neonatal-team decision (see the section on who should check with a clinician first) (AGA, 2020). That is a US guideline, written for US clinical practice, and it is a good illustration of the principle rather than a shopping list.

Fermented foods are not the same thing as probiotics

Yoghurt, kefir, kimchi, sauerkraut, sourdough and cheese are fermented foods. Whether the microbes are still alive on your plate depends on the food. The ISAPP panel's classification puts yoghurt, kefir, most cheeses, miso, natto, tempeh, non-heated fermented vegetables and most kombuchas in the "live microorganisms present" group, and bread, pasteurised fermented vegetables, wine and most beers in the "absent" group. The panel says plainly that there is no expectation that fermented foods must contain live microbes (Marco et al., 2021).

So a jar of pasteurised sauerkraut is fermented, but it is not alive. The panel also says "contains probiotics" should only appear on a fermented food when the strains are defined, present at an appropriate number through shelf life, and shown to benefit health, and it expects most fermented foods on sale today not to qualify. It notes that apart from yoghurt and other cultured dairy, few well-designed randomised trials of fermented foods exist.

A worked example: the Stanford fermented-food trial

The study people quote most is a 17-week randomised trial in Cell. Thirty-six healthy adults (average age 52) were analysed. One group raised their fibre from about 21.5 g to 45.1 g a day. The other group raised fermented foods from about 0.4 servings a day to 6.3, a lot of yoghurt, kefir, kimchi, kombucha and vegetable-brine drinks. In the fermented-food group, microbiome diversity rose steadily and inflammatory markers fell. The pre-specified primary outcome, a cytokine response score, did not change (Wastyk et al., 2021, Cell).

Here are its limits. It was a small trial in healthy people, and the headline result came from secondary measures. Six servings a day is a lot of kimchi. The paper lists university seed money and philanthropic donations as funding, and several authors declared founder or shareholder roles in biotech companies. It tells you that diet can change gut markers; it does not tell you fermented foods treat inflammation or any digestive condition.

Fermented foods also carry their own cautions. The ISAPP panel lists histamine and other biogenic amines in some cheeses, meats, vegetables, soybeans and wine; salt (in soy sauce and kimchi) and alcohol, which should be eaten in moderation; and a food-safety risk from some cheeses and low-acid fermented foods contaminated with Listeria, Salmonella or Clostridium botulinum. If you ferment at home, use a reputable recipe.

Prebiotics: fibre first, then ingredients with a trial

Fibre and prebiotic overlap, but they are not synonyms. The ISAPP definition needs selective use by gut microbes and a documented benefit, so a fibre can feed gut bacteria and still not qualify.

The Dutch Voedingscentrum gives adult fibre recommendations of at least 24 g a day for women and 30 g for men, scaled to energy intake (3.0 to 3.5 g per megajoule), and says many people in the Netherlands get too little (Voedingscentrum, Vezels). Vegetables, fruit, pulses, nuts and wholegrains are the usual places to find it. A tin of beans closes more of that gap than a tub of powder.

Evidence for a specific prebiotic ingredient looks like this: a 12-week trial gave 44 people with irritable bowel syndrome a galactooligosaccharide at 3.5 g a day, 7 g a day or placebo. Bifidobacteria increased at both doses. At 3.5 g a day, stool consistency, flatulence, bloating, a composite symptom score and the subjective global assessment improved against placebo; at 7 g a day, only the global assessment and anxiety scores did (Silk et al., 2009, Alimentary Pharmacology & Therapeutics). One of its authors, Glenn Gibson, also helped write the 2017 prebiotic definition above, and the trial's funding isn't stated in the sources behind this page. It is one small trial of one ingredient in one condition. Two things carry over: more bacteria is not the same as feeling better, and, in our reading, the higher dose did not do better on the symptoms measured.

Prebiotic fibres work by being fermented, and fermentation makes gas. A 2026 review of rapidly fermentable substrates (inulin, fructo-oligosaccharides, galacto-oligosaccharides) describes them as helpful in tolerant people and capable of causing gas, osmotic effects and distension in susceptible ones, and notes that clinicians restrict the same compounds as FODMAPs in irritable bowel syndrome (Ibrahim, 2026, Clinical Nutrition ESPEN). The NHS lists hard-to-digest foods such as beans, onions and cabbage among the things people with IBS may want to avoid for bloating and wind (NHS, IBS diet, lifestyle and medicines), and NIDDK says some people get more gas when they eat too much fibre (NIDDK). The Voedingscentrum says the same in plain terms: a sudden big jump in fibre can cause wind, cramps or constipation, because gut bacteria make gas as they process it, so build up step by step (Voedingscentrum, Vezels). In practice that means going gradually and not starting several new products at once, so you can tell what caused what.

Postbiotics: the newest word, the thinnest shelf

The ISAPP panel behind the 2021 definition said the term was "inconsistently used" and needed one. Its answer is narrow: effective postbiotics must contain inactivated microbial cells or cell components, with or without metabolites. A purified metabolite on its own does not meet it. A "heat-treated probiotic" label only helps if the benefit has been shown for that preparation.

The same paper offers a worked example. Two pooled randomised trials (537 healthy children aged 12 to 48 months in day-care) of heat-inactivated Lacticaseibacillus paracasei CBA L74, given in fermented milk or rice, suggest it might reduce the risk of diarrhoea and some other infections (Salminen et al., 2021). The authors' word is "might", and it is one strain in one age group for one use, so treat a postbiotic claim as a question to ask the maker rather than an established category with known uses.

Reading a probiotic label

A label does one job well: it lets you match a product to a study. These are the things to look for.

• The full strain name, code included. Genus, species and a strain identifier (a made-up example: Genus species ABC-123). "Lactobacillus blend" gives you nothing to look up. If the maker cites a study, check that the strain and form in your pack are the ones tested.
• A count at the end of shelf life. CFU (colony-forming units) counts organisms that were alive and able to multiply in the test. The questions that matter are whether the figure is per capsule or per daily portion, whether it holds at the end of shelf life or only at manufacture, and whether it matches the study dose. Nothing we cite says a higher count means a bigger benefit, and NCCIH says the right doses are still unknown for most uses.
• The portion and warnings. EU food-supplement labels must show the recommended daily portion, a warning not to exceed it, and a statement that supplements are not a substitute for a balanced diet (Your Europe, Food supplements: EU labelling rules). They may not claim to prevent, treat or cure a disease.
• The storage instructions. Refrigerate if the pack says so. Cold storage is an instruction, not a quality score.
• The claim wording. In December 2024 the European Ombudsman recorded the Commission's position that "probiotic" implies a health effect, so statements such as "contains probiotics" count as health claims needing authorisation, and that the Commission has rejected all applications for probiotic claims after negative EFSA opinions (European Ombudsman, decision 2273/2023/MIK). So the word on a pack and the science behind it are separate questions. We don't know how Dutch authorities treat individual products, and rules change.

NCCIH also notes that some probiotic products have been reported to contain microorganisms other than those on the label, which in some cases may pose serious health risks. That is a reason to look at what testing claims actually cover. A claim like "survives stomach acid" needs its own evidence, and a laboratory result is not the same as a symptom improvement in people. For a general method that works on any supplement, see our label guide.

Who should check with a clinician first

NCCIH says the risk of harm from probiotics is greater in people with severe illness or compromised immune systems, and that severe or fatal infections have been reported in premature infants given probiotics. It adds that the US Food and Drug Administration warned healthcare providers in 2023 about that risk. Seriously ill people and premature infants belong with their treating team, not a shop shelf (the AGA's suggestion for preterm infants concerns hospital care, not a purchase), and NCCIH advises talking to a healthcare provider before using a probiotic supplement, particularly if you have health problems.

This page has no source on pregnancy or breastfeeding, so ask a clinician or pharmacist first. Persistent digestive symptoms, blood in the stool or unexplained weight loss need proper assessment rather than a trial of products; that is an editorial caution, not a diagnosis. If you take medication, keep an exact product list for your pharmacist (our interaction guide explains why).

Questions people ask

Is a multi-strain product better? Not by default. The guidance and trials above attach benefits to named strains and named combinations for particular uses, so a long list of strains is only useful if that combination was tested for what you want. A bigger blend can cost more without meaning more evidence.

Can I take prebiotics with probiotics? Nothing we cite shows that a pairing works better than either alone. The practical issue is tolerance: prebiotic fibres ferment and make gas, so start one thing at a time.

How long until I notice anything? There is no universal answer. NCCIH says doses are still unknown for most uses. The trials we cite ran for 12 weeks (the prebiotic trial) and 17 weeks (the Stanford diet trial), and fibre advice is to build up over days and weeks.

Why am I bloated? More fibre, and especially fast-fermenting prebiotics such as inulin, produces gas. Building up slowly helps for most people; persistent symptoms need proper assessment, not a bigger dose.

One test for any biotic on any shelf

Think of the three words as a vocabulary lesson, not a shopping list. Eat fibre and food you like. Consider a named probiotic only for a use where that strain has been tested, and treat a prebiotic or postbiotic claim the same way: ask what was tested, in whom, at what dose, and for what outcome. A short, specific answer to those four questions is worth more than any number of "biotics" on the front of the pack.

Written by Smart Supplements with AI assistance from the sources linked in the text. It has not been reviewed by a clinician.

Last updated: 12 October 2026

This article is for informational purposes only and is not intended as medical advice. Always consult a healthcare professional before starting any new supplement, especially if you take prescription medication.