What Are Postbiotics? Definition, Trials and Pasteurised Kombucha

A postbiotic is "a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host": microbes that have been killed on purpose, usually by heat, and shown in a controlled trial to do something useful in the people who take them.¹ The definition comes from a 2021 expert panel convened by the International Scientific Association for Probiotics and Prebiotics, the body whose earlier panels defined probiotics and prebiotics. A probiotic is the live organism, a prebiotic is the fibre or similar substance that gut microbes feed on, and a postbiotic is the organism after it has been killed, with or without the acids and cell fragments it made while alive.^{1, 2, 3} The six randomised placebo-controlled trials of five named inactivated preparations cited below, all published since 2020, cover 41 to 110 people each; two were funded by the maker, a third was run by the maker's own researchers, and one found no difference from placebo.^{4, 5, 6, 7, 8, 9} Great Britain has no authorised health claim for any postbiotic, and the only live-culture wording it authorises is yoghurt's lactose claim, so "postbiotic" on a pack is a statement about health and "contains live cultures" is a statement about contents.^{10, 11}

Evidence at a glance

• The definition: a consensus statement in Nature Reviews Gastroenterology and Hepatology, 2021, from a panel convened in 2019; several panel members declare industry fees.¹
• Human trials of inactivated preparations: six randomised placebo-controlled trials of five preparations, 41 to 110 people each, three weeks to a year; two maker-funded, one maker-run, one with no difference from placebo (literature searched to 2 November 2026).^{4, 5, 6, 7, 8, 9}
• The law: no postbiotic, heat-killed or inactivated preparation appears on the Great Britain nutrition and health claims register, last updated 19 May 2026.¹⁰

Sources checked on 2 November 2026.

What are postbiotics?

The panel's working gloss on its own definition is the clearest version: "Postbiotics are deliberately inactivated microbial cells with or without metabolites or cell components that contribute to demonstrated health benefits."¹ Three parts of that do the work.

Inanimate. The panel chose "inanimate", meaning lifeless, over "inactive", because a killed cell can still do things: its cell wall, pili and surface molecules stay recognisable to the immune cells that line the gut, and the acids, enzymes and peptides it made while alive come with it.¹ Killing is by heat most often, or by high pressure or spray-drying. The panel does not demand that every last cell is dead, but it does demand "intentional and deliberate processing designed to inactivate" the organism; cells that happened to die in a jar do not count.¹

Preparation. A postbiotic is a specific, described thing: which organism, grown on what, killed how, containing what. Change the strain or the kill step and you have a different preparation whose effects have to be shown again, because the panel notes that intact cells and cell fragments may behave differently.¹

Health benefit. The benefit has to be shown "in the target host", meaning the species and the group of people the product is for, in a controlled trial. A postbiotic is not assumed to work because its live parent did, and the panel says a postbiotic "does not have to be derived from a probiotic" at all.¹

Two things are excluded by name. "Purified microbial metabolites and vaccines are not postbiotics", so a capsule of butyrate or lactic acid on its own is not one, however microbial its origin.¹ Injections are outside the scope; a postbiotic is swallowed or applied to a surface such as the skin, mouth or nose. The older names for the same idea, paraprobiotics, ghost probiotics, tyndallised probiotics, heat-killed probiotics and bacterial lysates, are what the panel wanted to replace with one word.¹ Psychobiotics are a different coinage, live cultures studied for mood, covered on what do psychobiotics do.

Postbiotics vs probiotics vs prebiotics

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}

The three are not rungs on a ladder. A trial result belongs to the preparation that was tested: a live strain's evidence does not transfer to the same strain killed, and a killed preparation's evidence does not transfer back.¹ What live cultures have been measured doing is on the probiotics guide, use by use.

Are postbiotics better than probiotics?

Neither is better; they are different preparations with separate trials, and the useful question is what each has been measured doing.¹ Live cultures have the larger body of evidence, listed use by use on the probiotics guide. The six placebo-controlled trials below cover five preparations, and none of the six papers reports the same preparation retested by an independent group.^{4, 5, 6, 7, 8, 9} What a postbiotic offers as a format is that it does not need to be kept alive: it tolerates heat, long storage and inclusion in a processed food, which is why the panel and the trial authors say the category attracts interest.^{1, 6} That is a property of the format, not evidence of a benefit, and the panel is as firm on this side as the other: a postbiotic "cannot be presumed to be safe solely based on the safety profile of the progenitor microorganism", and each preparation needs its own safety assessment.¹

What have trials of postbiotics found?

Other heat-killed strains have been tested in trials these papers cite; six trials of five preparations are set out below. Two of the six used the same preparation: soya powder, skimmed milk powder and sodium citrate fermented by three named strains, Lacticaseibacillus paracasei Zhang, Lactiplantibacillus plantarum P-8 and Bifidobacterium animalis subsp. lactis V9, then pasteurised and spray-dried to 30 billion inactivated cells per gram.^{4, 5}

The designs matter as much as the figures. Both fermentate trials were crossovers: in the diarrhoea trial the effect carried over into the placebo period, so the authors compared the first 21 days only, and in the constipation trial no difference remained two weeks after stopping. The constipation paper does not say whether its 0.81 and 0.68, from about 0.6 at the start, are counts per week or averages of daily counts. In the overweight-adults trial, cholesterol rose in the placebo group over the study, which the authors attribute to the season, so the difference is a smaller rise, not a fall. The older-adults record gives no strain, dose or funder, and its full text is not freely available.^{4, 5, 7, 9}

Is pasteurised kombucha a postbiotic?

Not under the definition, unless a specific pasteurised kombucha has been characterised and trialled as one, and none has.^{1, 12} Kombucha is tea fermented by a mixed culture of acetic acid bacteria and yeasts, a fermented food in the sense of "desired microbial growth and enzymatic conversions of food components".^{12, 13} Unpasteurised, it carries live cultures, which is why what drinks have probiotics counts it; pasteurised, the cultures are killed so the drink keeps on a shelf. What is left is inanimate cells and the fermentation acids, which is the raw material of a postbiotic rather than a postbiotic. The panel's criteria ask for identified organisms (a kombucha culture is a mixed community of several bacterial and yeast genera), a described and confirmed inactivation step, a described composition, a safety assessment and a controlled trial showing a benefit; it notes that soy sauce and sourdough also carry large numbers of killed microbes, and reserves the word for specific, described preparations.¹

One controlled study has used a pasteurised kombucha drink: an open-label Russian pilot in 40 women with constipation-predominant irritable bowel syndrome, who drank 220 mL a day of a pasteurised kombucha enriched with 2.53 g of inulin and five B vitamins, or water, for ten days. Stool frequency rose from 0.60 to 0.85 a day on the drink (p = 0.004) and did not change significantly on water. There was no placebo and no blinding, and the authors write that pasteurisation left the drink unable to act as a probiotic and that the inulin and the organic acids may explain the result. Funded by the Russian Science Foundation; no conflicts reported.¹³ None of this is a mark against a pasteurised fermented tea, which keeps its acids and its flavour; it is a statement about what a word means.

Can a label say "postbiotic" in Great Britain?

Not as a claim about health, because no such claim is authorised.¹⁰ The Great Britain nutrition and health claims register has no entry for any postbiotic, heat-killed or inactivated preparation; its only authorised live-culture wording is that live yoghurt cultures "improve lactose digestion of the product", and every named-strain "probiotic" claim on the register is listed as non-authorised.¹⁰ A health claim in law is any claim that "states, suggests or implies that a relationship exists between a food category, a food or one of its constituents and health".¹¹ The panel saw this coming: writing about the European Union, whose rule Great Britain kept, it expected that because its definition requires a health benefit, using the word on a food or supplement "would require health claim approval".¹ What a label can say is what is in the pack. "Contains live cultures", with the species and the count, is a statement of fact; "heat-treated cells of Bifidobacterium longum CECT 7347, 2.5 billion a day" would be the same kind of statement. Neither says what the contents do, and that is the line. On a pack, the things worth looking for are the ones the definition asks for: the organism named to strain level, how it was inactivated, how many inactivated cells a dose holds, and a controlled trial of that exact preparation in people like you.¹

What foods contain postbiotics?

Under the definition, very few, because a food has to be a described and trialled preparation, not a food whose microbes have died.¹ Pasteurised sauerkraut, kimchi and kombucha, heat-treated yoghurt, soy sauce and sourdough all contain inanimate cells and the metabolites of fermentation; the panel gives soy sauce and sourdough as its examples of foods that carry killed microbes without being postbiotics, and the others are in the same position.¹ The one food category the panel discusses as able to qualify is fermented infant formula, milk fermented with lactic acid bacteria and then dried so that it holds no substantial live bacteria, "to the extent that the microorganisms used to ferment these formulas are characterized adequately".¹ The six trial preparations were capsules, tablets or a powder of a single heat-killed strain or a spray-dried fermentate.^{4, 5, 6, 7, 8, 9}

What we don't know

• Whether any of the six results holds in a second, independent trial: none of the six papers reports any of the five preparations tested by a group other than the one that first tested it, and the two trials of the spray-dried fermentate were run by the same group in young adults in China.^{4, 5}
• The strain, dose and funder of the older adults trial, which the record does not give and whose full text is not freely available (literature read to 2 November 2026).⁹
• How many live cells survive different inactivation methods: the panel says such comparisons "have not yet been published".¹

Frequently asked questions

Are postbiotics the same as heat-killed probiotics?

Usually, but the term is wider: it covers deliberately killed cells of any characterised microbe, which need not have been a probiotic, plus their fragments and the metabolites made before killing, as long as a trial has shown a benefit.¹

Do the dead cells in a live-culture capsule count as postbiotics?

No. A postbiotic requires a deliberate, designed step that inactivates the organism, and a trial of the result; cells that died in storage meet neither test.¹ The panel does raise the open question of what those cells contribute to a live product's effect, and asks trials of live cultures to count the live and the inactivated cells over the course of the study.¹

Are short-chain fatty acids postbiotics?

Not on their own. A postbiotic preparation may contain the butyrate, acetate and propionate its cells made, but the panel excludes purified metabolites by name.¹ Short-chain fatty acids made in your colon from fibre are a product of your own microbes, not a preparation you take.^{1, 3}

Are postbiotics safe?

Preparation by preparation. The panel's view is that a killed cell cannot multiply in the body, but that safety still has to be assessed for each product, because fragments of some bacteria can themselves provoke a strong immune reaction.¹ Five of the trials report on adverse events, and none reports a serious event linked to the preparation. The constipation trial reports none serious and mostly mild gut symptoms in both phases, the diarrhoea and infant trials none linked to the preparation, the overweight-adults trial all mild and none judged related, and the digestive-symptom trial two mild events judged related, stomach pain and wind, and none serious.^{4, 5, 6, 7, 8} The NHS asks anyone with an existing health condition or a weakened immune system to talk to a doctor before taking live-culture supplements, and the panel's point means the same conversation is worth having about an inactivated product.^{1, 14}

Sources

1. 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 an expert panel convened in 2019; several panel members declare industry speaking and consulting fees; two errata published, no retraction. PubMed 33948025.
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. 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. PubMed 28611480.
4. Ma T, et al. Efficacy of a postbiotic and its components in promoting colonic transit and alleviating chronic constipation in humans and mice. Cell Reports Medicine, 2025. Randomised double-blind placebo-controlled crossover trial, 110 adults with chronic constipation, two 3-week periods with a 2-week washout, twelve 0.6 g tablets a day of a spray-dried fermentate of three named strains (30 billion inactivated cells per gram); funded by Chinese government research programmes, no competing interests declared, tablets supplied by their maker. PubMed 40286792.
5. Guo S, et al. Effects of postbiotics on chronic diarrhea in young adults: a randomized, double-blind, placebo-controlled crossover trial assessing clinical symptoms, gut microbiota, and metabolite profiles. Gut Microbes, 2024. Randomised double-blind placebo-controlled crossover trial, 72 adults aged 18 to 35 randomised and 69 completing, two 21-day periods with a 14-day washout, twelve 0.6 g tablets a day of the same fermentate; funded by Chinese government science funds, no conflict of interest reported. PubMed 39189588.
6. Naghibi M, et al. Effect of postbiotic Bifidobacterium longum CECT 7347 on gastrointestinal symptoms, serum biochemistry, and intestinal microbiota in healthy adults: a randomised, parallel, double-blind, placebo-controlled pilot study. Nutrients, 2024. Randomised double-blind placebo-controlled pilot trial, 60 adults aged 18 to 65 with mild to moderate digestive symptoms, 8 weeks, 2.5 billion heat-treated cells (50 mg) a day; part-funded by the maker, which took part in the study design and the writing, with ten authors its employees, and by a Spanish government grant. PubMed 39599737.
7. Tanaka Y, et al. Daily intake of heat-killed Lactobacillus plantarum L-137 improves inflammation and lipid metabolism in overweight healthy adults: a randomized-controlled trial. European Journal of Nutrition, 2020. Randomised double-blind placebo-controlled trial, 100 adults with a body mass index of 23 to 29.9, 12 weeks, 10 mg of heat-killed cells a day; the paper states no funding and no conflict of interest, and the authors are employed by the strain's maker and a university. PubMed 31620886.
8. Takeshita K, et al. The effectiveness of heat-killed Pediococcus acidilactici K15 in preventing respiratory tract infections in preterm infants: a pilot double-blind, randomized, placebo-controlled study. Nutrients, 2024. Randomised double-blind placebo-controlled pilot trial, 41 infants born before 36 weeks, one year, 9.1 mg of heat-killed powder (50 billion cells) in 1 g of dextrin a day; funded by the strain's maker, three authors its employees, with a Japanese research grant. PubMed 39519468.
9. Bravo M, et al. Postbiotic intervention improves bowel function and promotes systemic health in institutionalized older adults. Molecular Nutrition and Food Research, 2026. Randomised double-blind trial, 48 institutionalised older adults, 8 weeks, inactivated bacterial cells; strain, dose and funding not stated in the record, first author affiliated to a commercial company, no free full text. PubMed 42757563.
10. 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 and non-authorised lists searched for postbiotic, heat-killed, heat-treated and inactivated entries (none) and for the live yoghurt cultures entry and the probiotic strain entries. gov.uk.
11. Regulation (EC) No 1924/2006 as retained in GB law, Article 2(2)(5): "health claim" means any claim that states, suggests or implies that a relationship exists between a food category, a food or one of its constituents and health. legislation.gov.uk.
12. Marco ML, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nature Reviews Gastroenterology and Hepatology, 2021. Consensus statement from a panel convened in September 2019; several authors declare industry fees. PubMed 33398112.
13. Isakov VA, et al. Evaluation of the efficacy of kombucha-based drink enriched with inulin and vitamins for the management of constipation-predominant irritable bowel syndrome in females: a randomized pilot study. Current Developments in Nutrition, 2023. Open-label randomised controlled pilot, 40 women, 10 days, 220 mL a day of a pasteurised kombucha drink with 2.53 g inulin and five B vitamins against water; funded by the Russian Science Foundation, no conflicts reported. PubMed 38149073.
14. NHS. Probiotics. Page last reviewed 24 November 2022 (review overdue). nhs.uk.

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 or inactivated-culture 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.

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