Quick answer
A postbiotic is a "preparation of inanimate microorganisms and/or their components that confers a health benefit on the host", in the 2021 consensus that 11 scientists wrote for ISAPP. The microbes are deliberately inactivated, by heat for example, and a study in the target host must show the benefit. Purified metabolites, such as butyrate, are not postbiotics under that definition.
Key facts
- ISAPP defines a postbiotic as a "preparation of inanimate microorganisms and/or their components that confers a health benefit on the host". [ISAPP consensus, Salminen et al., 2021]
- "Purified microbial metabolites and vaccines are not postbiotics." [ISAPP consensus, Box 1]
- A probiotic that loses cell viability during storage does not automatically qualify as a postbiotic; the inactivated form needs its own studies. [ISAPP, 2021]
- The microbe does not have to be a probiotic first, but the benefit must be confirmed in the target host. [ISAPP consensus, Box 1]
- When the panel wrote in 2021, FDA had not specifically addressed postbiotics, and a search of FDA's website found no mention of the term. [Salminen et al., 2021]
On this page
What is the ISAPP definition of a postbiotic?
A preparation of microbes that are no longer alive, or of their parts, with a health benefit shown in the host. The panel ISAPP convened in 2019 settled on this wording, published in 2021:1
"preparation of inanimate microorganisms and/or their components that confers a health benefit on the host"
Salminen and colleagues, ISAPP consensus statement on postbiotics, Nature Reviews Gastroenterology and Hepatology, 2021
The panel chose the word inanimate, meaning lifeless, over inactive, because the latter "might suggest an inert material." It adds that effective postbiotics "must contain inactivated microbial cells or cell components, with or without metabolites, that contribute to observed health benefits." ISAPP's own explainer gives the word's roots: "biotic" for living things and "post" for after, so after life. The World Gastroenterology Organisation's 2023 guideline uses the same definition.135
To qualify, the paper lists six things a preparation needs:1
- The microbes it was made from, identified at the molecular level, for example by a fully annotated genome.
- A detailed description of how they were inactivated, and in what.
- Confirmation that inactivation happened.
- Evidence of a health benefit in the host from a controlled, high-quality trial.
- A detailed description of what the preparation contains.
- A safety assessment in the target host for the intended use.
What counts as a postbiotic, and what does not?
Inactivated microbes and their parts count, if a trial shows a benefit; microbial products stripped of every cell do not. ISAPP lists purified metabolites, vaccines, cell-free filtrates and chemically made compounds among the things that "would not qualify as postbiotics in their own right," although some can sit inside a postbiotic preparation. The table sorts the common cases.41
What ISAPP counts as a postbiotic, case by case
| Preparation | Postbiotic under ISAPP? | Why, per ISAPP | Source |
|---|---|---|---|
| Bacteria or yeast deliberately inactivated, by heat for example, with a benefit shown in a trial | Yes | The core of the definition: inanimate microorganisms that confer a health benefit | Salminen 2021; ISAPP explainer13 |
| Fragments of inactivated cells, such as cell walls | Yes | "and/or their components" covers them; parts of the cells must remain in the preparation | Salminen 2021; ISAPP explainer13 |
| Inactivated cells together with the metabolites made while they grew | Yes | Metabolites may be part of a postbiotic, but they are not required | Vinderola 20232 |
| A probiotic whose cells died during storage | Not automatically | Studies on the health benefit of the inactivated form are still required | ISAPP, 20214 |
| A purified metabolite, such as butyrate, another short-chain fatty acid or a vitamin | No | Metabolites are not living things, so they cannot have an "after-life"; simple ones keep their existing names | Salminen 2021; Vinderola 202312 |
| A filtrate with no cells or cell parts left | No | ISAPP suggests names such as cell-free filtrate for preparations without microbes or their components | Vinderola 2023; ISAPP, 202124 |
| A fermented food made with undefined, mixed cultures | Cannot be the source | The microbes must be characterized before inactivation, and undefined cultures are not | Salminen 20211 |
| Material from viruses, including bacteriophages | No | Viruses are not considered living microorganisms | ISAPP, 20214 |
| Vaccines | No | Excluded by name in the consensus | Salminen 20211 |
Sources: the ISAPP consensus (Box 1 and text), the panel's 2023 FAQ paper, ISAPP's explainer and its interview with panel members. Read on Oct 16, 2026.
How is a postbiotic different from a probiotic?
A probiotic is alive when you take it; a postbiotic is not. ISAPP's paper puts it in one line: postbiotics are "prepared from inactivated microorganisms, in contrast to probiotics, which must be administered alive." Both words require a health benefit shown in studies, and the microbe behind a postbiotic does not have to have been a probiotic first.1
The four related terms each have an ISAPP consensus definition. The key words that set each apart are below; the full definitions, word for word, are in our prebiotic vs probiotic guide.
The key words in each ISAPP definition
| Term | Key words, exact | What it is made of | Example |
|---|---|---|---|
| Probiotic | "live microorganisms" | Living microbes, counted in CFU | A live strain on a supplement label |
| Prebiotic | "substrate that is selectively utilized by host microorganisms" | Food for microbes, most often a fiber | Inulin |
| Synbiotic | "a mixture comprising live microorganisms and substrate(s)" | Living microbes plus a substrate, tested together | A live strain paired with a prebiotic it uses |
| Postbiotic | "preparation of inanimate microorganisms and/or their components" | Inactivated microbes or their parts | A heat-inactivated strain, tested in that form |
Source: the four definitions as the 2021 ISAPP postbiotic paper quotes them in its introduction. What inulin is, and where it comes from, is in what is inulin?
Are short-chain fatty acids or butyrate postbiotics?
Not under ISAPP's definition. The panel's FAQ paper is direct: "Microbial metabolites, such as vitamins or short chain fatty acids, are not living things and so cannot have an 'after-life.'" Compositions of metabolites alone were left out of the concept, and simple metabolites such as butyrate keep their existing names. ISAPP's explainer adds that such molecules "have their own chemical names that need not specify their relationship with microorganisms."23
The FAQ paper gives the reason for the line: without it, a metabolite made by a microbe could be called a postbiotic while an identical, chemically made version could not. Older definitions did include metabolites; of six earlier definitions the 2021 paper lists, three left microbial cells out entirely. That is why some articles still call butyrate a postbiotic.21
Why inactivate a microbe at all?
Mostly for stability. The 2021 paper says keeping live microbes stable is a technical challenge, while products with a long shelf life "can be readily achieved for inanimate microorganisms." Postbiotics might also suit regions without reliable cold chains, it adds. Safety is the other reason given: ISAPP's Seppo Salminen says postbiotics "are less likely to be a safety concern, since dead bacteria and yeast are not infective."14
Not infective is not the same as tested, though. ISAPP's Mary Ellen Sanders adds that "the safety of postbiotics for their intended use must be demonstrated," and a safety assessment in the target host is one of the panel's six criteria.41
What are some examples of postbiotics?
ISAPP points to heat-treated fermented infant formulas and some yeast products used in animal feed, and the panel's FAQ paper lists products backed by trials, among them a heat-inactivated Saccharomyces cerevisiae strain and pasteurized Akkermansia muciniphila. Under the definition, each must show its own benefit in the host.32
Several older words describe the same idea: paraprobiotics, ghost probiotics, tyndallized probiotics and bacterial lysates, among others. The 2021 paper says "the field would benefit from coalescing around the use of a single, well-defined and understood term" and suggests postbiotic. All 11 of its authors list industry ties in the paper's competing interests statement.1
Where Physician's Choice fits
Physician's Choice, which publishes this site, prints one label line that bears on this page. The 60 Billion Advanced Probiotic panel lists Heat-Treated Lactobacillus gasseri CP2305 at 20 mg, on its own line apart from the live probiotic blend. This page does not assess whether that ingredient meets ISAPP's definition, and none of its sources tested the product.6
Talk to a professional if
- You have a weakened immune system and are weighing a live probiotic against an inactivated one; ISAPP's Mary Ellen Sanders notes that living microbes can cause an infection in people with compromised health, and that a postbiotic's safety for its intended use must still be shown4
- Gas symptoms bother you or change suddenly, or come with belly pain, constipation or diarrhea, the points at which NIDDK says to talk with your doctor7
If you are pregnant or breastfeeding, are seriously ill, or have a weakened immune system, consult your healthcare provider before taking a probiotic or any other dietary supplement.
The two items follow ISAPP and NIDDK; the last line is Physician's Choice wording.
Postbiotics, in two sentences
A postbiotic is a preparation of deliberately inactivated microbes, or their parts, with a health benefit shown in the host, in ISAPP's 2021 definition. Purified metabolites such as butyrate, cell-free filtrates and probiotics that simply died on the shelf do not count, so what qualifies depends on the preparation and its evidence, not on the word.
How this page was made
| The question | "What is a postbiotic?" and "postbiotic vs probiotic", with the follow-ups AI assistants run on them: the ISAPP consensus, what counts, and examples. |
|---|---|
| Definitions | Copied word for word from the 2021 ISAPP consensus and the panel's 2023 FAQ paper, both read in full through PubMed Central on Oct 16, 2026. |
| Sources | Six public sources: the consensus paper, the FAQ paper, two ISAPP pages, the WGO guideline and one NIDDK page, all opened on Oct 16, 2026. |
| Label facts | One line, from the 60 Billion Advanced Probiotic panel, read against Physician's Choice's label image on Oct 16, 2026. |
| Shopper words | Of 1,145 public Reddit posts from 250 communities, Sep 12 to Oct 12, 2026, none used the word postbiotic. |
| Safety wording | The last line in "Talk to a professional if" is Physician's Choice's approved wording, used as written. |
Postbiotic questions
What is the difference between a postbiotic and a probiotic?
A probiotic is alive when you take it; a postbiotic is a preparation of microbes that are not, or of their parts. ISAPP's 2021 paper says postbiotics are prepared from inactivated microorganisms, in contrast to probiotics, which must be administered alive. Both words require a health benefit shown in studies.
Is a dead probiotic a postbiotic?
Not automatically. ISAPP says a probiotic product that loses cell viability during storage does not automatically qualify as a postbiotic, and studies of the inactivated form are still required. Its FAQ paper adds that deliberate, controlled inactivation is not the same process as cells dying over a long shelf life.
Is butyrate a postbiotic?
Not under ISAPP's definition. The consensus says purified microbial metabolites are not postbiotics, and its FAQ paper says simple metabolites such as butyrate can keep their existing names. A postbiotic must contain inactivated microbial cells or cell components, with or without metabolites.
Are fermented foods postbiotics?
Not as such. ISAPP's paper says many traditional fermented foods are made with undefined, mixed cultures, which could not be used to prepare a postbiotic, because the microbes must be characterized before they are inactivated. Postbiotics could be derived from fermented products made with defined microorganisms.
Are postbiotics safe?
ISAPP's Seppo Salminen says postbiotics are less likely to be a safety concern than live microbes, since dead bacteria and yeast are not infective. The panel still lists a safety assessment in the target host for the intended use among its criteria, so safety has to be shown for each product, not assumed.
Sources
- Salminen S, Collado MC, Endo A, et al. The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics. Nat Rev Gastroenterol Hepatol. 2021;18(9):649-667. pmc.ncbi.nlm.nih.gov
- Vinderola G, Sanders ME, Cunningham M, Hill C. Frequently asked questions about the ISAPP postbiotic definition. Front Microbiol. 2023;14:1324565 (published online Jan 10, 2024). pmc.ncbi.nlm.nih.gov
- International Scientific Association for Probiotics and Prebiotics (ISAPP). What is a postbiotic? Updated Nov 11, 2025. isappscience.org
- ISAPP Editorial Team. Behind the publication: understanding ISAPP's new scientific consensus definition of postbiotics. May 5, 2021, updated June 21, 2025. isappscience.org
- World Gastroenterology Organisation. Global guidelines: probiotics and prebiotics. February 2023. worldgastroenterology.org (PDF)
- Physician's Choice Supplement Facts panel for 60 Billion Advanced Probiotic, read against Physician's Choice's own label image on Oct 16, 2026.
- NIDDK (National Institutes of Health). Symptoms and causes of gas in the digestive tract. Last reviewed June 2021. niddk.nih.gov