What Is a Postbiotic? ISAPP 2021 Definition, Examples, and Evidence
Updated April 2026 · Based on ISAPP Postbiotic Consensus 2021 · Salminen et al., Nature Reviews Gastroenterology & Hepatology
The ISAPP 2021 Definition
"A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host."
Salminen et al., Nature Reviews Gastroenterology & Hepatology, 2021
Why Postbiotics Matter
The postbiotic concept emerged from a fundamental observation: some of the health benefits attributed to "probiotic foods" cannot be explained by live bacteria alone. Sourdough bread (baked at 200C, killing all bacteria) still affects blood glucose response. Fermented dairy products heated to 90C retain anti-inflammatory properties. The health benefit survives the death of the microorganism.
The ISAPP (International Scientific Association for Probiotics and Prebiotics) formalised the postbiotic category in 2021 (Salminen et al., Nat Rev Gastroenterol Hepatol) to distinguish these inanimate preparations from live probiotics. The definition requires that the preparation be formally characterised (you must know what's in it), inanimate (not live), and that there is evidence of health benefit.
Critically, postbiotics are not simply "dead probiotics" - the ISAPP definition explicitly excludes crude fermented food extracts unless the active component has been characterised. A heat-killed L. acidophilus preparation with documented immune-modulating activity qualifies; a generic "spent probiotic broth" without characterisation does not.
Categories of Postbiotics with Evidence
Short-Chain Fatty Acids (SCFAs)
Strong EvidenceButyrate, propionate, and acetate are the primary SCFAs produced when gut bacteria ferment prebiotic fibres. These are the most studied postbiotics. Butyrate is the primary energy source for colonocytes (colon cells), is anti-inflammatory (inhibits NF-kB signalling), promotes intestinal barrier integrity, and stimulates peristalsis. Butyrate supplementation has evidence for reducing inflammation in IBD and improving gut barrier function. Tributyrin (a butyrate prodrug) and sodium butyrate are available as supplements.
Cite: Canani et al., J Nutr Biochem 2011; Hamer et al., Aliment Pharmacol Ther 2008
Heat-Killed L. helveticus MCC1848 (Lactobacillus MCC1848)
Emerging EvidenceHeat-killed (tyndallised) L. helveticus MCC1848 is produced by Kirin Holdings (Japan) and is one of the first commercially characterised postbiotics. RCTs have shown improvements in sleep quality and psychological stress scores. The mechanism involves interaction with immune cells via bacterial cell wall components (peptidoglycan, lipoteichoic acid) even in the dead state. Unlike live probiotics, it requires no refrigeration and has no risk of bacteraemia.
Cite: Nakakita et al., Benef Microbes 2016; Takada et al., Benef Microbes 2017
Bacterial Cell Wall Fragments (Peptidoglycan, Lipopolysaccharide)
Emerging EvidenceCell wall fragments from Gram-positive bacteria (peptidoglycan, lipoteichoic acid) and Gram-negative bacteria (lipopolysaccharide in beneficial amounts) interact with pattern recognition receptors (TLRs, NODs) in intestinal immune cells. This interaction "trains" the innate immune system without requiring live bacterial replication. The immune education role of dead bacteria is an active area of microbiome immunology research.
Cite: Salminen et al., Nat Rev Gastroenterol Hepatol 2021
Exopolysaccharides (EPS) from Fermented Foods
Limited EvidenceSome Lactobacillus and Bifidobacterium species produce exopolysaccharides - polysaccharide coatings that protect cells and interact with the host immune system. EPS from kefir and some fermented dairy products retain biological activity even after pasteurisation. Human clinical evidence is currently limited to mechanistic studies and small pilots.
Cite: Salminen et al., Nat Rev Gastroenterol Hepatol 2021
Postbiotics in Food: What Counts
Several commonly consumed fermented foods contain meaningful postbiotic components even though their live bacteria are killed during processing:
- Sourdough bread: Contains organic acids (lactic, acetic), bioactive peptides, and SCFA precursors from fermentation. No live bacteria survive baking. The fermentation metabolites explain sourdough's lower glycaemic index versus standard wheat bread.
- Pasteurised kimchi and sauerkraut: Heat kills live bacteria. However, organic acids, bacteriocins, and cell wall fragments retain some immune-modulating activity. Postbiotic rather than probiotic.
- Miso soup (miso added to boiling water): If miso is added to boiling water, bacteria are killed. The resulting soup contains SCFA precursors and bacterial cell wall components from the koji fermentation - postbiotic properties.
- Vinegar (especially raw ACV "with mother"): The "mother" contains dead Acetobacter. Acetic acid (the active component) is a postbiotic SCFA with evidence for blood glucose modulation at high concentrations.
The Safety Advantage of Postbiotics
The primary clinical motivation for the postbiotic category is safety in vulnerable populations. Live probiotics carry a very small but documented risk of bacteraemia or fungaemia in immunocompromised patients, those with central venous catheters, and severely premature infants. These cases are rare, but they are real.
Postbiotics, containing no live microorganisms, cannot cause infection. For patients who want the gut-health benefits of probiotic-type supplementation but are immunocompromised, on high-dose immunosuppressants, have a CVC, or are recovering from major surgery, heat-killed preparations and butyrate supplements may be preferable to live probiotics.
This is an evolving area. Always consult a clinician for supplement decisions in immunocompromised states.
How Postbiotics Relate to the Probiotic-Prebiotic-Postbiotic Cycle
The three categories form a functional cycle. Prebiotics (non-digestible fibres) pass through the small intestine undigested and are fermented by probiotics (and resident gut bacteria) in the colon. This fermentation produces postbiotics (SCFAs like butyrate, propionate, and acetate), which then act on colonocytes, immune cells, and other organs via the portal circulation. Consuming a synbiotic (probiotic + prebiotic together) optimises this production chain.
You can increase postbiotic production endogenously by eating prebiotic-rich foods (garlic, oats, legumes, cooked and cooled potatoes) which feed gut bacteria, which produce SCFAs. You don't need to take a postbiotic supplement if your diet consistently provides prebiotic substrate and your resident microbiome is diverse. The highest need for postbiotic supplementation is in people with severely depleted gut bacteria (after heavy antibiotics, chemotherapy, or major GI surgery).