Prebiotics, Probiotics, Synbiotics and Postbiotics: The Terminology
Probiotics are live beneficial organisms; prebiotics are substrates selectively used by host microbes (inulin, FOS, GOS, resistant starch — selectivity is why not all fibre qualifies); synbiotics combine both, either complementary or synergistic; postbiotics are preparations of inanimate microbes or their components. The distinctions matter when reading trials: a study of a fibre-enriched fermented product tests a synbiotic, and its results can't be attributed to the fermented part alone.
Four related terms are routinely used interchangeably and mean different things. The definitions below follow the International Scientific Association for Probiotics and Prebiotics consensus statements. **Probiotic** — *live microorganisms which, when administered in adequate amounts, confer a health benefit on the host*. The organisms themselves, alive, at a defined dose, with strain-specific evidence. See Probiotic: Why the Strict Definition Excludes Most Fermented Foods. **Prebiotic** — *a substrate that is selectively utilised by host microorganisms conferring a health benefit*. Not an organism but food for organisms already present. The word "selectively" matters: a prebiotic must feed beneficial microbes specifically rather than everything indiscriminately, which is why not all dietary fibre is prebiotic. The best-established examples are inulin, fructo-oligosaccharides, galacto-oligosaccharides and resistant starch. Fermenting them is the main source of Short-Chain Fatty Acids: What Gut Bacteria Make From Fibre. **Synbiotic** — a combination of the two, in which the components act together. ISAPP distinguishes *complementary* synbiotics, where each part works independently, from *synergistic* synbiotics, where the substrate is chosen specifically to feed the co-administered organism. This distinction matters when reading trials. A study of a fibre-enriched fermented drink is a study of a synbiotic, and its results cannot be attributed to the fermented drink alone — the added prebiotic may be doing most of the work. **Postbiotic** — *a preparation of inanimate microorganisms and/or their components that confers a health benefit*. Deliberately includes killed cells and cell fragments, since some effects do not require viability. This is the category that heat-treated products belong in — they are not probiotics, but they are not necessarily inert either. ## Why the precision is worth keeping The terms describe different mechanisms, and conflating them produces claims the evidence does not support. "Contains live cultures" is not "is a probiotic". A benefit shown for a synbiotic is not a benefit shown for its fermented component. A pasteurised product cannot be probiotic regardless of what it contained before heating.