Probiotic, prebiotic, synbiotic, postbiotic: what the words mean

Prebiotic soda. Postbiotic skin cream. Synbiotic gummies. Walk down any supplement aisle and the "-biotic" words pile up fast, usually in big letters and rarely explained.

These words actually have precise scientific definitions, written by international expert panels. Knowing them won't tell you whether a product works. It does make it much easier to notice when a label is using a word loosely. Here's a plain-English guide.

Probiotic: live microbes with a shown benefit

The standard definition comes from the Food and Agriculture Organization of the United Nations and the World Health Organization: probiotics are "live microorganisms which when administered in adequate amounts confer a health benefit on the host." In 2014, an expert panel from the International Scientific Association for Probiotics and Prebiotics (ISAPP) reviewed that definition and confirmed it still fits (Hill et al., 2014).

Three parts of that sentence do a lot of work:

• Live. The microbes have to be alive when you take them.
• Adequate amounts. There has to be enough of them. That's why labels list colony-forming units, or CFU.
• A health benefit. The benefit has to be demonstrated, not assumed.

That last point is the one marketing tends to skip. "Contains live cultures" and "probiotic" are not the same claim. A capsule of bacteria with no evidence behind it is, strictly speaking, just a capsule of bacteria.

Prebiotic: food for the microbes you already have

A prebiotic isn't a microbe at all. ISAPP's updated definition describes it as "a substrate that is selectively utilized by host microorganisms conferring a health benefit" (Gibson et al., 2017). In everyday terms, it's something your own gut microbes can use as fuel, in a way that leads to a documented benefit for you.

The key word is "selectively." A prebiotic is meant to be used by specific beneficial microbes, not just any bacteria. The panel also kept the requirement that the benefit has to be shown (Gibson et al., 2017).

Many familiar prebiotics are fibers that gut bacteria ferment, and fermentation makes gas. That's one reason a prebiotic that suits one person can leave another feeling puffy by mid-afternoon. Inulin is the classic example.

Synbiotic: two approaches in one product

A synbiotic combines live microbes with something microbes can use. ISAPP defines it as "a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host" (Swanson et al., 2020).

The panel described two kinds:

• Complementary synbiotics are a probiotic plus a prebiotic that weren't designed to work together. Each part has to meet its own definition.
• Synergistic synbiotics are designed so the added substrate is used by the microbes in the same product. Here, the parts don't each need to qualify on their own, but the combination still has to show a benefit (Swanson et al., 2020).

In practice, plenty of products labeled "synbiotic" are simply a probiotic with some inulin or FOS added. Whether that pairing was designed and tested together is a fair question to ask the company.

Postbiotic: inactivated microbes with a shown benefit

This is the newest term, and the one most often stretched. In 2021, an ISAPP panel defined a postbiotic as a "preparation of inanimate microorganisms and/or their components that confers a health benefit on the host" (Salminen et al., 2021).

Two details matter. First, a postbiotic has to contain inactivated microbial cells or cell components, with or without the substances they produce. Microbial by-products on their own, without any cells or cell parts, don't meet the definition. Second, as with every other term here, a health benefit has to be demonstrated (Salminen et al., 2021).

So a probiotic whose bacteria died on the shelf doesn't automatically become a postbiotic. Nobody has shown that particular inactivated preparation does anything.

Fermented food: not automatically a probiotic

Yogurt, kefir, sauerkraut and kimchi often get called "probiotic foods." ISAPP's 2021 consensus defines fermented foods as "foods made through desired microbial growth and enzymatic conversions of food components," and it draws a clear line between fermented foods and probiotics (Marco et al., 2021).

The reason is practical. Some fermented foods, like sourdough bread, are baked, so no live microbes are left by the time you eat them. Others do contain live microbes, but those microbes haven't been tested for a specific benefit. A fermented food can still be a good part of your diet. It just isn't a probiotic by default.

A quick cheat sheet

• Probiotic: live microbes, enough of them, with a shown benefit.
• Prebiotic: a substance your own microbes selectively use, with a shown benefit.
• Synbiotic: live microbes plus a substance microbes use, with a shown benefit.
• Postbiotic: inactivated microbes or their parts, with a shown benefit.
• Fermented food: food made with the help of microbes, which may or may not still contain live ones.

Notice the pattern. Every definition ends with a benefit that has to be demonstrated. So when a label uses one of these words, the useful follow-up question is always the same: which microbe or ingredient, how much of it, and what did studies of that exact thing find? Our science page explains how we approach that question.

Sources

1. Hill C, Guarner F, Reid G, 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. Nat Rev Gastroenterol Hepatol. 2014;11(8):506-14. PMID 24912386
2. Gibson GR, Hutkins R, Sanders ME, et al. Expert consensus document: The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol. 2017;14(8):491-502. PMID 28611480
3. Swanson KS, Gibson GR, Hutkins R, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of synbiotics. Nat Rev Gastroenterol Hepatol. 2020;17(11):687-701. PMID 32826966
4. 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. PMID 33948025
5. Marco ML, Sanders ME, Gänzle M, et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol. 2021;18(3):196-208. PMID 33398112

This article is for general information and isn't medical advice. If you have a digestive condition, a weakened immune system or take prescription medication, talk to your healthcare provider before starting a new supplement.