Prebiotics, probiotics, postbiotics

Three Related Terms That Get Mixed Up Constantly

These are well-established, formally defined terms, though the definitions themselves are more recent than most people assume.

Prebiotics, probiotics, and postbiotics all describe different ways of interacting with the gut's bacterial community, and the three words get used interchangeably far more often than they should.

Getting the distinction straight makes it much easier to judge what a product or a habit is actually likely to do.

Live bacteria, their food, and what they leave behind

Probiotics are live microorganisms, most often specific strains of Lactobacillus or Bifidobacterium, that can survive digestion and temporarily take up residence in the gut when consumed in the right amounts.

Prebiotics aren't bacteria at all. They're specific fibres and compounds, such as inulin and fructooligosaccharides, that exist specifically to feed the bacteria already living in the gut, rather than to add new ones.

Postbiotics are the newest term of the three, and the International Scientific Association for Probiotics and Prebiotics only settled on a formal, agreed definition for the word in 2021, after more than a year of expert debate. They're defined as preparations of inactivated microorganisms, or components of them, that still confer a measurable health benefit, even though the microbes themselves are no longer alive.

Source: The International Scientific Association of Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of postbiotics

Why the postbiotics definition took so long to agree on

Before 2021, researchers used a scattered mix of terms, non-viable probiotics, heat-killed probiotics, paraprobiotics, ghost probiotics, for what are now all grouped under the single postbiotics label.

The expert panel specifically drew a boundary around what counts. A postbiotic must contain actual inactivated microbial cells or their components. A purified metabolite alone, with no cell material present at all, doesn't meet the definition, even if it came from a microbe originally.

This kind of precision matters for research and regulation. Without a shared definition, it becomes almost impossible to compare results from one study to the next, or to know what a commercial product actually contains when the label says "postbiotic".

Source: Frequently asked questions about the ISAPP postbiotic definition

None of these work in isolation from the rest of the gut

A probiotic strain introduced into the gut still has to compete for space and resources with whatever bacterial community is already established there. Simply adding more bacteria doesn't guarantee they'll stay, thrive, or do anything useful once they arrive.

This is where prebiotics and probiotics genuinely work together. A probiotic strain survives and establishes itself far better when the diet also supplies the specific fibre and plant compounds it needs to keep feeding on, once it's arrived.

Introducing bacteria without feeding them afterwards is a bit like restocking a zoo enclosure without arranging ongoing food deliveries. The new arrivals won't last long on their own, no matter how well-suited they were to begin with.

Source: A roundup of the ISAPP consensus definitions: probiotics, prebiotics, synbiotics, postbiotics and fermented foods

What this means for you

None of these three categories is inherently better than the others, they simply do different jobs, and a probiotic supplement is never a stand-alone fix without the diet to support what it's trying to establish. In addition, the research above shows a genuine, agreed scientific basis now exists for all three terms, replacing what used to be a confusing tangle of overlapping labels. A varied, plant-rich diet supplies the prebiotic fibre this research ties to helping any probiotic actually establish itself and do useful work. That's the same idea behind GutLab, everyday gut health starts with regularly feeding the microbiome a broad range of plant-rich ingredients.