Prebiotics, Probiotics, Postbiotics, and Synbiotics: What's the Difference?

If you type "prebiotic vs" into Google, one of the first suggestions is "prebiotic vs probiotic vs postbiotic." Keep going and you'll get "are prebiotics just fiber" and "what is a postbiotic energy drink." One supplement brand sells a gummy for ages 13 to 17 that it calls a "tribiotic," made with "pre+ post+ and probiotics," which is three of the four words on one label plus a fourth word that isn't in any of the scientific definitions.

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The four words do have precise meanings. All of them were written by the same group, the International Scientific Association for Probiotics and Prebiotics (ISAPP, an organization of researchers who study gut bacteria), in four papers published between 2014 and 2021. Each definition requires a health benefit, so a product that uses one of these words is supposed to have been shown to help you. How much checking stands behind each word varies a lot, and postbiotics have the least.

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Probiotics: live, and shown to help

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The ISAPP panel defined probiotics as "live microorganisms that, when administered in adequate amounts, confer a health benefit on the host" (you're the host). Both halves count. The bacteria have to be alive when you take them, and they have to have been shown to help.

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That second requirement leaves out a lot of what people call probiotic. The panel said that fermented foods whose live cultures have no evidence of a health benefit should be described as "containing live and active cultures," and "should not be called probiotic." The British Dietetic Association (the professional body for dietitians in the UK) says the same about kefir and kombucha: "these are not considered probiotics due to lack of scientific evidence regarding their health benefits."

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Whether probiotic supplements work is its own question, and we went through the evidence here. For a healthy person, the BDA's answer is short: "If you are healthy, it is unlikely a probiotic supplement will be beneficial."

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Prebiotics: food for your bacteria, with a catch

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A prebiotic is "a substrate that is selectively utilized by host microorganisms conferring a health benefit." A substrate is just something that gets used up, in this case eaten by your gut bacteria, and "selectively" means that a prebiotic has to feed specific helpful bacteria, where most food feeds whatever happens to be there.

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So if you've searched "are prebiotics just fiber," most fiber doesn't meet the definition. The ISAPP panel wrote that prebiotics "differ from most dietary fibres such as pectins, cellulose and xylans, which encourage growth of a wide variety of gut microorganisms." Fiber is still good for you for other reasons (it's the first thing on our list for improving your gut health), but only some types count as prebiotics. The BDA lists onions, garlic, asparagus, artichoke, and chicory root as sources, and inulin (often listed on labels as chicory root fiber) is the prebiotic you'll see added to bars, gummies, and sodas.

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Researchers at King's College London checked the health-benefit half in a 2019 review that pooled 11 trials covering 729 adults with IBS (irritable bowel syndrome, a common condition that causes belly pain and changes in how you poop) or similar bowel problems. Prebiotics did what they're designed to do and raised the number of bifidobacteria, one of the groups of bacteria usually counted as helpful. They didn't do any better than a placebo on pain, bloating, gas, or quality of life, and the inulin-type prebiotics made gas worse. The authors concluded that prebiotics "do not improve gastrointestinal symptoms or QoL [quality of life] in patients with IBS or other FBDs [functional bowel disorders], but they do increase bifidobacteria."

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Those were adults with gut problems, so the review can't tell you what happens in a healthy teenager. It does show that a rise in a "good" bacterium doesn't guarantee that you'll feel any different. If you want to know how much inulin your stomach can handle, the fiber-maxxing article goes through the doses.

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Synbiotics: both in one product

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Synbiotic is spelled with an n, and the ISAPP panel made a point of it: "'symbiotic' is not a synonym of synbiotic and is incorrect in this context." Its definition is "a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host," meaning live bacteria plus something for bacteria to eat, sold together.

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The panel also set a rule that plenty of products on the shelf wouldn't pass. A product that only has evidence for each part on its own, "and not as a combination product, should not be called a synbiotic." The selling point of a synbiotic is that the two parts work better together, so the combination itself has to be tested.

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When researchers at the University of Nebraska and Rush University tested exactly that, the combination didn't beat its parts. In a three-week trial in adults with obesity, the prebiotic and the probiotic strains each improved lab measures of how leaky the colon was, and the authors reported "no synergistic effects" when they were combined. Two of the authors co-own a company that develops synbiotic products, and they reported the lack of synergy anyway.

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In healthy adults, a 2025 review of 26 trials from the University of Wollongong in Australia found that synbiotics raised counts of Lactobacillus (another group of bacteria often called helpful) but found no significant change in any of the inflammation markers it measured.

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Postbiotics: dead on purpose

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The teen gummy from earlier lists a "heat-treated Lactobacillus Casei 327 Postbiotic," which means bacteria that were killed with heat before going into the gummy. The ISAPP definition of a postbiotic is "a preparation of inanimate microorganisms and/or their components that confers a health benefit on the host," and inanimate means dead. The panel chose that word because it "simply captures the fact that live microorganisms were present but have now been killed, without implying a loss of function." The idea is that some bacteria can still do something useful after they've been killed.

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Chemicals that bacteria make don't count on their own. Butyrate (a fatty acid your gut bacteria produce when they eat fiber) sold as a purified supplement isn't a postbiotic under the ISAPP definition, and the panel wrote that purified molecules like it "should instead be named using existing, clear chemical nomenclature, for example, butyric acid or lactic acid." Not every scientist agrees with where that line sits. A group of researchers wrote back in the same journal that the new definition "generates confusion," and ISAPP's own follow-up paper in 2024 admitted that the boundary "may seem to be semantics."

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The ISAPP postbiotics paper says outright that "data from human studies are limited." NCCIH (the US government's research center for complementary and integrative health) has a page defining probiotics, prebiotics, and synbiotics, and the word postbiotic doesn't appear on it (the page was last updated in 2019).

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Who wrote the definitions?

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ISAPP isn't a government agency, and the people in it are close to the industry that sells these products. In that 2024 follow-up paper, the authors disclosed that ISAPP's executive director "has previously been employed by manufacturers and distributors of probiotic and postbiotic products," and that its scientific advisor has consulted for companies including PepsiCo. The researchers who wrote in to disagree about postbiotics disclosed industry funding too, from companies including Danone.

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What the label promises

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In the European Union, "probiotic" and "prebiotic" on a food label each count as a health claim, and health claims need approval. Ireland's food safety regulator says that applications for both have gone to the EU's food safety agency, that "no application has received a positive opinion," and that "no claims on probiotics are listed on the EU register as authorised for use."

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In the US, NCCIH notes that probiotics sold as supplements "don't require FDA approval before they are marketed," so nobody has to sign off on a word like "synbiotic" or "postbiotic" before it goes on a bottle.

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So which one do you actually need?

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If you're healthy, you probably don't need any of them in a bottle. The BDA's line on probiotic supplements is above, and prebiotics are the easiest of the four to get from food. The BDA calls them "like fertiliser for our gut microbes," and the foods on its list (onions, garlic, asparagus, and the rest) are already on our best foods list.

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