Probiotics, Prebiotics & Fermented Foods
Plain-language definitions of probiotic, prebiotic, synbiotic, postbiotic and fermented food from the ISAPP expert panels, and why a fermented food is not automatically a probiotic.
Content last updated . This page has not been medically reviewed.
Safety first. Natural products can cause side effects and interact with medicines. Read functional food safety before you use anything on this page.
Words like "probiotic" and "fermented" are used loosely on food packages. This page explains how the International Scientific Association for Probiotics and Prebiotics (ISAPP), an expert group, defines them, and what the research says about each. It is information only, not personal advice.
The definitions
| Term | How the ISAPP panels define it |
|---|---|
| Probiotic | "Live microorganisms which when administered in adequate amounts confer a health benefit on the host." [1] |
| Prebiotic | "A substrate that is selectively utilized by host microorganisms conferring a health benefit." Beneficial health effects must be documented. [2] |
| Synbiotic | A mixture of live microorganisms and substrate(s) selectively used by host microorganisms that confers a health benefit. [3] |
| Postbiotic | A "preparation of inanimate microorganisms and/or their components that confers a health benefit on the host." [4] |
| Fermented food | "Foods made through desired microbial growth and enzymatic conversions of food components." [5] |
Fermented is not the same as probiotic
The fermented-food panel defined fermented foods and separately clarified how they differ from probiotics. [5] A probiotic, by definition, is a live microorganism given in adequate amounts that confers a health benefit. [1] So a fermented food is a probiotic food only if the microbes in it are alive and a benefit has been shown for them. Being fermented does not by itself show either.In an 87-person crossover trial that compared fresh with pasteurized sauerkraut, pasteurized sauerkraut changed gut bacteria and blood short-chain fatty acids more than fresh sauerkraut did. [6] That is one trial in healthy adults, so it does not mean heated foods are better. See Sauerkraut and Live cultures on labels.
What research shows
The first two rows below are about probiotic supplements, not foods. A result for a capsule does not carry over to a food, or the reverse.| Question | What the source found | Strength of evidence |
|---|---|---|
| Probiotic supplements and irritable bowel syndrome (IBS) | A network meta-analysis of 54 probiotic trials in adults reported improved symptoms, especially with Bifidobacterium and Lactobacillus strains; prebiotics and synbiotics did not show significant improvement. The authors said the best strain or combination is unclear. [7] | Promising but limited (supplements, not foods) |
| Probiotic supplements and depression or anxiety | A meta-analysis of 23 trials in people with a clinical diagnosis reported lower depression and anxiety scores with probiotics, but with high heterogeneity; prebiotics showed no significant effect on depression. [8] | Promising but limited (supplements) |
| Fermented foods in the diet | Trials of specific foods are small or low quality, for example kefir (16 trials, 12 at high risk of bias) and kimchi (small crossover trials). [9][10] | Insufficient evidence for most health outcomes |
Details are on the pages for yogurt, kefir and kimchi, and in Fermented foods and gut health: what trials found.
Frequently asked questions
Is yogurt a probiotic?
Not automatically. A probiotic is defined as live microorganisms that confer a health benefit when given in adequate amounts. [1] Whether a given yogurt meets that depends on its microbes and evidence, so check the label and see Yogurt.
What is a prebiotic food?
Foods rich in fibers that gut microbes use are often called prebiotic, but ISAPP's definition requires a substrate selectively used by host microorganisms with a documented health benefit. [2]
What is a postbiotic?
ISAPP defines it as a preparation of inanimate microorganisms and/or their components that confers a health benefit. [4]
Do fermented foods work like probiotic capsules?
Not shown. The ISAPP panel treats fermented foods and probiotics as different things, [5] and results for one product type do not transfer to another.
Who should be careful with live-culture products?
NCCIH says harm risk is greater in people with severe illness or weakened immune systems. [11] Ask a clinician or pharmacist.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Limits and uncertainty
- "Prebiotic" requires documented benefit, so calling a high-fiber food prebiotic is a label, not proof. [2] See Prebiotic foods.
- "Postbiotic" is described by ISAPP as a term used inconsistently in science and on products. [4]
- Food research is mostly short or observational, so ask what the comparison was.
Safety and who should ask a clinician
- NCCIH notes that probiotics can be found in yogurt and other fermented foods as well as supplements. [11] It also says the risk of harmful effects from probiotics is greater in people with severe illnesses or compromised immune systems, and that severe or fatal infections have been reported in premature infants given probiotics. [11] Those statements concern probiotic products generally; sources we read did not give a separate risk figure for foods. People in these groups should ask a clinician before using live-culture products. More in Probiotic safety.
- Some fermented foods may contain biogenic amines such as histamine and tyramine. [12] See Histamine, tyramine and fermented foods.
- In pregnancy, an extension fact sheet advises against foods made from raw or unpasteurized milk. [13]
- Home fermentation has its own risks; see Fermented foods safety. We do not give methods.
- Supplement versions are covered in Probiotic supplements explained and Fermented foods vs probiotic supplements.
References
- Hill C 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. PMID 24912386. Read 2026-10-23.
- Gibson GR et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on the definition and scope of prebiotics. Nat Rev Gastroenterol Hepatol 2017. PMID 28611480. Read 2026-10-23.
- Swanson KS 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. PMID 32826966. Read 2026-10-23.
- Salminen S 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. PMID 33948025. Read 2026-10-23.
- Marco ML et al. The International Scientific Association for Probiotics and Prebiotics (ISAPP) consensus statement on fermented foods. Nat Rev Gastroenterol Hepatol 2021. PMID 33398112. Read 2026-10-23.
- Schropp N et al. The impact of regular sauerkraut consumption on the human gut microbiota: a crossover intervention trial. Microbiome 2025. PMID 39940045. Read 2026-10-23.
- Wu Y et al. The Efficacy of Probiotics, Prebiotics, Synbiotics, and Fecal Microbiota Transplantation in Irritable Bowel Syndrome: A Systematic Review and Network Meta-Analysis. Nutrients 2024. PMID 38999862. Read 2026-10-23.
- Asad A et al. Effects of Prebiotics and Probiotics on Symptoms of Depression and Anxiety in Clinically Diagnosed Samples: Systematic Review and Meta-analysis of Randomized Controlled Trials. Nutr Rev 2025. PMID 39731509. Read 2026-10-23.
- Kairey L et al. The effects of kefir consumption on human health: a systematic review of randomized controlled trials. Nutr Rev 2023. PMID 35913411. Read 2026-10-23.
- Kim EK et al. Fermented kimchi reduces body weight and improves metabolic parameters in overweight and obese patients. Nutr Res 2011. PMID 21745625. Read 2026-10-23.
- National Center for Complementary and Integrative Health (NCCIH). Probiotics: Usefulness and Safety. Read 2026-10-23.
- Saha Turna N, Chung R, McIntyre L. A review of biogenic amines in fermented foods: Occurrence and health effects. Heliyon 2024. PMID 38304783. Read 2026-10-23.
- Clemson Cooperative Extension. Food Safety for Pregnant Women and Their Babies. Extension fact sheet. Read 2026-10-23.