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Probiotics, Prebiotics & Fermented Foods

ISAPP definitions of probiotic, prebiotic, synbiotic, postbiotic and fermented food, with the difference between fermented foods and probiotics, and what the evidence is for each.

Content last updated . This page has not been medically reviewed.

Safety first. Supplements can cause side effects and interact with medicines. Read mushroom safety and interactions before you use any mushroom product.

These words are used loosely in marketing. ISAPP, the International Scientific Association for Probiotics and Prebiotics, published consensus definitions.

Definitions

TermISAPP meaning
ProbioticLive microorganisms that, when given in adequate amounts, confer a health benefit on the host. [1]
PrebioticA substrate selectively utilized by host microorganisms conferring a health benefit. [2]
SynbioticA mixture of live microorganisms and substrate(s) selectively used by host microorganisms that confers a health benefit. [3]
PostbioticA preparation of inanimate microorganisms and/or their components that confers a health benefit. [4]
Fermented foodFoods made through desired microbial growth and enzymatic conversions of food components. [5]

Fermented is not the same as probiotic

A fermented food becomes a probiotic food only if it contains live microorganisms of a defined strain with a shown health benefit. [5][1] Many fermented foods are pasteurized or cooked after fermenting, which kills the microbes. This is not necessarily a loss: in a trial, pasteurized sauerkraut changed the gut more than fresh. [6]

What the research shows

  • Probiotic supplements are studied strain by strain. In an irritable bowel syndrome network meta-analysis, probiotics improved symptoms, especially Bifidobacterium and Lactobacillus strains; prebiotics and synbiotics did not show significant improvement. [7]
  • A review in people with diagnosed depression or anxiety found up to 8 weeks of probiotics reduced symptoms; prebiotics appeared to help anxiety but need more research. [8]
  • Fermented foods are studied less directly: for example kefir (16 trials, mostly low quality) and kimchi (small trials). [9][10]

Results for a probiotic capsule do not apply to a yogurt, and the reverse. See Fermented foods vs probiotic supplements.

Safety

  • People with weakened immune systems or serious illness should ask a clinician before live-culture products.
  • Pregnancy: pasteurized dairy only. [11]

Frequently asked questions

Is yogurt a probiotic?

Only if a defined strain with a shown benefit is present. [1]

What is a prebiotic food?

Foods containing fibers that gut microbes use are often called prebiotic, but ISAPP's definition requires a substrate selectively used by host microorganisms with a shown health benefit. [2]

References

  1. 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.
  2. Gibson GR 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. PMID 28611480.
  3. 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.
  4. 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.
  5. 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.
  6. Schropp N et al. The impact of regular sauerkraut consumption on the human gut microbiota: a crossover intervention trial. Microbiome 2025. PMID 39940045.
  7. 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.
  8. 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.
  9. Kairey L et al. The effects of kefir consumption on human health: a systematic review of randomized controlled trials. Nutr Rev 2023. PMID 35913411.
  10. Kim EK et al. Fermented kimchi reduces body weight and improves metabolic parameters in overweight and obese patients. Nutr Res 2011. PMID 21745625.
  11. Clemson Cooperative Extension. Food Safety for Pregnant Women and Their Babies. Extension fact sheet.
Education, not medical advice. This page is for general information. It does not diagnose or treat any condition and is not a substitute for care from a qualified health professional. Natural does not automatically mean safe. Ask a clinician or pharmacist before changing your care, especially if you take medication, are pregnant or breastfeeding, or have a health condition.