The two words sound alike and are often used loosely, but they mean quite different things – and neither is the same as “fermented food”. The clearest definitions come from the International Scientific Association for Probiotics and Prebiotics (ISAPP), whose expert panels set them out in a series of consensus statements.
The Definitions
- Probiotics are “live microorganisms that, when administered in adequate amounts, confer a health benefit on the host.” The benefit has to be shown, for a defined strain, at a defined dose (Hill et al., 2014).
- Prebiotics are “a substrate that is selectively utilized by host microorganisms conferring a health benefit.” Most are fibres or fibre-like carbohydrates – such as inulin and fructo- and galacto-oligosaccharides – but the definition isn’t limited to fibre, and not every fibre qualifies (Gibson et al., 2017).
- Synbiotics are “a mixture comprising live microorganisms and substrate(s) selectively utilized by host microorganisms that confers a health benefit on the host.” A complementary synbiotic is simply a probiotic plus a prebiotic; a synergistic one is designed so that the substrate feeds the microbes it’s paired with (Swanson et al., 2020).
- Fermented foods are foods “made through desired microbial growth and enzymatic conversions of food components.” Many contain live microbes, but most aren’t tested probiotics, and some – bread, pasteurized sauerkraut, most beer and wine – contain none (Marco et al., 2021). Our world fermented foods table shows which traditional ferments still contain live microbes when eaten.
Where Each Comes From
- Probiotics are mostly sold as supplements or added to specific products such as some yogurts and fermented milks, with the strain named on the label. Everyday fermented foods – yogurt, kefir, raw sauerkraut and kimchi – carry live microbes but usually aren’t probiotics in the strict sense; see probiotic supplements vs fermented foods.
- Prebiotic-type carbohydrates occur naturally in foods such as chicory root, Jerusalem artichokes, onions, garlic, leeks, asparagus, bananas, legumes and whole grains, and are added to some foods and supplements.
What the Evidence Shows
The best-studied use of probiotics is preventing diarrhoea during antibiotic treatment. A meta-analysis of 63 randomised trials with nearly 12,000 participants found that probiotics reduced antibiotic-associated diarrhoea (relative risk 0.58) – about one case prevented for every 13 people treated – though the strains used were poorly documented and the results varied (Hempel et al., 2012). For most other claims the picture is strain-specific and less clear, which is why the definitions insist on a demonstrated benefit. For eating more fermented foods in general, see how much fermented food per day; for the gut after antibiotics, see restoring your gut after antibiotics.
Who Should Be Careful
For healthy people, probiotics and prebiotic-rich foods are generally well tolerated, though a sudden increase in fibre can cause wind and bloating. Live microbes can, rarely, cause infections in very ill people: lactobacillus bloodstream infections have been reported in liver-transplant patients (Patel et al., 1994), and the FDA has warned against giving probiotic products to hospitalized preterm infants (FDA, 2023). If you’re immunocompromised, seriously ill, or caring for a premature baby, ask your doctor first. People with SIBO or IBS may react to fermentable carbohydrates; see fermented foods and SIBO.
