Probiotics vs prebiotics vs postbiotics: what each means and whether you need one

The American Gastroenterological Association suggests specific probiotic strains for adults and children taking antibiotics, and recommends them for IBS, Crohn’s disease and ulcerative colitis only inside clinical trials.⁵ Before you buy anything labeled “biotic”, it helps to know which of four terms the label is using, because probiotic, prebiotic, postbiotic and synbiotic describe four different things.

Do you need a supplement? Five questions

1. Are you taking antibiotics? The American Gastroenterological Association (AGA) guideline suggests that adults and children on antibiotics use one of several named products, to prevent C. difficile infection: Saccharomyces boulardii, or specific two-, three- or four-strain combinations of Lactobacillus and Bifidobacterium. It rates the evidence as low quality and notes that people who value avoiding cost, or who weigh the small absolute risk lightly, could reasonably choose none.⁵ Ask whoever prescribes the antibiotics.

2. Do you have IBS, Crohn’s disease or ulcerative colitis? The same guideline recommends probiotics for each of these only in the context of a clinical trial, because evidence is not yet good enough to say.⁵ Dietary help is a conversation for a gastroenterologist or dietitian, not a supplement shelf.

3. Are you healthy and curious? No guideline recommends a probiotic supplement for general wellness in healthy adults. Food is the better place to begin: fiber that feeds the microbes you have, and live fermented foods.

4. Have you read “contains probiotics” on a food label? The consensus panel on fermented foods says a product should only be labeled as containing probiotics when there is evidence that its live components give a health benefit and the microbiological content is defined.⁶ If the pack says it without naming a strain, that is a marketing word, not a finding.

5. Is a product selling “postbiotics” or “synbiotics”? Read the definitions below first.

The four terms

Two details are worth knowing. A probiotic benefit belongs to a strain, such as S. boulardii, not to “probiotics” as a class.¹ And a postbiotic is not a leftover or a waste product. The 2021 panel says effective postbiotics must contain inactivated microbial cells or cell components, with or without metabolites. A metabolite on its own, such as the butyrate your gut bacteria make, falls outside the definition.³

What food does for each

Prebiotics: Most are fibers. Fructans in wheat, rye, onions and garlic, and galacto-oligosaccharides in legumes, are common ones.⁹ Gut bacteria ferment fibers into short-chain fatty acids such as acetate, propionate and butyrate.⁸ A diet with many plants supplies many such fibers at once, which is part of why variety matters. The catch is gas: these are also the fermentable carbohydrates that give some people trouble, so see the bloating guide before you start adding them fast.

Probiotics from food: Live fermented foods bring live microbes, but only products with studied strains are probiotics in the strict sense. That is fine. In the Stanford trial, a diet rich in fermented foods raised microbiome diversity in 36 healthy adults during a 10-week diet phase (17 weeks in total), whatever the labels said.⁷ Our guide to live ferments shows which ones are live.

Postbiotics and synbiotics: These are mostly supplement-industry categories for now. A bowl of yogurt with oats is a good breakfast. It is a meal, not a synbiotic in the panel’s sense, which is a defined mix with a documented benefit.