Probiotic vs Prebiotic

Probiotics and prebiotics are constantly conflated, but they're entirely different products with different evidence bases. Knowing which is which saves a lot of supplement spend.

Probiotics

Live microorganisms that, when taken in adequate amounts, may benefit health. Strain-specific. The strain code on the label matters more than the genus or species. Most pass through the gut in a week and don't permanently colonise. Common sources: capsule supplements, kefir, yoghurt with live cultures, sauerkraut, kimchi, kombucha.

Prebiotics

Substrates (almost always specific fibres) that feed beneficial gut bacteria. Most prebiotics are oligosaccharides or polysaccharides that humans can't digest but gut bacteria can. Common sources: inulin (onion, garlic, leek, asparagus, chicory), GOS (legumes), resistant starch (cooked-and-cooled potato/rice), beta-glucan (oats), fructans (wheat, banana). Whole-food prebiotics outweigh supplement powders for most people.

• Lactobacillus rhamnosus GG: reduces antibiotic-associated diarrhoea, shortens infectious diarrhoea in children.
• Saccharomyces boulardii: the same around-antibiotics picture; some of its further claims are more mixed.
• Bifidobacterium infantis 35624: improves IBS bloating in trials (Whorwell 2006).
• Lactobacillus reuteri DSM 17938: reduces colicky baby crying.
• A few specialist gut situations use high-dose multi-strain preparations, but there the specialist chooses the product, not the shopper.
• Lactobacillus crispatus: bacterial vaginosis recurrence prevention.

• General immunity boost. Most population-level claims aren't strain-validated.
• Daily long-term use in healthy adults. No outcome trials show benefit beyond food sources.
• 'Bloating' as a generic complaint without a recognised IBS pattern. Strain-specific evidence is narrow.
• Weight loss. No probiotic strain has reliable weight loss data.
• Acne. Some early evidence for specific strains; not yet something worth buying for it.
• Mental health. The 'psychobiotic' field is exciting in research but not clinically actionable yet.

• Daily maintenance of microbiome diversity.
• Increasing Bifidobacterium and Akkermansia abundance over weeks.
• Producing butyrate to strengthen the gut barrier.
• Cardiovascular benefit (soluble fibre lowers LDL cholesterol).
• Constipation relief (some prebiotics like inulin add bulk and water-holding capacity).
• Recovery after antibiotic courses (alongside or after probiotic courses).

Synbiotics pair a probiotic strain with the prebiotic that feeds it. Bifidobacterium with inulin, for example. The logic is sound: arrive in the gut with food and you might colonise better. Real-world evidence for synbiotics over either alone is mixed and depends entirely on the strain-prebiotic pair. Most consumer 'synbiotic' products are loosely combined and don't show clinical benefit beyond their probiotic component.

1. For most healthy people the evidence points one way: prebiotics from food. 30 g of fibre a day with variety, including 30+ different plants per week.
2. Around antibiotics, the two strains with trial support for antibiotic-associated diarrhoea are L. rhamnosus GG and S. boulardii, taken alongside the course in the trials; a pharmacist can advise whether either fits you, and a prebiotic-rich diet afterwards helps diversity recover.
3. For IBS-type bloating, the strain with trial support is B. infantis 35624 (Whorwell 2006); in the trials, people who saw nothing by four weeks rarely saw more later.
4. Recurrent bacterial vaginosis: L. crispatus reduced recurrence in the Lactin-V trial, worth raising with your doctor. The evidence for thrush and urinary tract infections is much weaker, and probiotics are not the answer for either.
5. Don't pay £30/month for a generic Lactobacillus blend. The strain-blind ones rarely deliver.
6. One safety note: if you are immunocompromised, critically ill, or have a central venous line, live probiotics carry a rare but real infection risk. Ask your clinical team before taking any.