Probiotic vs prebiotic
Probiotics are live bacteria (capsules, yoghurt, kefir). Prebiotics are food (mostly fibre) that feeds the bacteria already living in your gut. Most healthy people get more value from prebiotics, they support the bacteria that already work for you. Probiotics are for specific situations: antibiotic recovery, IBS bloating with B. infantis 35624, infant colic. Strain-specific. Most consumer probiotics aren't worth the cost.
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.
What each one is
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.
Side by side
| Probiotic | Prebiotic | |
|---|---|---|
| What it is | Live bacteria | Food for bacteria |
| Stays in the gut? | Mostly no, transient | It's just food, gets fermented |
| Strain-specific? | Yes, very | No, broad effect |
| Best food source | Kefir, live yoghurt | Onion, garlic, oats, beans |
| Supplement form | Capsules with CFU count | Inulin/FOS powder |
| Strongest evidence | C. diff prevention, infant colic, antibiotic-associated diarrhoea | Maintaining microbiome diversity, cardiovascular health, gut barrier |
| Daily use case | Specific clinical situation | General gut health |
| Cost effectiveness | Often poor | Usually high |
When probiotics genuinely help
- Lactobacillus rhamnosus GG: reduces antibiotic-associated diarrhoea, shortens infectious diarrhoea in children.
- Saccharomyces boulardii: same antibiotic and infection picture, prevents C. diff recurrence.
- Bifidobacterium infantis 35624 (Align): improves IBS bloating in trials.
- Lactobacillus reuteri DSM 17938: reduces colicky baby crying.
- VSL#3 / Visbiome / Vivomixx (multi-strain): pouchitis and ulcerative colitis under specialist care.
- Lactobacillus crispatus: bacterial vaginosis recurrence prevention.
When probiotics probably don't help
- 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 IBS diagnosis. Strain-specific evidence is narrow.
- Weight loss. No probiotic strain has reliable weight loss data.
- Acne. Some early evidence for specific strains; not yet a treatment recommendation.
- Mental health. The 'psychobiotic' field is exciting in research but not clinically actionable yet.
When prebiotics shine
- 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: combining the two
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.
What to actually do
- Default for most healthy people: prebiotics from food. 30 g of fibre a day with variety, including 30+ different plants per week.
- After antibiotics: 2-week course of L. rhamnosus GG or S. boulardii at the start, then prebiotic-rich diet to recover diversity.
- IBS with bloating: try B. infantis 35624 (Align) for 4 weeks. If no effect, stop.
- Recurrent thrush, BV, or UTIs: L. crispatus oral or vaginal probiotics worth discussing with a doctor.
- Don't pay £30/month for a generic Lactobacillus blend. The strain-blind ones rarely deliver.
Common questions
- Should I take both?
- Probiotics for a defined situation (antibiotic recovery, IBS, etc) plus prebiotics from food daily is the most evidence-based combo. Daily generic probiotics with random prebiotic powder is mostly unnecessary expense for healthy adults.
- What about kefir? Probiotic or prebiotic?
- Kefir is a probiotic, it contains live bacteria and yeasts. Yoghurt with live cultures is the same. Sauerkraut, kimchi, and kombucha too. The bacteria in these foods are mostly transient but they do contribute to short-term diversity.
- Are inulin powders worth it?
- Sometimes. Inulin from chicory root is the most common supplement form. It feeds Bifidobacterium reliably. Useful if dietary fibre is hard to maintain, or specifically targeting low Bifidobacterium. Start with 5 g a day and ramp up, too much too fast causes severe gas.
- What about postbiotics?
- Postbiotics are the bioactive compounds bacteria produce (short-chain fatty acids, peptides, etc) sold without the live bacteria. The category is new and the evidence is early. Most consumer postbiotic products are speculative; food-based prebiotics are still the highest-use move.