Probiotics are live microorganisms you swallow. Prebiotics are the fibres that feed the microorganisms already living in your gut. The simplest way to hold it: probiotics are the seeds, prebiotics are the fertiliser. There is a detail almost every product page omits — the scientific definitions of both require a demonstrated health benefit, which means a supplement containing bacteria is not automatically a probiotic. It is a supplement containing bacteria.
Medical note
General information, not medical advice. Probiotics are not appropriate for everyone — people who are severely immunocompromised, critically ill, or have central venous catheters should not take them without medical supervision. If you are managing a digestive condition, talk to your doctor or a registered dietitian rather than self-treating.
Key takeaways
- Probiotic: live microorganisms that, in adequate amounts, confer a health benefit.
- Prebiotic: a substrate selectively used by your gut microbes that confers a health benefit — in practice, specific fibres.
- Synbiotic: both together. Postbiotic: inactivated microbes or their components.
- All four definitions require a demonstrated health benefit. That is a high bar many marketed products do not clear.
- Probiotic effects are strain-specific. “Contains lactobacillus” tells you very little on its own.
- You can get both from food — prebiotics from onions, garlic, oats and bananas; probiotics from live-culture yoghurt, kefir, kimchi and sauerkraut.
The difference in one line
Probiotics add microbes. Prebiotics feed the microbes you already have.
Your large intestine hosts a large and varied microbial community. Two broad strategies exist for influencing it: introduce new organisms, or feed the existing ones preferentially so the helpful ones do better. Probiotics take the first route, prebiotics the second.
| Probiotics | Prebiotics | |
|---|---|---|
| What it is | Live microorganisms | A fibre or substrate |
| Alive? | Yes | No — it is food for microbes |
| Mechanism | Adds organisms | Feeds existing organisms |
| Food sources | Yoghurt, kefir, kimchi, sauerkraut, miso, tempeh | Onions, garlic, leeks, asparagus, oats, bananas, chicory root |
| Survives cooking? | No — heat kills the organisms | Yes — fibre is heat-stable |
| Shelf stability | Often needs refrigeration; potency declines | Very stable |
That “survives cooking” row has a practical consequence people miss constantly: cooking kimchi into a stew, or baking with yoghurt, destroys the live organisms. You keep the flavour and the nutrition, not the probiotic content. Prebiotic fibre, by contrast, is unaffected — roasted garlic feeds your microbes exactly as raw garlic does.
The definitions, precisely
These terms have formal consensus definitions from the International Scientific Association for Probiotics and Prebiotics. They are worth reading closely, because the wording does real work.
| Term | Consensus definition |
|---|---|
| Probiotic | Live microorganisms that, when administered in adequate amounts, confer a health benefit on the host |
| Prebiotic | A substrate that is selectively utilised by host microorganisms conferring a health benefit |
| Synbiotic | A mixture comprising live microorganisms and substrate(s) selectively utilised by host microorganisms that confers a health benefit on the host |
| Postbiotic | A preparation of inanimate microorganisms and/or their components that confers a health benefit on the host |
The phrase every product page skips
Look at what all four share: “confers a health benefit.” It is not decoration. It is the load-bearing part of each definition.
Under these definitions, a capsule full of live bacteria is only a probiotic if those specific strains, at that dose, have been shown to produce a health benefit. Bacteria alone do not qualify. Neither does inulin alone qualify as a prebiotic without demonstrated selective utilisation and benefit.
This matters commercially because the word “probiotic” is used far more loosely on packaging than the science supports. A product listing impressive-sounding strain names and a large CFU count has told you what is in the bottle. It has not told you that anyone demonstrated a benefit from it.
Practical upshot: look for products naming specific strains — a genus, species and strain designation, like Lactobacillus rhamnosus GG — rather than just a genus. Strain designations are what appear in the research, and they are what you can actually look up.
Why “strain-specific” is the most important idea here
Probiotic effects do not generalise across species, let alone across the category. Two strains of the same species can behave completely differently.
This is the reason so much probiotic marketing is vague. Saying a product “contains Lactobacillus” is roughly as informative as saying a meal “contains plants.” The genus tells you almost nothing about what it will do.
What this means in practice:
- Match the strain to the goal. Evidence for one strain in one condition does not transfer to a different strain, or to a different condition.
- More strains is not better. A 12-strain blend is not automatically superior to a well-evidenced single strain. It may just be twelve things nobody studied together.
- CFU count is not a quality score. Higher colony-forming unit counts are not inherently better; the studied dose for that strain is what matters, and it varies.
If you are buying a probiotic for a specific reason, the useful question is not “which is the best probiotic” but “which strain has evidence for the thing I am trying to address, and does this product contain that strain at that dose?” Our guide to probiotics for women goes into product specifics.
Where to get prebiotics from food
Prebiotics are the easier of the two to get from an ordinary diet, and they are cheap. The relevant compounds are specific fibres — inulin, fructo-oligosaccharides and galacto-oligosaccharides — which occur in everyday produce.
- Alliums: onions, garlic, leeks, shallots
- Chicory root — the most concentrated source, and where most supplemental inulin comes from
- Jerusalem artichoke and asparagus
- Slightly under-ripe bananas — greener bananas contain more resistant starch
- Oats and barley
- Legumes — beans, lentils, chickpeas
- Apples, for pectin
Note the overlap with ordinary good eating. A diet with onions, garlic, oats, beans and fruit in it is already a prebiotic-rich diet. There is no separate shopping trip required.
Go slowly
Prebiotic fibre is fermented by gut bacteria, and fermentation produces gas. Increasing intake abruptly — particularly with a concentrated inulin supplement — reliably produces bloating and wind. This is not a sign something is wrong; it is the mechanism working faster than your gut has adapted to.
Increase gradually over a few weeks. And if you have IBS, be aware that many prebiotic foods are high-FODMAP and may worsen symptoms — worth discussing with a dietitian rather than pushing through.
Where to get probiotics from food
Fermented foods are the traditional route, with an important caveat we come to below.
- Yoghurt with live active cultures
- Kefir — typically more diverse than yoghurt
- Sauerkraut and kimchi, unpasteurised
- Miso and tempeh
- Kombucha
Fermented does not automatically mean probiotic
This trips up almost everyone, and it follows directly from the definitions above.
Many fermented foods are pasteurised after fermentation to make them shelf-stable. That kills the organisms. Shelf-stable sauerkraut in a jar in the ambient aisle has typically been heat-treated; the live version is in the refrigerated section. Same for many kimchi products.
Second, even a food with live organisms in it does not strictly meet the probiotic definition unless those organisms have been shown to confer a health benefit at that dose. ISAPP maintains a separate definition for fermented foods precisely because the two categories are not the same thing.
None of which means fermented foods are not worth eating — they are, and the broader evidence on fermented food and gut health is encouraging. It means the label “probiotic” is doing more work in marketing than in the science.
Do you need a supplement, or is food enough?
For most healthy people, food is enough — and prebiotics in particular are better addressed by eating more fibre than by buying a powder.
Where a supplement has a stronger case:
| Situation | Worth considering |
|---|---|
| During or after a course of antibiotics | Probiotic — one of the better-evidenced uses, though strain matters |
| A specific diagnosed condition | A strain with evidence for that condition, ideally on medical advice |
| Travelling somewhere with a high risk of traveller’s diarrhoea | Certain strains have evidence here |
| You eat very little fibre and cannot change that quickly | Prebiotic supplement, increased slowly |
| General “gut health” with no specific problem | Food first. Fibre, variety, fermented foods. |
That last row is where most supplement money goes and where the evidence is weakest. If nothing specific is wrong, the intervention with the best evidence is not a capsule — it is eating a wider variety of plants.
Who should be cautious: people who are severely immunocompromised, critically ill, or have central venous catheters should not take probiotics without medical supervision. Live organisms behave differently in a compromised immune system.
Frequently asked questions
What is the difference between a prebiotic and a probiotic?
Probiotics are live microorganisms you consume; prebiotics are fibres that feed the microorganisms already in your gut. Probiotics add organisms, prebiotics feed existing ones. Think of probiotics as seeds and prebiotics as fertiliser.
Should I take a prebiotic or a probiotic?
For general gut health with no specific problem, neither — increase fibre and eat a wider variety of plants first. A probiotic has the strongest case around antibiotic courses, specific diagnosed conditions, or travel. A prebiotic supplement makes sense mainly if your fibre intake is low and hard to change.
Can you take prebiotics and probiotics together?
Yes. A product combining both is called a synbiotic, formally defined as a mixture of live microorganisms and substrates selectively used by host microorganisms that together confer a health benefit. There is no need to separate them in time.
What is a postbiotic?
A preparation of inanimate microorganisms and/or their components that confers a health benefit. Unlike probiotics, the organisms are not alive. That makes postbiotics more shelf-stable and potentially more suitable for people who should avoid live organisms.
Are fermented foods the same as probiotics?
No. Many fermented foods are pasteurised after fermentation, which kills the organisms — shelf-stable sauerkraut has usually been heat-treated, while the live version is refrigerated. Even with live organisms present, the probiotic definition requires a demonstrated health benefit at that dose.
What foods are high in prebiotics?
Onions, garlic, leeks, asparagus, chicory root, Jerusalem artichoke, oats, barley, legumes, apples and slightly under-ripe bananas. Unlike probiotics, prebiotic fibre is heat-stable, so cooking does not destroy it.
Why do probiotics say “strain-specific”?
Because effects do not generalise. Two strains of the same species can behave entirely differently, so evidence for one does not transfer to another. Look for products naming a full strain designation — genus, species and strain — rather than just “contains Lactobacillus.”
Do prebiotics cause bloating?
They can, especially if you increase intake quickly. Gut bacteria ferment prebiotic fibre and fermentation produces gas. Increase gradually over a few weeks. If you have IBS, note that many prebiotic foods are high-FODMAP and may worsen symptoms — worth discussing with a dietitian.
Is a higher CFU count better in a probiotic?
Not necessarily. CFU counts indicate how many live organisms are present, not whether they help. What matters is whether the specific strains have evidence at the dose provided. A well-evidenced single strain can outperform a twelve-strain blend nobody has studied together.