Probiotics vs. Prebiotics: What is the difference?

Probiotics vs. Prebiotics: What is the difference?
Photo of Kate Hilton RD
Written by Kate Hilton RD Published 2 May 2024
Sonia Khan, Pharmacist
Medically reviewed by Sonia Khan, Pharmacist Last reviewed 4 September 2026

Probiotics are live bacteria and yeasts you take in, through fermented foods or supplements. Prebiotics are fibres that feed the good bacteria in our gut. The difference in one sentence is that probiotics add microbes to your gut, while prebiotics feed the ones already living there.

This guide covers what each one does for your health, which claims the research supports, who should be careful with them, and how to take them.

What is the Difference Between Probiotics and Prebiotics?

  • Probiotics are living microorganisms. The research definition, agreed in a 2014 expert consensus, is live microorganisms which, taken in adequate amounts, provide a health benefit. They arrive in fermented foods and supplements.
  • Prebiotics are not alive. They are substances, mostly plant fibres, that your own gut bacteria feed on, defined in a 2017 consensus statement as substrates selectively used by gut microbes with a health benefit.

A rough analogy is seeds and fertiliser. Probiotics sow new microbes, and prebiotics feed the garden you already have. Most people get both from an ordinary varied diet without ever buying a supplement.

In the UK, advertising rules do not permit general health claims for the word "probiotic", so products sold here usually say "live cultures" on the pack. The wording differs but the meaning is the same.

What are Probiotics?

Probiotics are found in fermented foods such as yoghurt, kefir, sauerkraut, and kimchi, and in supplement form. Look for "live and active" cultures on the label, because processing can kill the microbes off.

The most studied groups are these.

  • Lactobacillus species. Common in yoghurt and fermented foods, and the most familiar name on supplement labels.
  • Bifidobacterium species. Found in dairy products. Bifidobacterium is considered a beneficial gut bacteria and is one of the groups a gut test measures directly.
  • Saccharomyces boulardii. A yeast rather than a bacterium, with the strongest evidence of the three for preventing diarrhoea during a course of antibiotics.

What are Prebiotics?

Prebiotics pass through your stomach and small intestine undigested and reach the colon intact, where your gut bacteria ferment them. That fermentation produces short-chain fatty acids (SCFAs), compounds your gut bacteria make from fibre that help keep the gut lining healthy and are being studied across digestion, immunity and metabolism.

Sources of Prebiotics

Prebiotic-rich foods include garlic, onions, leeks, soybeans, asparagus, bananas, barley, oats, apples and cocoa. The list is far from exhaustive. Any diet with a wide variety of plants will supply prebiotic fibre without anyone having to think in those terms.

What Can They Help With?

The evidence differs sharply from claim to claim.

  • Diarrhoea during antibiotics. The strongest single result in the field. A review of 27 trials in just over 5,000 adults found Saccharomyces boulardii clearly reduced the risk of antibiotic-associated diarrhoea.
  • IBS symptoms. Some strains and combinations may improve symptoms of irritable bowel syndrome (IBS), but a 2023 review of 82 trials from a UK research group rated the certainty of that evidence as low. For some people it may be worth trying with professional guidance.
  • Gut bacteria diversity. Probiotic supplements are widely assumed to diversify the gut, but a review of seven trials in healthy adults found supplements made no measurable difference to the overall mix. Diet did better. In a 17-week Stanford trial, eating more fermented foods increased bacterial variety and lowered inflammation markers. If diversity is the goal, and it is a sensible goal for anyone trying to fix a gut imbalance, fermented foods and plant variety are where the evidence points.
  • Mood and the gut-brain axis. Researchers are investigating the link between gut bacteria and mood, including anxiety via the gut-brain axis. This is a research area, not a treatment. Nothing here replaces speaking to a GP about mental health.
  • General health in healthy people. A 2024 evidence review concluded the evidence is not strong enough to recommend probiotics to healthy people across the board. If you feel well and eat a varied diet, a supplement probably has little to add.

When and How Should I Take Them?

  • Probiotic supplements. Follow the manufacturer's label for amount, frequency and timing, because products differ in strains and strength. Consistency matters more than clock time. One laboratory digestion study found bacteria survived stomach acid better when taken with or just after food, which has not been confirmed in people but is a reasonable pointer.
  • Prebiotics. Timing makes little difference. Most people are better served getting them from food than from a supplement, and increasing fibre gradually is kinder on the gut than a sudden jump.
  • Foods for both. A varied, balanced diet built around gut friendly foods covers probiotic and prebiotic bases at once, and it is the option the evidence supports most strongly.

Can I Take Them Together?

Yes. Taking both at once is fine, and some supplements combine them in one product. The research term for the combination is synbiotics, formally defined in a 2020 expert consensus. Whether a combined product outperforms its parts is still an open research question.

Who Should Not Take Them?

People with weakened immune systems, for example those on chemotherapy or immunosuppressant medicines, should speak to their healthcare team before taking probiotics. Live microbes that are harmless to most people carry a small infection risk for this group, a risk highlighted in a 2023 expert safety review.

People with gut conditions such as IBS, inflammatory bowel disease or diverticular disease should get professional advice before significantly increasing prebiotic fibre, because in large amounts it can make symptoms worse.

Do not stop or change any prescribed medicine to make room for a supplement, and see your GP rather than a supplement aisle if you have blood in your stools, unexplained weight loss, or a change in bowel habit lasting more than three weeks. Symptoms like these need proper investigation first.

Do probiotics need to be stored in the fridge?

Some strains need refrigeration and others are stable at room temperature. The manufacturer's label will say which, so read it before assuming either way.

Bitesize summary

Prebiotics feed the gut bacteria you have; probiotics add more. The best supported route to both is food rather than supplements, with a couple of specific exceptions where supplement evidence is real. If you want to know where your gut actually stands before changing anything, a gut microbiome test can measure the current levels of beneficial bacteria, such as Bifidobacterium, in your gut, which turns the general advice above into something specific to you. And if you have ongoing digestive symptoms, introduce any of this under the guidance of a registered dietitian rather than alone.

Sources

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