Prebiotic
Also called: prebiotics, prebiotic fibre
A prebiotic is a fibre that your own digestive system cannot break down but that feeds beneficial gut bacteria in the colon. The bacteria ferment the prebiotic and produce helpful compounds like butyrate. The most studied prebiotics are inulin, fructo-oligosaccharides (FOS) and galacto-oligosaccharides (GOS).
What makes something a prebiotic
The 2017 ISAPP consensus defines a prebiotic as a substrate that is selectively used by gut microorganisms and provides a health benefit. The key word is selective. A general fibre that feeds any bacteria is not strictly a prebiotic. A prebiotic preferentially feeds species linked to health, like Bifidobacterium and Faecalibacterium.
Prebiotic vs probiotic
A probiotic is a live microorganism, taken in food or supplement form, that confers a health benefit when consumed in adequate amounts. A prebiotic is the food the bacteria eat, whether they came in via a probiotic or are already living in your gut. The two work together. Eating prebiotics without probiotics still helps; eating probiotics without prebiotics often does not, because the bacteria run out of fuel.
Top prebiotic foods
- Onion, garlic, leek and spring onion (high in inulin and fructans).
- Asparagus and Jerusalem artichoke (very high inulin).
- Chicory root, the source of most commercial inulin extract.
- Bananas, especially when slightly green.
- Oats, especially porridge oats (beta-glucan and resistant starch).
- Apples and pears (pectin).
- Beans, lentils, chickpeas (resistant starch and GOS).
- Cooled cooked potato, rice and pasta (resistant starch type 3, formed when starches retrograde on cooling).
How much prebiotic to aim for
There is no UK official prebiotic intake target. Studies showing benefit typically use 5 to 10 grams a day of specific prebiotic fibre. Hitting the NHS-recommended 30 grams of total fibre per day, with variety, will deliver this naturally without supplementing. Most UK adults sit at around 19 grams of total fibre daily.
When prebiotics cause symptoms
Many prebiotics are also FODMAPs. Inulin, FOS and GOS all sit in the oligosaccharide group. People with IBS, especially IBS-D, often react to higher doses with bloating, wind and loose stools. The clinical recommendation is to start low and build slowly. People in the elimination phase of a low-FODMAP protocol should restrict added prebiotic fibres until reintroduction.
Common questions
- Are prebiotic supplements worth it?
- If your diet is already varied and high in fibre, no. If you struggle to hit 30 grams of fibre a day, a 5-gram inulin or partially-hydrolysed guar gum supplement can help. Start at 1 gram and build slowly to avoid bloating.
- What's the best prebiotic for someone with IBS?
- Partially-hydrolysed guar gum (PHGG) has the best evidence for IBS specifically. It feeds beneficial bacteria but ferments slowly, so it produces less gas than inulin or FOS. NICE-affiliated dietitians often suggest PHGG for IBS patients who do not tolerate other prebiotics.
- Are prebiotic drinks any good?
- Most contain only 1 to 3 grams of prebiotic fibre per serving. Useful as a top-up, not a substitute for a fibre-rich diet. Read the label: many also contain added sugar or sweeteners that can offset the benefit.
- Can you take a prebiotic and probiotic together?
- Yes, and the combination is sometimes called a synbiotic. The pairing works best when the prebiotic specifically feeds the probiotic strain. Not all combinations are matched; many products throw both in without tested compatibility.