What Are Prebiotics?

Prebiotics are substrates that are selectively utilised by host microorganisms, conferring a health benefit. The 2017 ISAPP consensus definition refined this from earlier definitions to emphasise three requirements: the substrate must resist digestion by human enzymes (reaching the colon intact), it must be fermented by gut microbiota, and it must selectively stimulate the growth or activity of beneficial microorganisms. The 'selectively' element is important - indigestible fibres that are fermented non-selectively (stimulating all bacteria equally, including potentially harmful ones) do not meet the prebiotic definition. The most studied prebiotics are inulin-type fructans (inulin and FOS) and galactooligosaccharides (GOS), both of which have well-documented bifidogenic effects at the genus and species level.

Inulin is extracted from chicory root (15-20% inulin content by weight) and is the most studied prebiotic. It selectively stimulates Bifidobacterium species via inulinase enzymes that other bacteria lack. Long-chain inulin (DP 10-60) ferments slowly in the distal colon; short-chain FOS (DP 2-8) ferments faster in the proximal colon - their combination (sold as Synergy1) provides whole-colon prebiotic coverage. GOS (galactooligosaccharides) is produced enzymatically from lactose and is also strongly bifidogenic - found naturally in human breast milk as the primary infant microbiome-shaping prebiotic. PHGG (partially hydrolysed guar gum) is FODMAP-tolerant and the recommended prebiotic for IBS patients who cannot tolerate inulin/FOS (which are FODMAP-positive fructans). Resistant starch produces more butyrate than inulin-type prebiotics via Ruminococcus and Faecalibacterium fermentation.

All prebiotics are dietary fibres, but not all dietary fibres are prebiotics. Insoluble fibres (cellulose, lignin) add bulk and may support bowel regularity but do not selectively stimulate beneficial bacteria and are not prebiotic. The prebiotic designation requires demonstrated selectivity for beneficial microorganisms. This distinction matters for supplement selection: a fibre supplement labelled as 'gut health support' that contains only psyllium husk or cellulose provides different - and generally more modest - microbiome effects than a true prebiotic supplement containing inulin/FOS or GOS. For microbiome-targeted interventions, look for the specific prebiotic fibre named on the label rather than generic 'dietary fibre'.

The synbiotic approach - combining a prebiotic with a compatible probiotic - is mechanistically superior to either alone. The probiotic provides the strain-specific clinical effects; the prebiotic provides the fermentable substrate that supports its colonisation, growth, and activity post-ingestion. Complementary synbiotics pair any probiotic with a generally bifidogenic substrate (e.g., Bifidobacterium with inulin). Synergistic synbiotics are specifically designed combinations where the prebiotic preferentially feeds the co-administered probiotic strain. A 2011 meta-analysis in the British Journal of Nutrition found synbiotics significantly improved faecal Bifidobacterium concentrations compared to either prebiotics or probiotics alone. LOOM Gut Health is formulated as a synbiotic - the multi-strain probiotic is paired with prebiotic inulin-FOS at a dose that supports colonisation efficiency.