Prebiotics can be regarded as ‘food for probiotics’ and prebiotic products are often based on certain dietary fibres such as inulin. ‘Microbiota-accessible carbohydrates’ (MACs) are carbohydrates that cannot be digested by humans, but which are available to bacteria in the colon.19,21,22

However, there is no internationally agreed definition of ‘prebiotic’. In 2007, the FAO defined a prebiotic as “a non-viable food component that confers a health benefit on the host associated with modulation of the microbiota.” This is now under review as many products sold as ‘prebiotics’ do not meet this description.21

The definition ISAPP adopted in 2016 is: “A prebiotic is a substrate that is selectively utilised by host microorganisms conferring a health benefit .” To qualify, a product should (at least in part) be metabolised by a select microbe or group of microbes but not the entire microbiota.22

Lactulose is a prebiotic as it enhances growth of Bifidobacterium, Lactobacillus and Anaerostipes species via anaerobic fermentation, increasing ‘healthy’ SCFAs (acetate, propionate, butyrate) levels and decreasing branched-chain fatty acids, pH and ammonia levels.23

In 2019, ISAPP defined ‘synbiotic’ as “a mixture comprising live microorganisms and substrate(s) selectively utilised by host microorganisms that confers a health benefit on the host” with the combined effects being separate but complementary and/or synergistic.24