Assessment of inter-individual variation in metabolism of flavonoids from bilberry and grape seed extracts using an in vitro digestion and faecal fermentation model

The gut microbiota plays an essential role in the conversion of anthocyanins and (epi-)catechins into smaller phenolic acids, which are then absorbed into the blood stream. The phenolic composition of a commercial bilberry extract and grape seed extract was assessed, as well as a formulation extract containing a combination of both extracts. The extracts were subjected to an in vitro salivary, gastric and intestinal digestion environment, based on the INFOGEST Model. The solid fraction end-product of the combined extract from the in vitro digestion was further fermented with faecal samples from six healthy donors, for 72 hours, to assess phenolic acid metabolism, short-chain fatty acid formation and changes in microbial composition. During the in vitro digestion, flavonoid content in all three extract samples (bilberry, grape seed and the formulation extracts) decreased significantly. In the process of anthocyanin and flavonoid digestion, smaller phenolic acid compounds such as benzoic acid, cinnamic acid and mandelic acid increased in bilberry, grape seed and formulation extract samples. All faecal donors harboured unique microbiota compositions, however all faecal microbiota were able to fully convert catechin/epicatechin, the most prominent flavonoids in the formulation extract sample, into smaller phenolic metabolites (phenylacetic, phenylpropionic and benzoic acids) within 24 hours. Using 16S rRNA gene amplicon sequencing, Anaerobutyricum and Enterocloster spp. were correlated with catechin/epicatechin metabolism in the fermentation procedure, however, in single bacterial strain fermentation experiments with the formulation extract or catechin standard, these bacteria were not capable of metabolising flavonoids. HighlightsO_LIFaecal microbiota converted (epi-)catechin to phenolic metabolites within 24 h. C_LIO_LI(Epi-)catechin correlated negatively with Anaerobutyricum and Enterocloster spp. C_LIO_LIFaecal bacterial cultures did not show (epi-)catechin metabolism capacity. C_LI