In vitro metabolism of anthocyanins by human gut microflora

Background Only a small part of the dietary anthocyanins are absorbed. Thus large amounts of the ingested compounds are likely to enter the colon. In vitro and in vivo studies have shown that colonic bacteria transform various flavonoids to smaller phenolic acids. However, there is very little information on bacterial transformations of anthocyanins. Aim of the study was to explore if anthocyanin glycosides were deglycosylated,whether the resulting aglycones were degraded further to smaller phenolic compounds by colonic bacteria, and to characterise metabolites. Methods Isolated cyanidin–3–glucoside and –rutinoside were fermented in vitro using human faecal microbiota as an inoculum. Metabolites were analysed and characterised by HPLC–DAS and LC–MS. They were identified by comparing their characteristics with those of available standards, and semi–quantified using the amount of substrate analysed from samples at initial timepoint. Results Cyanidin–3–glucoside and cyanidin aglycone could be identified as intermediary metabolites of cyanidin–3–rutinoside. At early timepoints (before 2 h), the formation of protocatechuic acid as a major metabolite for both cyanidin glycosides and detection of lower molecular weight metabolites show that anthocyanins were converted by gut microflora. Furthermore, reconjugation of the aglycone with other groups, non–typical for dietary anthocyanins, was evident at the later (after 2h) timepoints. Conclusion Bacterial metabolism of anthocyanins involves the cleavage of glycosidic linkages and breakdown of the anthocyanidin heterocycle.