Mutual Interaction of Phenolic Compounds and Microbiota: Metabolism of Complex Phenolic Apigenin-C- and Kaempferol-O-Derivatives by Human Fecal Samples

Human colonic bacteria have an important impact on the biotransformation of flavonoid glycosides and their conversion can result in the formation of bioactive compounds. However, information about the microbial conversion of complex glycosylated flavonoids and the impact on the gut microbiota are still limited. In this study, the ability of different donors’ human intestinal microbiota to convert selected flavonoid O- and C-glycosides was evaluated by performing in vitro fermentations. The results show that all flavonoid glycosides were metabolized via their aglycones yielding smaller substances. Main metabolites were 3-(4-hydroxyphenyl)propionic acid, 3-phenylpropionic acid, and phenylacetic acid. Differences in the metabolite formation due to different time courses between the donors were determined. Therefore, from all fermentations, the ones with a specific donor were always slower resulting in a lower number of metabolites compared to the others. For example, in those fermentations, tiliroside was totally degraded from 0h (105 ± 13.2 µM) within the first 24h, while in the other fermentations with fecal samples from other donors, tiliroside (107 ± 52.7 µM at 0h) was not detected after 7h anymore. In general, fermentation rates of C-glycosides were slower compared to the fermentation rates of O-glycosides. The O-glycoside tiliroside was degraded within 4 h while the gut microbiota needed 13 h for the degradation of the structurally related C-glycoside vitexin. However, significant changes (p < 0.05) in the microbiota composition and in short chain fatty acid (SCFA) levels as important marker products of carbohydrate fermentation were not detected between incubations with different phenolic compounds. Therefore, microbiota diversity was not affected and a significant prebiotic effect of phenolic compounds cannot be assigned to the effects of flavonoid glycoside concentrations in typical diets concentration alone.