Black tea is the most widely consumed tea worldwide. Thearubigins are polymers of tea catechins, which account for more than 20% of the black tea polyphenols. However, the chemical structures and the underlying mechanisms regarding how the thearubigins, being poorly bioavailable, generate the <i>in vivo</i> health benefits are still largely unknown. Using germ-free and specific pathogen-free husbandry conditions combined with LC/MS-based non-targeted and targeted metabolomic analysis, we investigated the role of intestinal bacteria in thearubigins metabolism. Theaflavins and theasinensins were identified as the major microbial metabolites of thearubigins, suggesting these molecules are the building units for the complex thearubigins. To further confirm this, thearubigins depolymerization was done and menthofuran conjugated theaflavins, theasinensins, and catechins, as well as their free forms were detected as the major degradation products of thearubigins, suggesting that theaflavins and theasinensins could be further polymerized through B-type proanthocyanidin linkages. This was confirmed by enzymatic synthesis of theaflavin-epicatechin trimer. Furthermore, four microbial degradation products were able to be detected in urine samples, suggesting they can be absorbed into the circulating system. Using the combination of microbial degradation, metabolomics, chemical degradation, and enzymatic synthesis, our results demonstrate that thearubigins are the complex polymers of theaflavins, theasinensins, and catechins and can be metabolized by gut microbiota to their corresponding bioactive and bioavailable smaller molecular metabolites.

Gut microbiota and metabolomics as useful tools to dissect the complex structures of black tea polymers

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