Elucidation of the Metabolism of Fruit Tannins by the Intestinal Microbiome

Abstract

Tannins are polymers of polyphenols that are abundant in the diet. Due to their large size, tannins are not bioavailable unless extensively metabolized by the intestinal microbiome into phenolic acid metabolites. Despite the identification of several phenolic acid metabolites that are potentially derived from tannins, there are still discrepancies in the metabolism of tannins by the microbiome, especially for those that are rarer in nature, such as A-type proanthocyanidins.

To elucidate the metabolism of tannins by the intestinal microbiome, a static ex vivo fermentation model was utilized, whereas tannins were fermented with fecal microorganisms and the changes in their chemical structures and resulting metabolites were characterized and quantified via LC-ESI-MS/MS. Due to the importance of the microbiome in tannin metabolism, fecal microorganisms were collected also from donors who had a form of inflammatory bowel disease, ulcerative colitis, in order to better understand the composition of the microbiota’s role in tannin metabolism. The metabolism of other polyphenolic classes were also evaluated in order to identify which metabolites were unique to tannins. Finally, a food processing method for the production of high-gallotannin mango juice with higher polyphenolic bioaccessibility was developed. Major differences occurred between the metabolism of a-type and b-type proanthocyanidins. B-type proanthocyanidins resulted in a larger number of phenolic acid metabolites that are bioavailable and have been identified in plasma and urine in vivo. A-type proanthocyanidins however resulted in predominantly epimers and microbially derived tannins that had undergone C-ring reduction. Both flavonoids and phenolic acids resulted in similar metabolites to tannins, however metabolite production from flavonoids was dependent on C-ring substitution and aromaticity. Tannase proved to successfully increase bioaccessibility of tannins and can be utilized as a novel processing aid for mango juice. Tannin metabolism by the microbiome is unique to each class of tannin and is affected by degree of polymerization and type of bond that exists between monomers. Ultimately, these findings will help food processors optimize production of high-tannin functional foods.