Microbial metabolism of caffeic acid and its esters chlorogenic and caftaric acids by human faecal microbiota in vitro

Caffeic acid and its esters, chlorogenic and caftaric acids, are major dietary polyphenols present in various foods and beverages. Although caffeic acid is easily absorbed in the small intestine, its esterification with quinic acid, as in chlorogenic acid, decreases its gut absorption and increases the quantities reaching the colon and its microbiota. The microbial conversion of caftaric acid, the tartaric acid ester of caffeic acid, has not been studied earlier. In this work we compared the direct action of a human faecal microbiota on the metabolism of caffeic, chlorogenic and caftaric acids in an in vitro fermentation model. All substrates disappeared quickly and none of the free acids (caffeic, quinic or tartaric acids) were detected after 2 hours of incubation. Two major microbial metabolites were identified by HPLC–ESI–MS–MS as 3-hydroxyphenylpropionic (3-HPP) and benzoic acids (BA). Maximal levels of 3-HPP were reached after 2 h of fermentation and accounted for 9–24% of the dose of caffeic acid and its esters. BA was formed steadily throughout the incubation, accounting for 4–5% of the initial dose of the substrates after 24 h of incubation. The similarities in the metabolic patterns observed for caffeic, chlorogenic and caftaric acids suggest that esterification does not influence the metabolism of caffeic acid by the gut microbiota.