Original Contribution COLONIC METABOLISM OF DIETARY POLYPHENOLS: INFLUENCE OF STRUCTURE ON MICROBIAL FERMENTATION PRODUCTS ANDREAS R. RECHNER,* MARTIN A. SMITH, y GUNTER KUHNLE,* GLENN R. GIBSON, y EDWARD S. DEBNAM, z S. KAILA S. SRAI, § KEVIN P. MOORE, b and CATHERINE A. RICE-EVANS* ,b * Antioxidant Research Group, Wolfson Centre for Age-Related Diseases, School of BiomedicalSciences, King’s College London, London SE1 9RT, UK; y Food Microbial Sciences Unit, School of Food Biosciences, University of Reading, Reading, UK; z Department of Physiology and § Department of Biochemistry and Molecular Biology, Royal Free and University College Medical School, London, UK; and b Centre for Hepatology, Department of Medicine, Royal Free and University College Medical School, London NW3, UK (Received 7 August 2003; Revised 24 September 2003; Accepted 26 September 2003) Abstract—The metabolism of chlorogenic acid, naringin, and rutin, representative members of three common families of dietary polyphenols, the hydroxycinnamates, the flavanones, and the flavonols, respectively, was studied in an in vitro mixed culture model of the human colonic microflora. Time- and concentration-dependent degradation of all three compounds was observed, which was associated with the following metabolic events after cleavage of the ester or glycosidic bond: reduction of the aliphatic double bond of the resulting hydroxycinnamate caffeic acid residue; dehydroxylation and ring fission of the heterocyclic C-ring of the resulting deglycosylated flavanone, naringenin, and of the deglycosylated flavonol, quercetin (which differed depending on the substitution). The metabolic events, their sequences, and major phenolic end products, as identified by GC-MS or LC-MS/MS, were elucidated from the structural characteristics of the investigated compounds. The major phenolic end products identified were 3-(3-hydroxyphenyl)- propionic acid for chlorogenic acid, 3-(4-hydroxyphenyl)-propionic acid and 3-phenylpropionic acid for naringin, and 3- hydroxyphenylacetic acid and 3-(3-hydroxyphenyl)-propionic acid for rutin. The degree of degradation of the compounds studied was significantly influenced by the substrate concentration as well as individual variations in the composition of the fecal flora. The results support extensive metabolism of dietary polyphenols in the colon, depending on substrate concentration and residence time, with resultant formation of simple phenolics, which can be considered biomarkers of colonic metabolism if subsequently absorbed. It is also apparent that a relatively small number of phenolic degradation products are formed in the colon from the diverse group of natural polyphenols. D 2003 Elsevier Inc. All rights reserved. Keywords—Colonic metabolism, Flavonoids, Hydroxycinnamates, Phenolic acids, Free radicals INTRODUCTION In recent years, major advances have been made in the understanding of structural changes occurring on the absorption of flavonoids, factors influencing the absorp- tion of flavonoids across the small intestine, and the extent to which they are metabolized in the gastrointes- tinal tract and undergo conjugation in the liver postab- sorption [1–7]. Flavonoids are substrates for several enzymes located in the small intestine and colon, and for hepatic metabolism: phase I hydrolyzing and oxidiz- ing enzymes, including cytochrome P450 and glucosi- dase enzymes, and phase II conjugating and detoxifying enzymes (UDP-glucuronosyl transferases, catechol-O- methyl transferases, sulfotransferases, glutathione trans- ferases, quinone reductases). Indeed many biochemical, cellular, animal, and human studies have demonstrated that most families of dietary flavonoids (with the excep- tion of anthocyanins [8]) are not only deglycosylated on absorption, but also appear in the circulation as glucur- onidated, sulfated, and, in the case of catechol structures, methylated and mixed forms [9–16]. In addition, a large proportion of ingested dietary flavonoids pass to the large intestine without degradation and undergo colonic bio- Address correspondence to: Professor Catherine Rice-Evans, Antioxidant Research Group, Wolfson Centre for Age-Related Diseases, London SE1 9RT, UK; Fax: +44-20-7848-6143; E-mail: catherine.rice-evans@kcl.ac.uk. Free Radical Biology & Medicine, Vol. 36, No. 2, pp. 212 –225, 2004 Copyright D 2003 Elsevier Inc. Printed in the USA. All rights reserved 0891-5849/$-see front matter doi:10.1016/j.freeradbiomed.2003.09.022 212