is further decarboxylated to phloroglucinol, which is dehydroxylated to resorcinol. Resorcinol is hydroxylated to hydroxyquinol. Protocatechuic acid and hydroxyquinol undergo intradiol cleavage through protocatechuate 3,4-dioxygenase and hydroxyquinol 1,2-dioxygenase to form β-carboxy cis, cis-muconic acid and maleyl acetate. Among fungi, degradation of catechin can be achieved by Chaetomium cupreum. Metabolism in humans Catechins are metabolised upon uptake from the gastrointestinal tract, in particular the jejunum, and in the liver, resulting in so-called structurally-related epicatechin metabolites (SREM). The main metabolic pathways for SREMs are glucuronidation, sulfation and methylation of the catechol group by catechol-O-methyl transferase, with only small amounts detected in plasma. The majority of dietary catechins are however metabolised by the colonic microbiome to gamma-valerolactones and hippuric acids which undergo further biotransformation, glucuronidation, sulfation and methylation in the liver. The stereochemical configuration of catechins has a strong impact on their uptake and metabolism as uptake is highest for (-)-epicatechin and lowest for (-)-catechin. Biotransformation Biotransformation of (+)-catechin into taxifolin by a two-step oxidation can be achieved by Burkholderia sp. (+)-Catechin and (-)-epicatechin are transformed by the endophytic filamentous fungus Diaporthe sp. into the 3,4-cis-dihydroxyflavan derivatives, (+)-(2R,3S,4S)-3,4,5,7,3',4'-hexahydroxyflavan (leucocyanidin) and (-)-(2R,3R,4R)-3,4,5,7,3',4'-hexahydroxyflavan, respectively, whereas (-)-catechin and (+)-epicatechin with a 2S-phenyl group resisted the biooxidation. Leucoanthocyanidin reductase (LAR) uses (2R,3S)-catechin, NADP+ and H2O to produce 2,3-trans-3,4-cis-leucocyanidin, NADPH, and H+. Its gene expression has been studied in developing grape berries and grapevine leaves. Glycosides (2R,3S)-Catechin-7-O-β-D-glucopyranoside can be isolated from barley (Hordeum vulgare L.) and malt. Epigeoside (Catechin-3-O-alpha-L-rhamnopyranosyl-(1-4)-beta-D-glucopyranosyl-(1–6)-beta-D-glucopyranoside) can be isolated from the rhizomes of Epigynum auritum. Research Vascular function Only limited evidence from dietary studies indicates that catechins may affect endothelium-dependent vasodilation which could contribute to normal blood flow regulation in humans. Green tea catechins may improve blood pressure, especially when systolic blood pressure is above 130 mmHg. Due to extensive metabolism during digestion, the fate and activity of catechin metabolites responsible for this effect on blood vessels, as well as the actual mode of action, are unknown. Adverse events Catechin and its metabolites can bind tightly to red blood cells and thereby induce the development of autoantibodies, resulting in haemolytic anaemia and renal failure. This resulted in the withdrawal of the catechin-containing drug Catergen, used to treat viral hepatitis, from market in 1985. Catechins from green tea can be hepatotoxic and the European Food Safety Authority has recommended not to exceed 800 mg/d. Other One limited meta-analysis showed that increasing consumption of green tea and