Flavonoids are important dietary polyphenols. Catechins (flavan-3-ols) are a group of flavonoids found mainly in green tea, but also in fruits such as grapes and apples. Catechins are also the structural units in proanthocyanidins, large molecular weight catechin oligomers found in foods including cocoa. Evidence in the nutritional literature suggest that both catechins and proanthocyanidins have particular health benefits, and this suggests they are biologically active in humans. In particular they appear to be protective of cardiovascular disease, with (-)-epicatechin being most heavily implicated as the causative factor in this relationship. Epicatechin is known to be absorbed in the small intestine of humans, and during its passage through the enterocytes of the gut it undergoes extensive metabolism to phase 2 conjugates (like all flavonoids). This results in a number of new metabolites of epicatechin including (-)-epicatechin-3’-glucuronide, (-)-epicatechin-3’-sulfate and 3’-methyl-(-)-epicatechin-5-sulfate.
Plasma concentrations of these phase 2 metabolites peaks at around 1 to 3 hours post ingestion and 21 to 50 % of these metabolites can be recovered from urine. Some of the metabolites may therefore undergo further postabsorptive metabolism in the liver which explains their absence from the urine. Plasma concentrations of metabolites reach only submicromolar levels following the ingestion of catechins. Much of the absorbed epicatechin (up to 50 %) appears to be effluxed back to the intestinal lumen, where its high water solubility likely precludes further absorption in the small intestine. The fate of these phase 2 conjugates is not known but evidence suggests they may be degraded to more simple phenolic compounds by colonic bacteria, and then absorbed, where they play a role in humans health. The main metabolites effluxed back to the lumen may be the epicatechin-3-sulfate conjugates, with higher amounts of the epicatechin-3-glucuronide making it to plasma and subsequently the urine.
Enterohepatic circulation is the excretion of substances from the bile duct to the lumen of the small intestine, followed by reabsorption of that compound to the small intestine. Evidence suggests that catechins undergo this enterohepatic recirculation route. However, perfusion experiments show that only around 0.005 % of perfused epicatechin is found in the lumen of the small intestine, suggesting that enterohepatic recirculation is a relatively minor event. The transport of catechin metabolites to the large intestine for metabolism into degradation products such as 5-(hydroxyphenyl)-γ-valeroacetones and hydroxyphenylvaleric acids is however much more important. These phenolics may be bioavailable in humans, and evidence suggests they may play a role in human nutrition. As with dietary fibre then, one of the main routes that provides health benefits from catechins may actually come from the ability of gram positive bacteria in the gut to synthesise a range of chemicals that are biologically important to humans.
Dr Robert Barrington’s Nutritional Recommendation: Evidence suggests that catechins are important compounds for human health. While not essential like vitamins and minerals, they clearly play a role in the maintenance of health. The best source of catechins is green tea, and the catechin content of tea likely explains at least some of its health benefits. Drinking green tea regularly therefore provides all the catechins necessary to benefit from these interesting phytonutrients.
RdB
