Proanthocyanidins

Proanthocyanidins are dimers, oligomers, and polymers of flavan-3-ols and are formed by enzymatic or chemical condensation. These so-called "condensed tannins" contribute to astringent tastes in fruits (e.g., grapes, peaches, apples, pears, berries etc.), beverages (e.g., wine, cider, tea, beer etc.) and chocolate. At a lower degree of polymerization they are colorless and bitter to taste, but with greater polymerization the taste becomes astringent and the color yellow to brown. Proanthocyanidins purely consisting of catechin and epicatechin monomers are called procyanidins, which are the most common type of proanthocyanidins. Less abundant are the prodelphinidins, which include both epicatechin and gallocatechin monomers.

Previous studies in rats have indicated that the bioavailability of procyanidins is low and characterized by a very low urinary recovery (0.5% ingested dose) (76). Procyanidin consumption in rats and in humans is associated with the production of several aromatic compounds including derivatives of phenylpropionic, phenylacetic, and benzoic acids (77,78). More recent studies have also established that consumption of proanthocyanidins from grape seed extract can result in a consistent increase in urinary excretion of 3-hydroxyphenylpropionic acid and 4-O-methylgallic acid. Inter-individual variation in excretion of 3-hydroxyphenylproionic acid was significant (79). The microbial metabolism of proanthocyanidins has never been studied in humans but the microbial origin of these compounds was established in vitro following incubation of proanthocyanidins with rat cecal contents (80) and human fecal microbiota (81). These studies utilized 14C labeled proanthocyanidin oligomers and led to the formation of m-hydroxyphenylpropionic acid, m-hydroxyphenylacetic acid and their p-hydroxy isomers, m-hydroxyphenylvaleric acid, phenylpropionic acid, phenyl acetic acid and benzoic acid. Attempts have been made in the past to identify intestinal bacteria that can degrade proanthocyanidins (82,83) although these studies actually failed. The impact of proanthocyanidins on colonic microbiota populations has been investigated in rat studies and revealed that there was a shift in the predominant bacteria present towards Gramnegative Enterobacteriaceae and Bacteroides species (84). Furthermore, proanthocyanidin intestinal absorption and microbial metabolism of some of the above metabolites fell as the degree of polymerisation increased (77,81). Thus studies on antioxidant and biological effects of proanthocyanidins are only useful when targeted at compounds with a low degree of polymerization. Larger compounds do not appear to be able to reach systemic circulation or be available for microbial metabolism that would result in significant production of readily absorbable phenolic acid metabolites. However, this does highlight that at least some of the purported health effects of proanthocyanidin-rich diets may be due to secondary metabolites rather than the original ingested compounds.

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