Flavan-3-ols are found in most plants and the stereo isomers (C)-catechin and ( — )-epicatechin are the most common monomeric flavan-3-ols in fruits. (+ )-gallocatechin and (— )-epigallocatechin are their corresponding O-3 gallates and are rarer but found in certain seeds of leguminous plants, in grapes and in tea. Catechins are found in many types of fruit and red wine but by far the most abundant sources are green tea and chocolate (56).

The bioavailability of flavan-3-ols differs markedly among the different catechins and appears to be related substantially to structure and degree of galloylation (12). Again due to their structure, a substantial proportion of ingested flavan-3-ols may persist to the colon where they encounter the colonic microbiota. The colonic degradation of flavan-3-ols such as catechin, epicatechin, and epicatechin gallate have been investigated previously (14,44,57-59) revealing that in contrast to other similar structures the heterocyclic C-ring is not cleaved per se. The hydroxylation pattern of flavan-3-ols (5,7,3,3',4'-) has instead been suggested to enhance the opening of the heterocyclic ring after hydrolysis (60,61) and this results in the production of a large number of metabolites from the colonic microbiota: 3,4-dihydroxyphenylacetic acid, 3-hydroxyphenylacetic acid, homovanillic acid and their conjugates are derived from the B-ring and phenolic acids from the C-ring (61). In animal studies, phenylvalerolactones, and phenylpropionic acids have also been identified as degradation products (61).

Antibiotic treatment in rats significantly alters the metabolism of catechin and decreases the urinary elimination of many of the compounds of flavan-3-ol metabolism indicating that an intact microbiota is necessary for the production of many of these compounds (57). At present there are limited studies (described by 8; see Flavonols) that have investigated the specific species that may be responsible for the conversion of flavan-3-ols, and data is limited on any possible inter-individual variation in final metabolic profiles. However, green tea catechins have been shown to cause a shift in bacterial populations in humans (62), pigs (63) and chickens (64), and this may have relevance to the overall polyphenol metabolic capabilities of the resident microbiota. Structurally, these compounds may exhibit substantial anti-oxidant activities and thus the influence of the composition of the resident microbiota and associated metabolic variations could impact on the overall health impact of flavan-3-ol ingestion.

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