small intestine, leaving the balance to persist to the colon where it is exposed to the gut microbiota (26,30). Inter-individual variations in excretion profiles in these studies suggest that hydroxycinnamates are substantially metabolized by the colonic microbiota. In vitro studies have also revealed that chlorogenic acid is extensively metabolized by the colonic microbiota (31). Using inocula from different volunteers, it was clearly demonstrated that degradation rates of chlorogenic acid and the production of the metabolites 3,4-dihydroxyphenylpropionic acid and 3-hydroxyphenylpropionic acid varied considerably between volunteers. Meanwhile in studies with healthy human volunteers, over 50% of the ingested dose was excreted as hippuric acid (a potential microbial metabolite of chlorogenic acid), whilst in the same study individuals without a colon excreted a much smaller amount of aromatic acids. The balance was not metabolized and excreted as chlorogenic acid in the latter volunteers. The absence of a colon and therefore a substantial colonic microbiota in the volunteers and the apparent excretion of intact chlorogenic acid effectively demonstrate the necessity for and the metabolic capacity of the colonic microbiota (27).

Successful attempts have been made to identify colonic microbiota species capable of metabolizing hydroxcinnamates. Inter-individual differences in excretion profiles of volunteers imply that the composition of the resident microbiota may be important in determining this final profile. Given that some of these metabolites are considered to be potentially protective of health, knowledge of the identity of species responsible for such metabolic activity is valuable. It has been demonstrated that at least three colonic microbiota species (Bifidobacterium lactis, Lactobacillus gasseri, and Escherichia coli) can release hydroxycinnamates from chlorogenic acid in the gut (32) as well as diferulic acid being released in the colon as a result of metabolism by esterase activity of the colonic microbiota (33,34). Given that free hydroxycinnmates (including ferulic, caffeic, and p-coumaric acids) exhibit antioxidant and anticarcinogenic properties in vitro and in animal models, and that various microbial metabolites can be absorbed readily (35), this supports the notion that some beneficial effects of hydroxycinnamtes can be ascribed to the metabolic activities and products of the colonic microbiota.

A more limited set of studies has been carried out for hydroxybenzoates. Ellagitannins are polyphenols made up of subunits of ellagic acid (a hydroxybenzoate) and are thought to possess chemopreventative properties that might contribute to health benefits in humans (36-38). Their fate has been studied in 40 volunteers consuming a variety of foodstuffs known to contain high levels of ellagitannins (39). In all cases the ellagitannin microbial metabolite urolithin B (which may be antiangiogenic) could be identified although inter-individual variation in excretion rates was large. Furthermore they were able to identify high and low excretors of this compound in much the same way that consumers of soya can be differentiated by their ability to excrete equol (40,41). Again this observation indicates that the gut microbiota is likely to be important in the bioavailability of these potentially health-promoting compounds and that variations in the composition of the microbiota may dictate the production of a potentially health-promoting metabolite. At present, we are unaware of any studies designed to identify components of the colonic microbiota that are potentially responsible for the metabolism of ellagitannins.

Pregnancy And Childbirth

Pregnancy And Childbirth

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