The flavonols and flavones are generally present in plants in the form of glycosides and as such are water-soluble. Thus, some of the flavonol glycosides may be absorbed intact in the small intestine or hydrolyzed by mucosal enzymes and absorbed as aglycones. However, those that pass through the small intestine unabsorbed or reenter the gut from the bile become available for bacterial metabolism in the colon.
The colon contains numerous microorganisms and as a result it has significant capacity for catalytic and hydrolytic reactions. These colonic bacteria produce enzymes that are capable of stripping flavonoid conjugates of their sugar moieties, enabling free aglycones to be absorbed. The enzymes produced by colonic bacteria can also break down the flavonoids into simple compounds, resulting in the production of a range of derivatives, some of which may be more biologically active than the parent compound. This is an important area for future research because the metabolism of flavonoids is influenced by intestinal microflora and these metabolic reactions may result in deactivation of bioactive compounds or activation of previously inactive compounds. It is therefore critical to identify the bacteria involved in these transformation reactions and define their relative importance and occurrence in the human gut to gain a better understanding of the transformation processes.
Other key body compartments that are important in defining the metabolism of flavonoids are the liver and, to a lesser extent, the small intestine and kidney, in which the biotransformation enzymes are located. Flavonols and flavan-3-ols are primarily metabolized in the colon and liver.
The evidence for absorption of intact flavonoid glycosides is weak. Recent data showing ^-glucosi-dase activity in the small intestine, together with the absence of intact glycosides in plasma and urine, strongly suggest that only free flavonoid aglycones are being absorbed. In addition, data also indicate that there is a more rapid and efficient absorption of flavonoids originating from glucosides than from other glycosides or free aglycones. This suggests that dietary sources containing high levels of glucose-bound flavonoids are more likely to have potential health benefits than foods containing other flavo-noid glycosides.
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