Absorption, Metabolism, and Excretion of Flavonoids

This biotransformation is common to many xenobiotics and diminishes the potential toxicity of the substrate and facilitates its biliary and urinary elimination by increasing its hydrophilicity. Flavonoid metabolites are transported to extrahe-patic tissues and eventually to the kidneys, where they are excreted in the urine or incorporated into bile and excreted in feces.13 Because phase II metabolism is highly efficient with respect to the flavonoids, aglycones (except for anthocyanins) 

For most flavonoids, the maximum concentration in human plasma occurs 1-6 hours after ingestion with an elimination half-life (t1/2) from 1 to 28 hours.15-21 Therefore, flavonoids can accumulate in the circulation and in tissues with frequently repeated intakes. Plasma quercetin in free-living subjects, measured after overnight fast, is typically 50-80 nmol/L, though smaller values are observed following a low-flavonoid diet.15 Conversely, supplementation with 500 mg/d rutin for 6 weeks increased plasma quercetin to 165 nmol/L22 and to 0.63 and 1.5 prnol/L following supplementation with 80 mg and 1 g quercetin equivalents daily for 7 and 28 days, respectively.23-24 

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Figure 2.3 Absorption, metabolism, and excretion of flavonoids. Glucuronidation, methyla-tion, and sulfation are catalyzed by UDP-glucuronyltransferase, catechol-O-methyltrasferase, and sulfotransferase, respectively. Abbreviation LPH, lactase phlorizin hydrolase.

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