Table 2.

Summary of the metabolism and bioavailability of flavonoid classes discussed.

Flavonoid class	Molecular Weight	Glycosylation	Metabolic conversion	Colonic microflora
General	Decreases bioavailability	Generally removed	Major factor in bioavailability; can take place in small intestine, liver and colon; usually to glucuronides but also sulphation and methylation [39]; facilitates urinary and biliary excretion [39].	Influence availability; catabolize compounds to low molecular weight compounds that are readily absorbed [58].
Flavan-3-ols (monomeric)			Major bioactive forms: conjugates of epicatechin [41]; catechin: methyl, sulfate and glucuronic acid conjugates; epicatechin: mainly to sulfate conjugates, no glucuronidation [41].
Proanthocyanidins	Decreases bioavailability [40].		Major bioactive forms: conjugates of epicatechin [41]; oligomeric procyanidins can absorb in small intestine [51].	Influences polymeric proanthocyanidin degradation [50].
Flavonols		Sugars and their position affects bioavailability [44].	Potentially active metabolites: glucuronides [44].	Facilitates glucuronidation [60].
Anthocyanins	Anthocyanin derivatives (flavan-3-ol-anthocyanin dimer) can potentially be absorbed with less efficiency [47].	Sometimes found with sugars intact in circulation [55].	Major intestinal metabolites: glucuronide and sulfate conjugates of protocatechuic acid and phloroglucinaldehyde [46]; anthocyanin derivatives metabolically more resistant than parent compounds [47].
Isoflavones		Aglycone more bioavailable; possible deglycosylation prior hepatic metabolism [48].		Metabolize daidzein to equol [58].
Flavanones		Rapid absorption, low bioavailability [49].		Extensive first-pass metabolism partly by intestinal bacteria degraded into phenolic compounds [49].