Flavonols, flavones and flavanols--nature, occurrence and dietary burden

Total flavonol and flavone contents of foods have been determined with validated state-of-the-art methods. Quercetin dominates, and flavonol levels found in vegetables and fruits are below 10 mg kg-1. However, high concentrations are found in onions (300 mg kg-1), kale (450 mg kg-1), broccoli (100 mg kg-1), beans (50 mg kg-1), apples (50 mg kg-1), blackcurrants (40 mg kg-1), and tea (30 mg l-1). The dietary intake of flavonols varies 10-fold between countries (6-60 mg day-1). Flavones are of minor importance in the diet. Tea, wine and fruits are the most important sources of flavanols, but there are gaps in our knowledge on flavanol levels of many foods. The absorption of dietary quercetin glycosides in humans ranges from 20 to 50%. The sugar moiety is an important determinant of the bioavailability of flavonols. The presence of a glucose moiety significantly enhances absorption. The extent of absorption of flavanols in humans seems similar to that of flavonols but has been little studied. Flavonols and flavanols are extensively metabolised, as only 1-2% of them are excreted with an intact flavonoid backbone. Hepatic biotransformations include glucuronidation and sulphatation of the phenolic hydroxyls and O-methylation of catechol groups. Bacteria of the colon cleave the C-ring of the flavonoid nucleus to phenolic acids which are subsequently absorbed. Apart from conjugates, virtually no metabolites have been characterised in humans. Absorption of flavanols is rather fast, with times to reach peak values between 0.5 and 4 h. Flavonols are rapidly excreted, with elimination half-lives of 1-6 h. Quercetin glucosides show rapid to slow absorption; peak values are reached between <0.5 and 9 h. The type of glycoside determines the rate of absorption. Excretion of quercetin glycosides is slow: elimination half-lives are 24 h, independent of the type of glycoside. Analytical data for flavanols in foods are needed. Tea, as an important dietary source, has to be studied. Research on the bioavailability of flavonols and flavanols has to be expanded. Attention is needed for the identification and quantification of their metabolites in body fluids.