Dietary flavonoids: intake, health effects and bioavailability.

Flavonoids are polyphenolic compounds that occur ubiquitously in foods of plant origin. Over 4000 different flavonoids have been described. They may have beneficial health effects because of their antioxidant properties and their inhibitory role in various stages of tumour development in animal studies. An estimation of the total flavonoid intake is difficult, because only limited data on food contents are available. It is estimated that humans ingest a few hundreds of milligram per day. The average intake of the subclasses of flavonols and flavones in The Netherlands was 23 mg/day. The intake of flavonols and flavones was inversely associated with subsequent coronary heart disease in most but not all prospective epidemiological studies. A protective effect of flavonols on cancer was found in only one prospective study. Flavonoids present in foods were considered non-absorbable because they are bound to sugars as beta-glycosides. However, we found that human absorption of the quercetin glycosides from onions (52%) is far better than that of the pure aglycone (24%). Flavonol glycosides might contribute to the antioxidant defences of blood. Dietary flavonols and flavones probably do not explain the cancer-protective effect of vegetables and fruits; a protective effect against cardiovascular disease is not conclusive.