Flavonoids: Dietary occurrence and biochemical activity

Flavonoids are plant phytochemicals that cannot be synthesized by humans. The six classes of flavonoids (flavanones, flavones, flavonols, isoflavonoids, anthocyanins, and flavans) vary in their structural characteristics around the heterocyclic oxygen ring. Flavanones occur predominantly in citrus fruits, flavones in herbs, isoflavonoids in legumes, anthocyanins and catechins in fruits and flavonols in all fruits and vegetables. Food preparation and processing of fresh fruits and vegetables may decrease flavonoid content by 50% owing to leaching into water or removal of portions of the plant that are rich in them. Grains and oils seeds have flavonoids, but processing removes or reduces them. Other plant food groups contain differing amounts of flavonoids. Honey, chocolate, and sweets that contain some plant constituents have flavonoids. Flavonoids are not present in animal foods. Dietary intake estimates vary from 23 mg/day to 1000 mg/day but the number of compounds and classes assessed vary, and all estimates currently are incomplete. In vitro and animal studies have demonstrated that flavonoids have antioxidant and antimutagenic activities. Case control studies suggest that flavonoids may reduce the risk of cardiovascular disease and stroke. Flavonoid classes vary in their absorption and their metabolism is still obscure. They are conjugated in the liver or kidney and excreted into bile or urine. Colonic bacteria split the heterocyclic ring and degrade the flavonoids to phenyl acids which may be absorbed, conjugated, and excreted or metabolized further by colonic bacteria. (C) 1998 Elsevier Science Inc.