Flavonoids are a group of polyphenolic compound present in plants as secondary metabolites (meaning they are synthesised for purposes other than primary metabolic activity). Many edible and commonly consumed plants contain flavonoids, and as a result human consumption of flavonoids may be significant. Flavonoids are bioavailable in humans, where they may have health effects. Tea, red wine, chocolate and fruits and vegetables are rich sources of flavonoids and likely supply most of the human intake. The reason that flavonoids are beneficial may relate to their antioxidant structure, although they may also have gene regulatory ability (although this may in turn be due to the control of reactive oxygen species levels in cells). However, the bioavailability and the antioxidant capacity of flavonoids is dependent on their structure, and so not all flavonoids confer the same protective effects. As a result researchers are interested in those flavonoids that confer the greatest protection from disease in humans.
Observational studies have shown that flavonoids belonging to the class flavonols and flavan-3-ols (catechins) may confer the greatest health effects. The most common dietary flavonol is quercetin and its glycoside derivatives, and the most common flavan-3-ols are the catechins found in tea (including catechin and epicatechin derivatives). However, recent studies comparing quercetin with epicatechin (here) showed a significant effect for epicatechin, but not quercetin, at improving insulin sensitivity in healthy individuals. While quercetin may be beneficial in other aspects of health, it does demonstrate the differing biochemical activity of isolated flavonoids. Of course, understanding flavonoids in the diet is complicated by the presence of multiple flavonoid types and classes within foods, and the interaction between different flavonoids is not understood. Consuming whole foods rather than individual flavonoids is therefore still the best strategy, as this is how they are found in foods that show health effects.
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