Flavonoids

The family is sorted by the oxidation state of the central ring and where the second aromatic ring attaches. Six subclasses cover most of what you will meet:

Flavonols

quercetin, kaempferol, myricetin, rutin

Flavones

apigenin, luteolin, baicalein, the skullcap flavones

Flavanones

naringenin, hesperetin, the citrus flavonoids

Flavan-3-ols

catechin, epicatechin, EGCG and the proanthocyanidins

Isoflavones

genistein, daidzein, the legume phytoestrogens

Anthocyanidins

cyanidin, delphinidin, malvidin and the fruit pigments

Flavonols, flavones, flavanols, proanthocyanidins, isoflavones and anthocyanins each have their own page. This page collects the family as a whole and the members that sit outside those six, chiefly the flavanones and dihydroflavonols.

In the living plant flavonoids almost always circulate with sugars attached — quercetin as rutin or isoquercitrin, apigenin as apiin, and so on. The sugar keeps them soluble and stored safely in the vacuole.

That matters enormously for how they behave in people. Most flavonoid glycosides are poorly absorbed intact; the gut microbiota cleave the sugar, and what actually reaches the bloodstream is often a bacterial metabolite rather than the compound named on the label. Absorbed aglycones are then rapidly glucuronidated and sulfated, so free circulating quercetin is scarce even after a large dose. This is why in-vitro potency so often fails to translate.

Direct radical scavenging is the effect flavonoids are famous for and probably the least important one in the body, given the plasma concentrations actually achieved. The better-supported mechanisms are indirect: activation of the Nrf2 pathway, which turns on the cell's own antioxidant enzymes; inhibition of NF-κB-driven inflammatory signalling; and modulation of enzymes and transporters.

Clinically the group carries real, specific effects — the citrus flavonoids and rutin in venous insufficiency and capillary fragility, quercetin and luteolin as mast-cell stabilizers, baicalein and apigenin at the benzodiazepine site of the GABA-A receptor, and the flavan-3-ols in vascular endothelial function.