Flavonols

Four parent molecules account for most of the flavonols in food and medicinal plants, and each one exists as dozens of glycosides.

Quercetin

as rutin, hyperoside, isoquercitrin and quercitrin

Kaempferol

as astragalin, nicotiflorin, tiliroside and kaempferitrin

Myricetin

as myricitrin; named after the bayberry genus Myrica

Isorhamnetin

a methylated quercetin; narcissin is its rutinoside

Icariin and its relatives

the prenylated flavonols of horny goat weed

Fisetin, galangin, morin and gossypetin are smaller members with a narrower spread: fisetin in strawberry and smoke tree, galangin in galangal and propolis, morin in the mulberry family, gossypetin in hibiscus and cotton flowers.

They are found in nearly every leaf, flower and fruit, and concentrate in the outer, sun-exposed tissues, where they screen ultraviolet light.

The sugar attached to a flavonol changes how much of it reaches the blood. Quercetin glucosides, the form in onions, are absorbed in the small intestine within an hour or so.

Rutin is not. Its rutinose sugar has to be removed by gut bacteria in the colon first, so it is absorbed later and less completely.

Once absorbed, flavonols are quickly converted by the gut wall and liver into glucuronide and sulfate forms. Very little free quercetin circulates even after a large dose, and much of the cell research uses concentrations the blood never reaches.

The flavonols share one chemistry and broadly one profile. In laboratory work nearly all of them act as antioxidants, damp down inflammatory signalling and inhibit a range of enzymes; the differences between members are mostly of degree.

The human evidence is narrower. Quercetin at 500 mg a day or more lowered blood pressure by about 3 mmHg in pooled trials. Rutin and its semi-synthetic derivatives, the hydroxyethylrutosides, are used for chronic venous insufficiency and fragile capillaries, with trial evidence for reduced ankle swelling.

Quercetin also stabilises mast cells in laboratory studies, the basis of its use for hay fever, though human trials are few.

A few members have effects of their own. Icariin weakly inhibits PDE5, the enzyme sildenafil targets, in laboratory tests. Fisetin clears ageing (senescent) cells in mice and has reached early human trials.