Quercetin

Bioflavonoid compounds are widely distributed in nature. Onion has a rich content of Quercetin. Flavonoids are phenolic compounds naturally occurring that are abundantly present in all plants and in some seaweed.

They are ingested when drinking fruits, vegetables, tea and red wine, for example. ^{(Serafini 1996)}

Most important types are: anthocyanins, chalcons, flavonols, flavones, flavonoids and tannins. 

Attributed anti-oxidant activity and other properties that have not been fully shown yet ^{(Martinez 2016, Morling 2015, Cohen 2012, AESAN 2012)}, as amelioration of capillary vessel function which is a reason to be used for treatment of venous insufficiency (varicose veins) and prevention of arteriosclerosis. 

They appear naturally in the breast milk in amounts related to mother's diet. ^{(Romaszko 2014, Song 2013)}

No risk for toxic nor side-effects in humans have been found ^{(AESAN 2012)}. No problems have been observed in infants whose mothers were treated with troxerutin. ^{(Krajnovic 1977)}

The maximum acceptable daily intake amount for quercetin is 75 mg. ^{(AESAN 2012)}