Dietary flavonoids are a large family of plant compounds sharing a structure, and there are 6 major subfamilies (anthocyanidins, flavan-3-ols, flavanones, flavones, flavonols, and isoflavones).  Quercetin is a flavonoid from the Flavanol subfamily.

 

Some of the functions quercetin provides (human or animal studies) include:

 

Main dietary sources (from the USDA database) are listed below.  The way food is prepared (boiled, fried, etc.) impacts quercetin concentration.

Excessive quercetin may cause headache, tingling of arms/legs, and kidney damage.  There are some possible medication interactions (antibiotics, cyclosporine, warfarin, liver medications), so always consult a doctor when changing dietary habits.