Quercetin belongs to a class of water-soluble plant coloring substances called bioflavonoids. Bioflavonoids have strong antioxidanteffects when they are studied in the test tube, and this is the basis for some of the health claims attached to them. However, growing evidence suggests that bioflavonoids do not in fact act as antioxidants in human beings.24 Nonetheless, as widely available plant substances, they are considered possible semi-nutrients, substances that are not essential for life but might help promote optimal health.

Quercetin is not an essential nutrient. It is found in red wine, grapefruit, onions, apples, black tea, and, in lesser amounts, in leafy green vegetables and beans. However, to get a therapeutic dosage, you'll have to take a supplement.

Quercetin supplements are available in pill and tablet form.