Quercetin is a dietary flavonoid found in many plants, including tea, apple, onion, ginkgo biloba and citrus.

Quercetin forms the "backbone" for many other flavonoids, including the citrus flavonoids rutin, hesperidin, naringin and tangeritin. In studies, Quercetin is found to be the most active of the flavonoids, and many medicinal plants owe much of their activity to their high quercetin content. Quercetin has demonstrated significant anti-inflammatory activity because of direct inhibition of several stages and processes of inflammation. In addition, it exerts potent antioxidant activity and boosts the effectiveness of vitamin C.

Quercetin has antioxidant, anti-inflammatory, nitric oxide inhibitor, and tyrosine kinase inhibitor (leading to inhibition of the division and growth of T-cells and some cancer cells) activity. The anti-inflammatory effects of quercetin appear to be due to inhibition of the production and activity of leukotrienes and prostaglandins, and inhibition of histamine. Quercetin also appears to reduce capillary fragility.

Quercetin’s broad spectrum anti-oxidant and anti-inflammatory actions make it one of the best cytoprotective and neuroprotective molecules found in nature.

References:

Abd-El-Gawad HM & Califa AE. Quercetin, coenzyme Q10 and L-canavanine as protective agents against lipid peroxidation and nitric oxide generation in endotoxin-induced shock in rat brain. Pharmacological Research 2001, 43: 257-263

Bastianetto S & Quirion R. Natural extracts as possible protective agents of brain aging. Neurobiology of Aging 2002, 23: 891- 897.

Esposito E, Rotilio D, Di Matteo V, Di Giulio C, Cacchio M & Algeri S. A review of specific dietary antioxidants and the effects on biochemical mechanisms related to neurodegenerative processes. Neurobiology of Aging, 2002, 23: 719-735

Jae Sue Choi, Hae Young Chung, Sam Sik Kang, Mee Jung Jung, Jung Won Kim, Jae Kyung No, Hyun Ah Jung. The structure-activity relationship of flavonoids as scavengers of peroxynitrite. Phytotherapy Research 2002:16(3);232 - 235

Oyama Y, Fuchs PA, Katayama N, Noda K. Myricetin and quercetin, the flavonoid constituents of Ginkgo biloba extract, greatly reduce oxidative metabolism in both resting and Ca(2+)-loaded brain neurons. Brain Res. 1994 Jan 28;635(1-2:125-9).