Quercetin

Quercetin is one of many flavonoids found in plant pigments, being orange to orange-yellow in color. It is the principal source flavonoid in human nutrition and is commonly used in food processing. Quercetin is found in high concentrations in asparagus, red onions, broccoli and buckwheat.

Other names include: Citrus bioflavonoid, Sophoretin; Meletin; Quercetine; Xanthaurine; Quercetol; Quercitin; Quertine; Flavin.

Function
Quercetin affects immunity and inflammation by acting mainly on leukocytes and targeting many intracellular signaling kinases and phosphatases, enzymes and membrane proteins often crucial for cellular specific function.

Food sources
Quercetin can be found in many foods including red onions, red wine, onions, green tea, apples, asparagus, berries, broccoli and Brassica vegetables, ginkgo biloba, St. John's Wort, American elder, tea and buckwheat tea.

Health uses
Quercetin supplements are often promoted for However, many of these uses have weak evidence supporting them, and for some uses there is evidence that quercetin supplementation do not improve symptoms.
 * prevention and treatment of certain types of cancer
 * atherosclerosis
 * high cholesterol
 * heart disease and circulatory problems
 * diabetes
 * hay fever
 * cataracts
 * peptic ulcer
 * schizophrenia
 * inflammatory conditions (asthma, gout)
 * chronic fatigue syndrome
 * chronic prostate

Quercetin is also taken by athletes to increase endurance and improve performance.

Quercetin is a potent anti-oxidant. Most of the information on flavonoids concerns quercetin because with only sight changes to the backbone of flavones and subtle cell behavior mechanisms and responsiveness, flavoinoids can be modulating, biphasic and exert regulatory action on immunity and inflammation. Only a few flavones and flavonols have been assayed mainly due to chemical similarity to quercetin.

Safety
The US FDA has issued warning letters to manufacturers of supplements containing quercetin to withdraw health benefit claims and emphasize that quercetin is not a defined nutrient nor an antioxidant, cannot be assigned a dietary content level and is not regulated as a drug to treat any human disease. The European Food Safety Authority evaluated possible health claims associated with consumption of quercetin, and found that no cause-and-effect relationship established for any physiological effect in human health or diseases.

Chronic fatigue syndrome
Quercetin's antioxidant and anti-inflammatory properties are thought to be important in resolving the pathophysiology of ME/CFS. Additionally, a 2009 study by J. Mark Davis, et al., showed markers of mitochondrial biogenesis in mouse skeletal muscle and brain, and on endurance exercise tolerance after a week of quercetin in their food.

Luteolin is another flavonoid antioxidant which is common place.

Flavonoids are believed to be poorly absorbed due to bring rapidly metabolized and excreted. There remains no evidence that polyphenols (of which flavonoids are subset of) have any relevance on the body.

Additionally, although antioxidants in the diet are required to be healthy, there is considerable debate on which antioxidants are health promoting, in what amounts and if they can prevent or help chronic diseases.

Learn more

 * Quercetin - Pubchem
 * Quercetin - drugs.com