quercetin

Quercetin is a plant pigment classified as a flavonol, a subclass of flavonoids in the polyphenol family. Its chemical formula is C15H10O7. In plants it occurs mainly as glycosides such as quercetin-3-O-rutinoside (rutin) and quercetin-3-glucoside, which are converted to the aglycone in the gut. Quercetin is widely distributed in fruits, vegetables and other plant foods.

Principal dietary sources include onions, apples, berries, capers, leafy greens, and tea; contributions also come from grapes, citrus fruits and red wine. Intake varies with diet and food preparation.

In vitro and animal studies associate quercetin with antioxidant, anti-inflammatory, antihistamine and potential anticarcinogenic activities. Mechanisms may involve free radical scavenging, inhibition of enzymes such as cyclooxygenase and lipoxygenase, and modulation of signaling pathways like NF-κB. Evidence from human trials is mixed; while some studies suggest modest cardiovascular or anti-allergic effects, results are inconsistent and not enough to establish definite health claims.

Bioavailability is relatively low due to poor absorption and rapid metabolism. After ingestion, quercetin is largely conjugated to glucuronides or sulfates in the intestine and liver and can be metabolized by gut microbiota. Consumption with fats may enhance absorption; foods containing quercetin glycosides are often more bioavailable than aglycone forms.

Quercetin is available as a dietary supplement in various formulations, usually as aglycone or glycoside forms. It is generally considered safe at typical dietary levels, but high-dose supplementation can cause adverse effects in some individuals and may interact with anticoagulants or other medications. Individuals should consult a healthcare professional before use, especially during pregnancy, lactation or while taking prescription drugs.