What is Quercetin?
Quercetin is a naturally occurring polyphenolic flavonoid compound found abundantly in plant-based foods that functions primarily as a dietary antioxidant with anti-inflammatory, antihistamine, and potential cardiovascular protective properties. 1, 2
Chemical Classification and Natural Sources
Quercetin belongs to the flavanol class of polyphenolic compounds and represents one of the most prominent dietary antioxidants consumed by humans 1, 3
It is ubiquitously present in common foods including green tea, cranberries, apples, onions, asparagus, radish leaves, buckwheat, blueberries, broccoli, and coriander 1
The typical adult consumes approximately 3-6 mg/day through dietary sources, though intake can reach up to 1 gram daily in diets rich in vegetables, legumes, seeds, and nuts 4, 5
Quercetin exists in multiple chemical forms, with the most abundant derivatives being quercetin 3-O-glycoside, quercetin 3-sulfate, quercetin 3-glucuronide, and quercetin 3'-methylether 1
Primary Biological Mechanisms
Antioxidant Activity:
Quercetin possesses strong antioxidant properties through its ability to scavenge highly reactive species including peroxynitrite and hydroxyl radicals, reducing oxidative stress and protecting against cellular damage 2, 3
It inhibits the biosynthesis of advanced glycation end products (AGEs) through multiple mechanisms: antioxidant properties, metal-chelating ability, protein interaction, methylglyoxal (MGO) trapping, and blocking the receptor for AGE (RAGE) 4
Anti-inflammatory Properties:
Quercetin inhibits the production of inflammatory cytokines and enzymes, making it a potential therapeutic agent for various inflammatory conditions 2
In the context of calendula extracts, quercetin is considered one of the most important bioactive molecules due to its anti-inflammatory and anti-oxidant properties 4
Antihistamine and Immunomodulatory Effects:
Quercetin has anti-allergic functions through inhibiting histamine production and pro-inflammatory mediators 6
It can regulate Th1/Th2 balance and decrease antigen-specific IgE antibody release by B cells 6
FDA-approved homeopathic formulations list quercetin as an ingredient that "calms immune system reactivity" and "calms histamine response" for allergy and sinus symptom relief 7
Potential Health Applications
Quercetin has been studied for cardiovascular benefits including lowering blood pressure, reducing cholesterol levels, and improving endothelial function 2, 8
Research suggests potential benefits against cardiovascular diseases, certain cancers, diabetes, neurodegenerative diseases, allergic asthma, allergic rhinitis, atopic dermatitis, peptic ulcers, osteoporosis, arthritis, and eye disorders 1, 3, 6
It demonstrates antimicrobial, antifungal, antiviral, and antibacterial effects in various studies 4, 1, 2
Critical Bioavailability Limitations
The clinical use of quercetin is significantly restricted due to poor water solubility, substantial first-pass metabolism, and consequently low bioavailability 1
The majority of dietary polyphenols including quercetin are processed by gut microbiota in the large intestine rather than being absorbed in the small intestine 4
Gut bacteria transform quercetin and related compounds (like rutin, which metabolizes to quercetin) into low-molecular-weight phenolic compounds with improved bioavailability 4
Recent nanoformulation approaches have been developed to circumvent limited bioavailability by increasing quercetin uptake by the epithelial system and enhancing delivery to target sites 1
Important Clinical Caveats
Despite widespread use in dietary supplements and homeopathic products, many health claims are based on traditional practice rather than accepted medical evidence and are not FDA evaluated 7
Individuals with known sensitivity to the Compositae (Asteraceae) plant family may be predisposed to allergic reactions when consuming quercetin-containing botanical products like calendula 4
Further studies are needed to elucidate precise mechanisms of action, optimize bioavailability, and assess long-term safety for widespread clinical utilization 2