What is Quercetin
Quercetin is a plant-derived polyphenolic flavonoid compound found abundantly in fruits, vegetables, and other plant foods, functioning primarily as a potent antioxidant with multiple therapeutic properties including anti-inflammatory, anti-glycation, and gut microbiota-modulating effects. 1
Chemical Classification and Dietary Sources
Quercetin belongs to the flavonol subclass of flavonoids, representing one of the most prominent dietary antioxidants consumed by humans 2, 3
It is ubiquitously present in common foods including onions, apples, grapes, berries, cherries, broccoli, citrus fruits, capers, lovage, tea, and wine 1, 4, 5
Typical adult consumption of dietary polyphenols (including quercetin) reaches up to 1 gram daily, which is approximately 100 times higher than carotenoid intake and 10 times higher than vitamin C intake 1
Quercetin exists in multiple forms in foods, with the most abundant derivatives being glycosides and ethers such as quercetin 3-O-glycoside (also called rutin), quercetin 3-sulfate, quercetin 3-glucuronide, and quercetin 3'-methylether 6
Biochemical Properties and Mechanisms of Action
Quercetin functions as a potent antioxidant by scavenging highly reactive species including peroxynitrite and hydroxyl radicals, thereby neutralizing free radicals and reducing oxidative stress 2, 3
It exhibits anti-glycation activity by inhibiting the formation of advanced glycation end products (AGEs) and trapping reactive dicarbonyl species such as methylglyoxal (MGO) and glyoxal (GO) 1, 7
Quercetin acts as an mTOR (mechanistic target of rapamycin) pathway inhibitor, which has implications for longevity and metabolic regulation 1
It possesses anti-inflammatory properties by inhibiting the production of inflammatory cytokines and enzymes, including effects on TLR4 gene expression and NLRP3 inflammasome activity 1, 3
Effects on Gut Microbiota and Metabolic Health
A large proportion of dietary quercetin accumulates in the colon where gut microbiota processes it into bioactive metabolites, including phenol derivatives with vicinal hydroxyl groups 1, 8
Quercetin supplementation modifies gut microbiota composition, specifically decreasing high-fat diet-induced increases in Proteobacteria and normalizing intestinal short-chain fatty acid (SCFA) production 1
It can lower the Firmicutes-to-Bacteroidetes ratio, which is elevated in obesity and associated with body weight regulation 1
Quercetin promotes the growth of beneficial bacteria such as Lactobacillus and Bifidobacterium while inhibiting pathogenic species like Clostridium histolyticum and Clostridium perfringens 1
Therapeutic Applications and Health Benefits
In animal models, 16-week quercetin supplementation partially reverses high-fat diet-induced nonalcoholic fatty liver disease (NAFLD) alterations including steatosis, ballooning, elevated liver triglycerides, and insulin resistance 1
Quercetin restores gut barrier function, reduces endotoxemia, and improves intestinal permeability that is compromised by high-fat diets 1
It demonstrates cardiovascular benefits including lowering blood pressure, reducing cholesterol levels (particularly LDL oxidation resistance), and improving endothelial function 1, 3
Quercetin exhibits anticancer effects by inhibiting cancer cell proliferation and inducing apoptosis, along with antiviral and antimicrobial properties 3, 4, 5
It shows potential neuroprotective effects against Alzheimer's disease through acetylcholinesterase inhibition and reduction of protein aggregation 1, 5
Bioavailability and Clinical Limitations
Clinical use of quercetin is restricted due to poor water solubility, substantial first-pass metabolism, and consequently low bioavailability 6
Quercetin accumulates primarily in the lungs, liver, kidneys, and small intestines, with lower concentrations in the brain, heart, and spleen 5
It is eliminated through renal, fecal, and respiratory systems 5
Quercetin has potential drug interactions due to its effects on cytochrome P450 enzymes and transport proteins, which must be considered in clinical applications 8
Nanoformulation approaches are being developed to circumvent bioavailability limitations by increasing epithelial uptake and enhancing targeted delivery 6
Practical Considerations
The Chinese Nutrition Society has not established specific proposed intake levels or upper limits for quercetin due to inadequate evidence, though it is recognized as a bioactive compound of interest 1
Quercetin-rich foods or supplements may be incorporated into the diet for potential health benefits, though further studies are needed to optimize dosing, assess long-term safety, and elucidate complete mechanisms of action 3, 6
When used as a functional food ingredient (such as in bread formulations at 1.2-3.6%), quercetin significantly inhibits the formation of carboxymethyllysine (CML) and total fluorescent AGEs while enhancing antioxidant and antibacterial activities 1