Beyond the Basics: The Science of Quercetin—9 Key Health Perks, Dietary Sources, and Safe Usage Tips

Classified as a flavonoid, quercetin possesses a characteristic tricyclic structure with hydroxyl groups. It appears as a bright yellow substance, soluble in alcohol and lipids but insoluble in water and hot water. Its aglycone form (non-glycosylated) exhibits higher lipophilicity than quercetin glycosides (e.g., rutin in tea) and is more readily absorbed; glycosylated forms require hydrolysis to release free quercetin.

1. Anti-inflammatory & Antioxidant Effects: As a flavonoid, quercetin counteracts the "oxidation" process associated with aging by scavenging free radicals—harmful particles that damage cell membranes, disrupt DNA function, induce cell mutations, and trigger healthy cell death. It also downregulates the expression of pro-inflammatory genes (e.g., interleukins).

2. Anti-allergic & Immunomodulatory Roles: Acting as a natural antihistamine, quercetin stabilizes histamine release from immune cells, alleviating symptoms of seasonal/food allergies, asthma, and skin reactions (e.g., coughing, tearing, rhinorrhea, urticaria).

3. Cardiovascular Protection: Through dual anti-inflammatory and antioxidant mechanisms, quercetin supports cardiovascular health. Epidemiological data link high intake of flavonoid-rich fruits/vegetables to reduced cardiovascular events and mortality in the elderly. Preclinical and preliminary human studies indicate it may reduce atherosclerotic plaques, inhibit LDL oxidation, lower "bad cholesterol," and assist in blood pressure regulation—though large-scale clinical trials are needed for validation.

4. Analgesic Potential: It may relieve pain associated with autoimmune diseases (e.g., arthritis) and infections (e.g., prostate, respiratory tract infections). Studies demonstrate its efficacy in reducing inflammatory pain and bladder pain symptoms (e.g., urgency, swelling, burning) caused by infections. Rheumatoid arthritis patients report less pain and fewer relapses when switching from a "typical Western diet" to one rich in antioxidants.

5. Energy & Endurance Enhancement: Incorporated into sports supplements for its positive effects on blood flow, quercetin has been shown to significantly improve maximal oxygen uptake (VO2 max) and endurance performance in humans (per research from the Institute for Applied Physiology, Georgia Institute of Technology).

6. Anti-cancer Properties: It exhibits potential chemopreventive activity and exerts antiproliferative effects on cancer cells, possibly via modulation of EGFR or estrogen receptor pathways. It may inhibit cell proliferation, tumor growth, and symptoms associated with cancer treatments (e.g., radiotherapy, chemotherapy). However, most studies to date are animal-based, requiring more human trials.

7. Skin Health Protection: By blocking mast cells—key mediators of allergies, inflammatory diseases, and autoimmune disorders—quercetin helps protect the skin against conditions like eczema and photosensitivity.

8. Liver Protection: In animal studies, quercetin protected rats from ethanol-induced acute liver injury. Its hepatoprotective effects are attributed to enhanced activity of ethanol-metabolizing enzymes, strengthened antioxidant defense against oxidative stress, and reduced expression of pro-inflammatory cytokines.

9. Neuroprotective Effects: It defends the brain against oxidative stress and inflammation, potentially lowering the risk of cognitive impairments (e.g., Alzheimer’s disease, dementia). A 2018 study found that quercetin administration during the early-to-moderate stages of AD pathology improved cognitive dysfunction, enhanced Aβ clearance, and reduced astrogliosis (linked to neuronal damage).

• Fruits: Red-skinned apples, black cherries, blueberries, blackberries, bilberries, cranberries, citrus fruits (oranges, grapefruits), grapes, and red wine.

• Vegetables: Red onions, raw onions, bell peppers, chili peppers, cruciferous vegetables (broccoli, cabbage, bean sprouts), leafy greens (spinach, kale), raw asparagus, and capers.

• Grains & Legumes: Whole grains (rye, oats), and legumes (soybeans, lentils, peas).

• Beverages & Condiments: Green tea, black tea, cocoa powder, dark chocolate, and extra virgin olive oil.

• Herbs & Plants: Sage, St. John’s Wort, ginkgo biloba, and elderberry (Sambucus nigra).

Note: Quercetin content is influenced by growing conditions (soil, climate), freshness (higher in fresher produce), and processing methods (raw or mild processing preserves more quercetin).

Drug Interactions

1. Cyclosporine: Inhibits hepatic metabolism (CYP3A4) and intestinal P-glycoprotein transport of cyclosporine, increasing blood concentrations and immunosuppressive effects (elevating toxicity risk). Avoid co-administration or monitor blood concentrations under medical guidance.

2. CYP450 Substrates (e.g., warfarin, midazolam, pravastatin, quetiapine): May enhance or reduce drug efficacy/toxicity by inhibiting or inducing CYP enzymes (e.g., CYP1A2, CYP3A4, CYP2C9). Monitor efficacy and adverse effects during co-administration.

3. P-glycoprotein Substrates (e.g., cyclosporine, digoxin, some anticancer drugs): Inhibits P-glycoprotein, increasing drug bioavailability or tissue distribution (enhancing efficacy/toxicity). Monitor drug concentrations.

4. Warfarin: Enhances anticoagulant effects by inhibiting CYP2C9, increasing bleeding risk. Avoid co-administration or closely monitor INR.

5. Antihypertensives: Exerts mild vasodilatory and hypotensive effects, potentially causing hypotension when combined with antihypertensives. Monitor blood pressure.

6. Antidiabetics: Improves insulin sensitivity and lowers blood glucose, risking hypoglycemia when combined with antidiabetics. Monitor blood glucose.

7. NSAIDs (e.g., diclofenac): Inhibits CYP2C9, slowing diclofenac metabolism and increasing risks of gastrointestinal bleeding and hepatorenal toxicity. Avoid long-term co-administration and ensure gastrointestinal protection.

8. Statins (such as Pravastatin): Inhibit liver transport proteins (such as OATP), affecting drug excretion. May enhance lipid-lowering effects, but increase the risk of muscle toxicity. Monitoring of creatine kinase (CK) levels is necessary to prevent rhabdomyolysis.

9. Psychotropics (e.g., quetiapine, midazolam): Inhibits CYP3A4 or P-glycoprotein, increasing drug concentrations and central nervous system side effects (e.g., sedation, drowsiness). Avoid co-administration or adjust doses.

10. Anticancer Drugs (e.g., mitoxantrone): Increases drug concentrations by inhibiting transporters (e.g., P-gp, OATP), enhancing efficacy but also toxicity. Use under medical supervision; avoid self-administration.

11. OAT/OATP Substrates (e.g., losartan, sulfasalazine, prazosin): Interferes with organic anion transporters, affecting drug distribution and excretion in the kidneys/liver. Monitor drug concentrations and adjust doses if necessary.

The average daily dietary intake of quercetin from plant-based foods ranges from 5 to 40 mg. Individuals on nutrient-dense diets may consume up to 500 mg/day. For healthy adults, quercetin supplementation (≤1000 mg/day for ≤12 weeks) is generally safe, with potential adverse effects including nausea, headache, stomach pain, and tingling in the extremities. Consult a healthcare professional before long-term supplementation. For pregnant/lactating individuals, quercetin is safe when consumed as food, but doses exceeding dietary levels are not recommended due to insufficient safety/efficacy data.

Quercetin exhibits multiple bioactivities (antioxidant, anti-inflammatory, etc.) and confers health benefits for the cardiovascular, immune, and nervous systems. Dietary intake via quercetin-rich foods (e.g., apples, onions, berries) is a safe and effective approach. For high-dose supplementation, potential drug interactions must be considered, and professional guidance is essential.

Important Reminder：All content in this article is for general reference only and is provided solely to offer information support for practitioners in the nutrition and health industry. Descriptions related to efficacy are supported by corresponding data, but they do not represent claims or guidance for consumers. Content related to health, medical care, and technological applications is for reference only. For medical matters, please consult professional medical institutions and follow medical advice. This article does not provide any medical recommendations.