What is Quercetin?
Quercetin is a flavonol polyphenol found ubiquitously in plant foods — onions (especially red onions), capers, apples, berries, broccoli, kale, and green tea contain the highest concentrations. It is one of the most abundant dietary flavonoids in the human diet and has been the subject of extensive research across cancer prevention, immune function, antiviral activity, cardiovascular health, and anti-aging. Quercetin acts through multiple molecular targets simultaneously — a property that makes it particularly relevant for complex multifactorial conditions.
Key Mechanisms
- Antiviral — viral protease inhibition: Quercetin inhibits the 3CL protease (Mpro) — the main replication enzyme of SARS-CoV-2 and other coronaviruses — as well as PLpro, NTPase/helicase, and the host ACE2 receptor. This multi-target antiviral activity has been demonstrated both computationally and in laboratory studies against multiple respiratory viruses
- NLRP3 inflammasome suppression: Quercetin inhibits the NLRP3 inflammasome — the intracellular sensor responsible for triggering cytokine storms in severe viral infections and chronic inflammatory conditions. This is the mechanism behind its anti-inflammatory activity in COVID-19 and other inflammatory diseases
- Senolytic activity: Quercetin is one of a small number of identified senolytics — compounds that selectively clear senescent ("zombie") cells that accumulate with age and drive chronic inflammation. This property links quercetin to longevity and age-related disease prevention
- Cancer prevention — Phase II enzyme induction + apoptosis: Quercetin activates Phase II detoxification enzymes, induces apoptosis in cancer cell lines, and in combination with curcumin, produced 60% adenoma reduction in a Phase II FAP trial — the strongest human cancer prevention signal for any flavonoid combination
Clinical Evidence — Antiviral
A 100-patient RCT (Di Pierro et al.) found quercetin supplementation (Quercetin Phytosome®) resulted in significantly faster SARS-CoV-2 clearance (p=0.0004) and symptom resolution compared to standard care alone. A systematic review of quercetin RCTs in COVID-19 found that 500–1500mg supplementation led to quicker discharge, reduced inflammation, faster viral clearance, and improved prognosis across multiple trials. A separate double-blind RCT found 36% lower URTI severity and 31% fewer sick days (p=0.020 and p=0.048 respectively) in adults over 40 taking 1000mg quercetin daily compared to placebo.
Bioavailability and Formulation
Standard quercetin powder has poor bioavailability — it is rapidly metabolised and has low solubility. Quercetin Phytosome (quercetin complexed with sunflower phospholipids) provides approximately 20-fold higher plasma levels than equivalent standard quercetin doses and is the most-used format in recent positive clinical trials. Isoquercetin (quercetin-3-O-β-D-glucopyranoside) shows over 4-fold higher bioavailability than quercetin. When choosing a quercetin supplement, phytosome or isoquercetin formulations significantly outperform standard quercetin powder.
Synergistic Combinations
Quercetin's activity is enhanced in combination with: Vitamin C (synergistic antiviral and antioxidant effects; vitamin C also regenerates oxidised quercetin), Vitamin D3 (complementary immune-modulation), zinc (quercetin acts as a zinc ionophore — facilitating zinc transport into cells where zinc inhibits viral RNA polymerase), and bromelain (enhances quercetin absorption). The Quercetin + Curcumin combination is the most evidenced for cancer prevention (FAP adenoma trial).
Dosing
- Prevention/general: 500mg daily (Quercetin Phytosome preferred)
- Acute antiviral support: 500–1000mg twice daily for 1–2 weeks
- Cancer prevention (with curcumin): 20mg quercetin + 480mg curcumin (from the FAP trial protocol)
- Safe up to 2000mg/day in clinical trials with no serious adverse events reported
