Quercetin is a plant pigment found in onions, apples, and tea, sold as a dietary supplement at doses many times higher, and taken for blood pressure, inflammation, and longevity. The clearest human signal is a small, repeatable drop in blood pressure and in a common marker of low-grade inflammation, at 500 mg a day or more for at least two months. Most tested products contain less than their labels claim. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Blood pressure (home, seated) | 110–120 / 70–78 mmHg | Primary efficacy endpoint; reproduces across meta-analyses |
| High-sensitivity C-reactive protein (hsCRP) | <1.0 mg/L, ideally <0.5 mg/L | Second efficacy endpoint; tracks the inflammatory mechanism |
| Estimated glomerular filtration rate (eGFR) and serum creatinine | eGFR ≥90 mL/min/1.73 m²; creatinine in the middle of the sex-specific range | Gates the intervention; the kidney is the only organ with documented toxicity |
| Urine albumin-to-creatinine ratio | <10 mg/g | Detects early glomerular damage before filtration rate falls |
| Alanine aminotransferase (ALT) | <25 U/L in men, <20 U/L in women | Screens for hepatic effects; tracks the fatty liver endpoint |
| Thyroid-stimulating hormone (TSH) and free thyroxine (free T4) | TSH 0.5–2.5 mIU/L; free T4 in the upper half of the reference range | Screens for the thyroid gene suppression demonstrated preclinically |
| Hemoglobin A1c (HbA1c) and fasting glucose | HbA1c 4.8–5.4%; fasting glucose 75–86 mg/dL | Metabolic endpoint where pooled evidence is null but subgroup signals exist |
| Fasting insulin | 2–5 µIU/mL | More sensitive than glucose to the insulin-resistance mechanism |
| Lipid panel with apolipoprotein B (ApoB) | ApoB <80 mg/dL; triglycerides <80 mg/dL; triglyceride-to-HDL ratio <1.5 | Secondary cardiovascular endpoints examined in the pooled trials |
| Serum uric acid | 3.5–5.5 mg/dL | Quercetin inhibits xanthine oxidase; a fall confirms biological activity |
| Ferritin with transferrin saturation | Ferritin 50–150 ng/mL; transferrin saturation 25–35% | Detects the iron chelation risk, silent until anemia develops |
Cadence: Home blood pressure twice weekly for four weeks, then monthly; a full laboratory panel at twelve weeks; thereafter every six to twelve months, six-monthly at 1,000 mg per day or above or with reduced kidney function. Thyroid markers at three months in higher-dose users.