Outcome
    Moderate Evidence

    Quercetin for Antioxidant Protection

    Quercetin is a potent antioxidant in vitro, but poor bioavailability means human trials show smaller and less consistent effects on oxidative stress markers than laboratory data suggest.

    Overview

    Quercetin is a potent antioxidant in vitro, but poor bioavailability means human trials show smaller and less consistent effects on oxidative stress markers than laboratory data suggest.

    Verdict

    Mixed evidence

    How It Works

    Quercetin scavenges reactive oxygen species directly through its catechol B-ring and, more importantly in vivo, activates the Nrf2 pathway, which upregulates endogenous antioxidant enzymes including glutathione peroxidase, superoxide dismutase and heme oxygenase-1. The indirect enzymatic route is likely more relevant at physiological concentrations than direct radical scavenging. Absorption is the limiting factor. Free quercetin aglycone is poorly absorbed and heavily conjugated during first-pass metabolism, so plasma levels after typical supplement doses are far below the micromolar concentrations used in cell culture. Phytosome and isoquercitrin formulations meaningfully improve exposure.

    Dosing & Protocol

    Typical dose

    Recommended dose
    500-1,000 mg/day quercetin in divided doses with food; antioxidant marker changes are inconsistent in humans
    Expected timeframe
    8-12 weeks

    Protocol

    form
    Bioavailability-enhanced forms - phytosome, enzymatically modified isoquercitrin, or with bromelain and vitamin C - since plain aglycone absorption is under 10%
    duration
    8-12 weeks
    co factor
    Take with food containing fat. Evidence is mixed. Quercetin is one of the most potent dietary antioxidants in vitro, scavenging free radicals directly and inducing Nrf2-mediated antioxidant enzyme expression. Human trials give an inconsistent picture: some report reduced markers of oxidative damage such as F2-isoprostanes or oxidised LDL, particularly in smokers, athletes under heavy load, and people with metabolic syndrome, while others show no change in healthy well-nourished adults. Poor bioavailability and rapid conjugation to metabolites with different activity are the likely explanation for the gap between test tube potency and human results.
    titration
    Up to 500 mg twice daily; 1,000 mg/day for up to 12 weeks is the well-studied ceiling
    starting dose
    500 mg once daily with a fat-containing meal

    Evidence

    What the studies say

    Randomised trials measuring oxidative stress biomarkers - malondialdehyde, oxidised LDL, total antioxidant capacity - have produced mixed results, with some showing improvement at doses of 500-1000mg daily and others no change. Meta-analyses find more consistent effects on inflammatory markers such as CRP than on oxidative markers. A further caution is that "antioxidant protection" is not itself a clinical endpoint. Large trials of other antioxidants, including beta-carotene and vitamin E, showed that improving biomarkers does not reliably improve health outcomes and can occasionally harm. Quercetin from food sources - onions, apples, capers, tea - comes without the interaction risks of high-dose supplementation.

    Bioavailability of Quercetin in Humans with a Focus on Interindividual Variation

    Score: 7/10
    2018
    systematic_review

    Almeida AF, Borge GIA, Piskula M +3 more

    there is less interindividual variation in metabolites which are derived from absorption in the small intestine compared to catabolites derived from the action of microbiota in the colon

    View source

    Impact of quercetin on systemic levels of inflammation: a meta-analysis of randomised controlled human trials

    Score: 7/10
    2020
    meta_analysis

    Ou Q, Zheng Z, Zhao Y +1 more

    No relevant overall effects on peripheral CRP, IL-6 and TNF-alpha were observed

    View source

    Effects of supplementation with quercetin on plasma C-reactive protein concentrations: a systematic review and meta-analysis of randomized controlled trials

    Score: 8/10
    2017
    meta_analysis

    Mohammadi-Sartang M, Mazloom Z, Sherafatmanesh S +2 more

    The meta-analysis of seven RCTs (10 treatment arms) showed a significant reduction of circulating CRP levels (WMD: -0.33 mg/l)

    View source

    Safety

    Caveats

    Antioxidant capacity in vitro has repeatedly failed to predict benefit - beta-carotene, vitamin E and vitamin C supplementation showed no benefit and in some cases harm in large trials, with beta-carotene increasing lung cancer in smokers. Reactive oxygen species also serve necessary signalling roles, and high-dose antioxidants around training have blunted exercise adaptations in some studies, which matters for athletes. Safety of quercetin: up to 1,000 mg/day for up to 12 weeks is well tolerated and is the practical ceiling; longer durations and higher doses are not adequately studied, and renal toxicity has been reported with high intravenous doses. Quercetin inhibits CYP3A4 and P-glycoprotein, raising levels of ciclosporin, some statins, calcium channel blockers, and other substrates; it may interact with warfarin and can reduce the efficacy of quinolone antibiotics. Use caution in kidney disease. Avoid in pregnancy and breastfeeding, since safety data are inadequate. A diet rich in vegetables, fruit, onions, apples, tea and berries provides quercetin alongside the food matrix, and has the evidence that isolated supplements lack.

    Less likely to help if

    Healthy well-nourished adults, in whom trials show little change.

    Medical Disclaimer

    The information provided on this website is for educational and informational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition or before starting any supplement regimen.

    Individual results may vary. The statements on this website have not been evaluated by the Food and Drug Administration. Products and information are not intended to diagnose, treat, cure, or prevent any disease.