Outcome
    Preliminary

    Quercetin for Immune Function

    Evidence for quercetin and immune function is preliminary. A trial in cyclists found reduced post-exercise illness, but broader immune benefit and the widely promoted antiviral claims are not established in humans.

    Overview

    Evidence for quercetin and immune function is preliminary. A trial in cyclists found reduced post-exercise illness, but broader immune benefit and the widely promoted antiviral claims are not established in humans.

    Verdict

    Insufficient evidence

    How It Works

    Quercetin stabilises mast cells and inhibits histamine release, which underpins its use for allergic symptoms rather than for infection defence. It also suppresses NF-kB-driven cytokine production, an anti-inflammatory action that is not the same thing as immune enhancement. In-vitro work has shown that quercetin acts as a zinc ionophore, increasing intracellular zinc, and inhibits replication of several viruses in cell culture. These findings drive much of the marketing but have not been reproduced at achievable plasma concentrations in humans, and should not be presented as clinical antiviral activity.

    Dosing & Protocol

    Typical dose

    Recommended dose
    Studied at 500-1,000 mg/day; general immune benefit has not been demonstrated in humans
    Expected timeframe
    Not established

    Protocol

    form
    Bioavailability-enhanced quercetin, since plain aglycone absorption is under 10%
    duration
    Trials ran 8-12 weeks
    co factor
    Evidence is insufficient. Quercetin has plausible immune-relevant actions in vitro - inhibition of NF-kB signalling, reduced pro-inflammatory cytokine production, mast cell stabilisation, and zinc ionophore activity that attracted attention during the pandemic. Human trials are limited and mostly negative or equivocal for infection outcomes: a large trial in ultramarathon runners found quercetin reduced upper respiratory infection incidence in one analysis, but subsequent work in broader populations did not replicate this convincingly, and trials in COVID-19 were small, poorly controlled and have not supported the claims made for them. Immune support is not a measurable endpoint in any case.
    titration
    Not applicable - no dose has shown a general immune benefit
    starting dose
    Not established for this outcome. Trials used 500-1,000 mg/day in divided doses with food

    Evidence

    What the studies say

    The most-cited human trial randomised trained cyclists to 1000mg of quercetin daily for three weeks around a period of heavy exercise and found a lower incidence of self-reported upper respiratory illness compared with placebo, without measurable change in immune function markers. Other trials in community adults have not found reduced infection rates. A meta-analysis of quercetin and upper respiratory infection concluded that evidence is limited and inconsistent. Trials of quercetin for COVID-19 have been small and methodologically weak, and no regulator has approved it for that use. Quercetin remains reasonable for allergic symptoms, where mast cell stabilisation is the better-supported mechanism.

    Quercetin supplementation and upper respiratory tract infection: A randomized community clinical trial

    Score: 9/10
    2010
    rct
    n=1002

    Heinz SA, Henson DA, Austin MD +2 more

    for all subjects combined, quercetin supplementation over 12 weeks had no significant influence on URTI rates or symptomatology compared to placebo

    View source

    Safety

    Caveats

    The largest trial (n=1002, 12 weeks) found no reduction in upper respiratory infection rates or severity overall; benefit appeared only in a post hoc subgroup of fitter adults aged 40+ taking 1000 mg/day.

    Less likely to help if

    Everyone - no general immune benefit has been demonstrated at any dose.

    Medical Disclaimer

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    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.