Quercetin [a bioflavonoid] has been investigated for its possible antiviral effect on several members of the Coronaviridae family. Quercetin glucosides, such as isoquercetin, occur naturally and have the same therapeutic effects in vivo as quercetin, but with better bioavailability. [absorption]
Of the multiple flavonoids shown in vitro to block the activity of MERS-CoV 3CLpro, [critical enzyme for coronavirus replication], quercetin showed the most promise. A cell has a lock, and the virus has a key [to enter and infect the cell], but quercetin puts glue in the lock. There is also significant longstanding interest in the anti-inflammatory activity of quercetin, and inhibition of platelet aggregation [blood clots].
Quercetin displays a broad range of antiviral properties that can interfere at multiple steps of pathogen virulence -virus entry, virus replication, protein assembly- and that these therapeutic effects can be augmented by the co-administration of vitamin C. Furthermore, due to their lack of severe side effects and low-costs, combined administration of vitamins C and D, may exert a synergistic antiviral action more potent for the prophylaxis and the early treatment of respiratory tract infections, especially including COVID-19 patients.
Quercetin stimulates T-helper cells to produce (Th-1)-derived Interferon, and downregulates Th2-derived IL-4 when added to cultured blood cells. [modulates immune system]
Optimal intake remains unknown., but oral supplementation with quercetin up to 1 g/day for 3 months has not resulted in significant adverse effects. Note: it is better with long-term versus short use. If you can find isoquercetin, this is the preferred form, but currently finding “any form” of quercetin is difficult. Remember that it is a bioflavonoid, so it will be included in those formulations.
Quercetin has shown to have:
Antiviral properties that can interfere at multiple steps of pathogen virulence -virus entry, virus replication.
An inhibiting action on lipoxygenase, an enzyme that contributes to problems with asthma.
Protects LDL cholesterol from damage.
Helps protect against cataracts by blocking sorbitol accumulation in the eye.
A popular treatment for hay fever and asthma.
Improvement of blood sugar control and prevention of complications of diabetes in animal research.
Improvement of glucose metabolism and reduced complications in animal models of type 2 diabetes.
Quercetin is found in onions, apples, green tea, or black tea. broccoli, buckwheat, capers, peppers, Brassica vegetables, apples, grapes, berries, wine, as well as many nuts, seeds.
A role for quercetin in coronavirus disease 2019 https://doi.org/10.1002/ptr.6887
Front. Immunol., 19 June 2020 | https://doi.org/10.3389/fimmu.2020.01451