Antiviral, Antioxidant Mechanisms

Quercetin has long been evaluated for its potential protective effects against cancers, heart disease, and cells that release histamines.

The agent promotes SIRT2, which then inhibits the NLRP3 inflammasome assembly involved with COVID-19 infection, said Samuel F. Yanuck, DC, of the Program on Integrative Medicine at the University of North Carolina Chapel Hill School of Medicine, who co-authored a review of emerging research on the subject. It also plays a role in facilitating zinc transportation across lipid membranes.

A plant flavonoid found in capers and green tea is being eyed by some as a potential adjunct therapy for patients with COVID-19, but whether quercetin will stand the test of rigorous trials remains unclear.

Proponents of the supplement say it could be one part of a treatment regiment along with interventions and that its over-the-counter availability and relatively good safety profile serve as advantages.

"It's not a bizarre or experimental substance and given it has these potential important biological roles, I think it's worth being considered as part of an overall strategy," Yanuck told MedPage Today, adding that quercetin would need to be one part of a multifactorial treatment regimen.

In cell cultures, quercetin has been shown to prevent viral entry and reduce the cytopathic effects of many viruses, including rhinovirus and poliovirus. In a 2016 animal study, rodents administered quercetin before being exposed to a lethal load of Ebola virus survived.

COVID-19 has been associated with high levels of interleukin-6, depleted levels of interferons, and a cytokine storm that damages the body and is related to respiratory failure, said Ruben Colunga Biancatelli, MD, of Old Dominion University in Norfolk, Virginia, and first author of a paper on quercetin and vitamin C as a potential therapy for treating SARS-CoV-2 in Frontiers in Immunology.

Given the antiviral activity that has been demonstrated in preclinical data, it would be reasonable to prescribe quercetin in the context of a properly designed clinical trial for treating COVID-19, commented David M. Aronoff, MD, of Vanderbilt University in Nashville, Tennessee.