The quercetin compound occurs widely in nature, found in berries, fruits, grains, and vegetables. Onions and kale are particularly rich. Quercetin is a “polyphenol” and belongs to the “flavonoid” family of compounds. “Polyphenol” is a description of molecular structure and refers to the multiplicity OH groups around the periphery of the molecule. It is these “functional groups” that allow polyphenols to neutralize potentially damaging free radicals through the donation of a hydrogen atom. This is what is referred to as “antioxidant” activity and is the reason that polyphenols have been promoted as useful dietary supplements.
The current interest in quercetin, however, has nothing to do with its antioxidant activity. Researchers have been applying computer modeling techniques to identify molecules with structures that could conceivably block the “ACE2 receptors” on cells to which the coronavirus that causes COVID-19 attaches, much like a key fits into a lock. Once the virus enters a cell it hijacks the cellular machinery and uses it to reproduce and crank out more viruses. Based on its molecular structure, quercetin could be a “key” that fits the lock and blocks other “keys.”Â
There is actually some laboratory evidence that this can happen. After the SARS epidemic, a couple of studies did show that quercetin could block entry of that particular virus into cells. However, that was an “in vitro” experiment and involved a virus that is different from the one that causes COVID-19, albeit it is in the coronavirus family. Without further studies, we can’t assume that quercetin will also block SARS-CoV-2.
A clinical trial is needed, and one is being organized. At this point, all one can say is that quercetin has the possibility of being useful. Since it occurs in nature, it is regulated as a “Natural Health Product” and dietary supplements have long been available, hyped for their antioxidant potential, usually in daily doses of 500-1000 mgs. The daily intake from our diet is, in general, less than 100 mgs, but quercetin supplements have not been linked with any adverse reaction. Although the information about possible efficacy is sketchy, at least taking quercetin supplements is not harmful.
