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Quercetin: A Safe, Natural Antiviral?

Nicholas Noble | March 12, 2020

Natural Antivirals for COVID-19

More widespread by the day, novel Coronavirus (COVID-19, 2019-nCov) is driving unprecedented infrastructural challenges. People are also fearful, as now the entire world takes measures to reduce infectious spread.

Now considered a pandemic by the WHO, COVID-19 is expected to reach a maximum rate of infection and begin tapering-down in a couple months, though little is certain.

Dr. Andrew Weil reviewed reasonable precautions you can take, including natural compounds which may improve your viral resilience. We also covered more surprising or effective immune boosting options in a recent journal article on the topic.

It remains clear that people want all the information they can get to help protect themselves. Rather than repeat standard recommendations (see the CDC), here we review ‘quercetin’.

Unfamiliar to most, this natural antiviral is likely hiding in your pantry right now!

The Antiviral Quercetin

Even the most common food item, like an onion, contains a variety of natural compounds. Polyphenols and other flavonoids (like quercetin) are popular examples.

These are often the pigments or bitter flavors in food and drink (e.g. tannins in red wine). The word you’ll hear most often is ‘antioxidant’, yet quercetin is an example of a polyphenol that may double against viruses (even allergies). [1]

Quercetin Protects against Coronavirus?

Both COVID-19 and SARS are viral infections of the coronavirus family. Following the SARS outbreak of 2002, researchers closely evaluated natural compounds such as quercetin and EGCG (both found in matcha), which may counteract the virus.

Unlike EGCG which is found near-exclusively in matcha green tea, quercetin is readily abundant. Foods like onions, capers, and green apples are examples that can help you increase your intake. Full list below.

Quercetin has many dietary sources, making it especially worthwhile to explore. It’s a practical, safe, and easily accessed option (likely in your kitchen right now) that may impart antiviral activity against the COVID-19 virus. [2]

Antivirals for Coronavirus (COVID-19)

Antivirals need to directly (and safely) target the infectious mechanisms. FDA approved drug options are limited due to concerns of side-effects, which may exacerbate cellular damage.

Many patients who’ve already recovered from COVID-19 have been given a mix of antiviral drugs, which may carry side-effects. [3]

How does COVID-19 Infect?

The virality of COVID-19 relies on similar mechanisms as SARS. These coronaviruses are genetic relatives, each encoding for protein enzymes (i.e. ‘proteases’, e.g. ‘3CLpro’) which enable viral replication.

Upon entering the cell, COVID-19 relies on the release of these proteases to continue the cycle of infection. They are essential for replication, and they may also bind (i.e. attach) to your cells’ own antiviral proteins that would normally fight infection. [4]

Is Quercetin an Antiviral?

These proteases are essential for the virus to function; any way that we can safely interfere with them is of primary research for COVID-19, just like it was with SARS. [5]

So where does quercetin come in? The latest research indicates that quercetin may not stop the immunosuppressive action of those protease enzymes. But instead, it might give our body more time to respond. [6]

Quercetin has an affinity to coronaviruses’ enzyme receptors, similar to the EGCG in matcha which is studied against influenza. Simply speaking, the enzymes these viruses release are also part of the signalling to replicate (and continue infecting).

When quercetin contacts the virus, it may stop this from happening. Possibly slowing infection, and giving your body more time to build immunity. [7]

Is there a Drug for Coronavirus?

Any safe drug or natural compound that can reduce COVID-19’s process of replication should be reasonably considered. Some FDA approved antiviral drugs have already showing signs of promise, but often carry side-effect risks. [8]

The way quercetin is thought to work is similar to many FDA approved antivirals. And although COVID-19 and SARS are similar, their structures are slightly different. Trial and error is necessary to test previous treatments.

During this tedious phase of research, rather than panic, we should genuinely consider safe (potential) antiviral compounds like quercetin. They may be the added layer of protection our immunity needs.

Is Quercetin Safe?

Quercetin has a high margin of safety. It’s widely available and may ultimately deter infection. You can choose foods with high natural levels, or make use of a dietary supplement. [9]

DIETARY SOURCES OF QUERCETIN

  • Grapes
  • Raspberries
  • Broccoli
  • Onions
  • Tomatoes
  • Red Leaf Lettuce
  • Cooked Asparagus
  • Matcha Green Tea

The Bottom Line

Although more research is underway, there is good evidence to suggest quercetin as a safe, supplementary measure against viral infection.

It’s been rigorously evaluated against the SARS strain of the coronavirus, and may yield comparable results to COVID-19. Due to the high margin of safety, there’s little risk to adding it to your antiviral regimen.

Eat a diet high in foods containing quercetin, or look for it in capsule form. Supplements commonly range from 250-500mg daily. Practice moderation, and if you have pre-existing kidney disease, there’s some evidence that amounts over 1000mg/daily may add to that condition.

Finally, it’s studied that quercetin synergizes with other flavonoids like EGCG (found in matcha green tea), or resveratrol (found in medicinal mushrooms, red wine). So you may also consider adding these to your immune-boosting repertoire.

References
[1,6,7] Chiow, K. H., Phoon, M. C., Putti, T., Tan, B. K., & Chow, V. T. (2016). Evaluation of antiviral activities of Houttuynia cordata Thunb. extract, quercetin, quercetrin and cinanserin on murine coronavirus and dengue virus infection. Asian Pacific journal of tropical medicine, 9(1), 1-7.
[2] Park, H. R., Yoon, H., Kim, M. K., Lee, S. D., & Chong, Y. (2012). Synthesis and antiviral evaluation of 7-O-arylmethylquercetin derivatives against SARS-associated coronavirus (SCV) and hepatitis C virus (HCV). Archives of pharmacal research, 35(1), 77-85.
[3,8] Touret, F., & de Lamballerie, X. (2020). Of chloroquine and COVID-19. Antiviral Research, 104762.
[4] Arya, R., Das, A., Prashar, V., & Kumar, M. Potential inhibitors against papain-like protease of novel coronavirus (COVID-19) from FDA approved drugs.
[5] Nguyen, T. T. H., Woo, H. J., Kang, H. K., Kim, Y. M., Kim, D. W., Ahn, S. A., ... & Kim, D. (2012). Flavonoid-mediated inhibition of SARS coronavirus 3C-like protease expressed in Pichia pastoris. Biotechnology letters, 34(5), 831-838.
[9] Lakhanpal, P., & Rai, D. K. (2007). Quercetin: a versatile flavonoid. Internet Journal of Medical Update, 2(2), 22-37.