Quercetin as a Coronavirus Immune Booster?

A plate with vegetables topped with capers. Capers are one of the richest natural sources of quercetin. They have a very high concentration of this flavonoid that has theoretical evidence for preventing and treating COVID-19 infection. They can be thought of as an immune booster against coronavirus.
What is quercetin and can it help in COVID-19? Let’s find out! Capers are a rich source of this flavonoid.

What is quercetin? How is it an “immune booster”?

Quercetin is a flavonoid, a type of chemical produced in certain plants. These chemicals protect plants from UV radiation and infection, and they pass benefit on to humans that eat them. They are yellow (from “flavus” in Latin) and some tout them as an immune booster. Before COVID-19, quercetin had been studied for antiviral and anti-inflammatory activity in:

Immune booster or modulator? Or both?

Quercetin can be though of as both an “immune booster” and an “immune modulator”. It can calm down overactive parts of our immune system while also protecting us from coronavirus:

Where can I find this immune booster, quercetin?

Quercetin occurs naturally across a variety of healthful plants (amounts are per 100mg serving, source):

  • Capers: 100-200 mg
  • Onions: 10-40 mg
  • Apples: (more in skin) 1-5 mg
  • Raw kale: 20mg (raw more than canned)
  • Broccoli, brussel sprouts: – 1-5 mg
  • Mexican Oregano (dried): 42 mg
  • Tarragon – 10 mg

It has also been isolated in supplement form (dosages from 500-1000mg).

Onions, and red onions in particular. Onions are a good source of quercetin. Quercetin may prove beneficial to preventing and reducing symptoms of COVID-19 infection. Onions may be a potential immune booster against COVID-19.
Red onions are a great source of quercetin, as well as other plant compounds!

What about quercetin and COVID-19 evidence, specifically?

While the theoretical evidence looks favorable, we don’t yet know if quercetin will work against COVID-19. It is reported that studies are underway in China, but nothing definitive has been shown yet.

Don’t be fooled by in vitro studies trying to sell you supplements. We don’t yet have any clinical data to prove efficacy of quercetin quite yet.

Quercetin remains exciting because of its immune boosting and modulating effects. Recall the overactive immune system wreaks havoc in COVID-19 with cytokine storm and ARDS. With its ability to modulate key inflammatory signalers in the body, quercetin may indeed help enhance our immune system.

How safe is quercetin?

As with any chemical we may consider supplementing with, we must consider safety:

  • From vegetables and fruits: consume away! The broad range of beneficial compounds from plants don’t pose safety risks, but the quercetin concentration is not particularly high.
    • Even though quercetin concentrations are modest in most foods, the absorption from whole foods appears much higher than supplements! When studied in 6 individuals, 47mg quercetin from onion soup appeared to have the same absorption as a 544mg tablet!
  • From supplements: with no formal COVID-19 studies to support a clear dosing, many will use 500-1000mg a day.
    • Side effects appear quite rare from quercetin supplementation.
    • Quercetin can interact with other medications. Another reason to always consult your doctor before starting any supplements.
  • Not all supplements accurately reflect their potency! Always refer to third party testing to verify the purity and potency of any supplements.
A bunch of kale. Kale is a good source of the flavonoid quercetin. Quercetin is a powerful flavonoid that has theoretical evidence for preventing and treating COVID-19 infection. It can hence be an immune booster!
Raw kale is another great source of quercetin that may help your immune system against COVID-19.

What’s the take home on quercetin, coronavirus, and our immune system?

Quercetin offers an exciting approach to COVID-19, but we don’t have direct evidence to support supplementation yet. However, quercetin is found in many healthful plant foods and is absorbed better from whole food than supplements! Like curcumin, quercetin represents a fantastic integrated approach to our health.

I advocate for integrating healthful nutrients that impact multiple aspects of our health, not just singular effects. We can do just that with quercetin:

  • Eating and emphasizing whole plant sources of quercetin can help reduce processed food intake.
  • Whole plants provide a wide variety of compounds that promote overall well being in ways that isolated compounds do not (consider highly concentrated caffeine drinks versus tea or coffee).
  • Incorporating onions, capers, kale, and broccoli into our cooking can encourage us to prepare more home-made meals. This is always a healthy benefit!
  • If we chose to supplement, there are few side effects from quercetin supplementation (though there are medication interactions). Always consult with your doctor before starting any supplements.
  • By choosing healthful, quercetin-rich foods we are empowering ourselves to take control of our health through our daily food choices.
  • Quercetin-rich foods are cheap.

I love conclusions like this: look at all the healthful effects from incorporating quercetin into our lifestyle. This is independent of any anti-viral effects!

Take home: quercetin is a plant compound and “immune booster” and “immune modulator”. It may inhibit viral-entry to help prevent or improve symptoms of COVID-19. While not yet formally studied, the safety profile, low cost, and healthy food sources of quercetin make it a safe consideration for a healthy lifestyle.


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The information provided in this post in intended for general education. It is not medical advice. While I make every effort to provide the most up-to-date information, please note that new data is continuously becoming available and may change the conclusions I present here.

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