New Studies Test Quercetin and COVID-19 Outcomes

Studies suggest this antiviral, anti-blood clotting, anti-inflammatory is an effective therapeutic

​​Quercetin is a potent antioxidant found in many plants. It’s safe, inexpensive, and does more than just get zinc into your cells to stop viral replication. KucherAV/Shutterstock

Since the beginning of the COVID-19 pandemic, good doctors have done what good doctors do and have given their patients safe and effective therapeutics to treat COVID-19.

While public health officials said nothing could be done, many doctors applied lessons learned from previous viral infections and helped their patients support their immune systems with safe measures that have a known history.

Common measures suggested by those doctors have included the use of vitamin D and antioxidants. As the pandemic wears on, mounting evidence points to the efficacy of many of these treatments, including quercetin.

Quercetin is a plant flavonol, a potent antioxidant found in many fruits, vegetables, seeds, and leaves. It’s safe, inexpensive, easy to obtain, and can help zinc get inside your cells, where it can work to stop viral replication.

Statistical Improvement in Clinical Outcomes

Two recent studies highlight the potential this safe plant-derived antioxidant has to treat COVID-19.

In the first study, published in the International Journal of General Medicine, 42 COVID-19 outpatients were divided into two groups. One group of 21 patients received standard medical therapy consisting of painkillers (acetaminophen) and an antibiotic for three consecutive days. The other 21 patients received standard therapy, plus the equivalent of 600 mg of quercetin per day (divided into three doses) for seven days, followed by another seven-day course of 400 mg of quercetin per day (divided into two doses).

The quercetin was used with sunflower lecithin, which has been demonstrated to increase absorption in the gut by as much as 20 times, compared to pure quercetin formulations.

The main outcomes being evaluated were virus clearance and symptoms. After one week of treatment, 16 of the 21 patients in the quercetin group tested negative for SARS-CoV-2 and 12 reported that all of their symptoms had diminished.

In the standard care group, only two patients tested negative and four reported partially improved symptoms. By the end of week two, the five remaining patients in the quercetin group tested negative. In the standard care group, 17 of the 19 remaining patients tested negative and one had died.

“These results are impressive, and hopefully additional studies will be conducted on hospitalized patients to see how quercetin might be helpful in more severe cases,” Dr. Michael Murray wrote in an Aug. 21 newsletter.

Can Quercetin Reduce Hospitalizations and Deaths?

The second study—a prospective, randomized, and controlled open-label trial—gave 152 COVID-19 outpatients a daily dose of 1,000 mg of quercetin for 30 days to evaluate its adjuvant effects in the treatment of early COVID-19 symptoms and the prevention of severe infection.

“The results revealed a reduction in frequency and length of hospitalization, in need of non-invasive oxygen therapy, in progression to intensive care units and in a number of deaths,” the study authors wrote.

“The results also confirmed the very high safety profile of quercetin and suggested possible anti-fatigue and pro-appetite properties.”

They described the branded form of quercetin being tested (Quercetin Phytosome) as “a safe agent and in combination with standard care, when used in early stage of viral infection,” saying that it could improve early symptoms and prevent severe COVID-19.

Mechanisms of Action

The authors of the first study wrote that they chose quercetin because it has antiviral, anti-blood clotting, anti-inflammatory, and antioxidant properties, all of which are important in the treatment of SARS-CoV-2 infection. In the second study, more detailed mechanisms of action are reviewed.

The authors looked at specific enzymes with the help of molecular docking studies, gaining insight into the intricate biochemistry at play.

“Quercetin is characterized by three crucial properties: antioxidant, anti-inflammatory and immunomodulatory. The combination of these actions allows quercetin to be a potential candidate to support all unhealthy conditions where oxidative stress, inflammation and immunity are involved,” they wrote.

Initially, quercetin gained attention because it’s a zinc ionophore, meaning it shuttles zinc—which has well-known antiviral effects—into your cells just like the drug hydroxychloroquine.

Some proposed that the primary reason hydroxychloroquine and quercetin worked was because of this feature. Of course, you also had to take zinc along with either of them. To effectively act as a zinc ionophore, quercetin also needs vitamin C.

Since then, other studies, including the two reviewed in this article, have shown that quercetin has other actions that make it useful against SARS-CoV-2 as well. As reported by Murray in his newsletter:

“In particular, quercetin exerts significant inhibition on the binding of specific spike proteins to ACE-2 receptors, thereby blocking the ability of the virus to infect human cells. Quercetin has also been shown to directly neutralize viral proteins that are critical in the replication of SARS-CoV-2.”

In some studies, quercetin has also been shown to inhibit the release of inflammatory cytokines, signaling molecules that summon immune cells to fight off invaders. Unfortunately, in COVID-19, too many cytokines send up a signal and the immune response turns deadly. Dampening the cytokine signal could help alleviate infection-related symptoms and suppress excessive inflammatory responses from occurring. Quercetin’s antioxidant effects may also help prevent tissue damage caused by scavenging free radicals, thereby aiding in the recovery process of viral infections.

Quercetin’s Antiviral Properties

Quercetin’s antiviral properties have been attributed to three main mechanisms of action: inhibiting the virus’ ability to infect cells; inhibiting replication of already infected cells; and reducing infected cells’ resistance to treatment with antiviral medication.

Research on mice funded by the U.S. Defense Advanced Research Projects Agency and published in 2008 found that quercetin lowers your risk of viral illnesses such as influenza and boosts mental performance following extreme physical stress, which might otherwise undermine your immune function and render you more susceptible to infections.

Another study looked at cyclists who received a daily dose of 1,000 mg of quercetin in combination with vitamin C (which enhances plasma quercetin levels) and the B vitamin niacin (to improve absorption) for five weeks. Those in the treatment group were significantly less likely to contract a viral illness after bicycling three hours per day for three consecutive days, compared to untreated controls. While 45 percent of the placebo group eventually got sick, that number was at just 5 percent of the treatment group.

Quercetin Works Against Many Common Viruses

Before the COVID-19 pandemic struck, several studies had highlighted quercetin’s ability to prevent and treat the common cold (which is often caused by a coronavirus) and seasonal influenza. By attenuating oxidative damage, quercetin also lowers your risk of secondary bacterial infections, which is actually the primary cause of influenza-related deaths.

Importantly, quercetin increases the growth in size and number of mitochondria (the powerhouse of a cell) in skeletal muscle, which suggests part of its antiviral effects are due to enhanced mitochondrial antiviral signaling. Quercetin has also been found to work against other viruses as well.

A 1985 study found that quercetin inhibits the infectivity and replication of herpes simplex virus type 1, polio-virus type 1, parainfluenza virus type 3, and respiratory syncytial virus (RSV).

A 2016 animal study found that quercetin inhibited mouse dengue virus and the hepatitis virus.

Other studies have confirmed quercetin’s power to inhibit both hepatitis B and hepatitis C infections.

A March 2020 study found that quercetin provides “comprehensive protection” against Streptococcus pneumoniae infection. As reported by the authors of this study: “Our results indicated that quercetin may be a novel potential drug candidate for the treatment of clinical pneumococcal infections.”

How Quercetin Combats Inflammation and Boosts Immunity

Aside from its antiviral activity, quercetin is also known for boosting immunity and combating inflammation. A 2016 study in the journal Nutrients describes the mechanisms of action, which included inhibiting a cytokine involved in systemic inflammation, dampening the production of inflammation-producing enzymes, and curbing other inflammatory agents.

According to the study, quercetin also stabilizes mast cells, has cytoprotective activity in the gastrointestinal tract, and has “a direct regulatory effect on basic functional properties of immune cells,” which allows it to inhibit “many inflammatory pathways and functions.”

Bioavailability

While quercetin does have potent antiviral effects, in order for it to work effectively, you need sufficiently high dosages to raise the level of quercetin in your body’s tissues. The relatively low absorption rate of quercetin is why a sunflower lecithin formulation was used.

Research published in the July–December issue of the Journal of Natural Health Products Research, found that a quercetin matrix has the same total absorption rate as quercetin phytosome—and higher peak blood levels.

“Since both of these forms of quercetin produce similar blood levels, they should produce the same effects at equal dosages based upon quercetin content,” Murray wrote in his newsletter.

“My dosage recommendation as part of a nutritional supplement program to support immune function is 250 mg twice daily.

“And in patients with active infection, my recommendation is ... six capsules twice a day providing a total of 3,000 mg of quercetin. This high dosage should be taken for at least 10 days and then reduced to a maintenance dosage of 250 mg twice daily.

“[This] high dosage may not be necessary. But my dosage calculations are based upon likely tissue concentrations needed to exert the strongest antiviral effects. And given the safety of quercetin, there is no harm at this level.”

Protocol Using Quercetin

One doctor who brought quercetin into the limelight early was Dr. Vladimir Zelenko. As hydroxychloroquine became difficult to obtain, Zelenko switched to recommending quercetin instead, as it’s readily available as an over-the-counter supplement. For a downloadable “cheat sheet” of Zelenko’s protocol for COVID-19, visit VladimirZelenkoMD.com.