Quercetin, Zinc, Vitamin C and Vitamin D3: Can They help Against Coronavirus? (2022)

The claim: Vitamin D, Vitamin C, Zinc and Quercetin can prevent or treat COVID-19?

Since the early stages of the pandemic, people have claimed supplements like quercetin, zinc, vitamin D and vitamin C can help treat and prevent COVID-19. But public health organizations and experts say there is little evidence these products are effective at treating or preventing COVID-19? 

Good, valuable and unbiased articles are hard to come by. Before you continue to read this rather long article, let's start with the end in mind and begin with the conclusion that you may have been told. Most of the studies on supplements are small and are of low quality? We shall wait for bigger and better quality evidence before we can make formal recommendations?

McCullough et al. Reviews in Cardiovascular Medicine, 2020

Science requires questioning and testing. The world does not exist in a 'black or white' manner and most of the time, things do fall into the 'gray' area. Medical science is dynamic and evidence development is constantly in a continuous work-in-progress mode. If a supplement has been shown to work in a small study, would you wait for a bigger study or should you just take it after considering the benefit and risk ratio; especially if the supplement is actually a nutrient that your body needs? Do your own research and the final decision should be yours, after a consultation with your trusted medical professional of course.

COVID-19 kills some people and spares others. How do you ensure that you are on the right side of the statistics? There are just too many self-proclaimed medical experts recommending all kinds of supplements for COVID-19 out there.  On one side are experts telling you that supplements don't work and you should avoid them and just rely on wholesome foods. On the other, are experts telling you to take all kinds of supplements that will help protect you against COVID-19. Do they actually work? Some supplements do have evidence and some don't. 

This guide is based on various references to scientific literature and hopefully, can help you make sense of the options and to separate the facts from fiction.

cytokine storm and COVID-19
Image credit: ClevelandClinic

As of December 2022, there are more than 100 published clinical studies that are related to vitamin D, vitamin C, zinc and quercetin:


The medical community themselves are battling over supplements on whether they should be used to treat and prevent COVID-19. On one side are experts telling you that supplements don't work and you should avoid them and just rely on wholesome foods. On the other, are experts telling you to take all kinds of supplements that will help protect you against COVID-19. Do they actually work? 

How do you deal with different expert groups dishing out conflicting guides? A common issue is that certain groups have pre-defined narrative that they would like to support. Therefore, only studies that support that pre-defined narrative are picked and cited as references. This is what we call as 'cherry-picking'. Cherry picking will naturally lead to a 'biased' decision. In order to avoid that, scientific information needs to be analyzed in a comprehensive, updated and non-biased manner in order to come up with the best 'evidence-based' decision.

The lung injury in COVID-19 patients is associated with ROS (reactive oxygen species) released by white cells in the blood, and thus the use of antioxidants is necessary for the management of COVID-19.

The largest observational study on self-reported dietary supplement use and SARS-CoV-2 infection found that, among 372,720 people in the U.K. who were tested for SARS-CoV-2, those who used probiotics, fish oil, multivitamins, or vitamin D had a 14%, 12%, 13%, or 9% lower risk of infection, respectively, compared to those who did not. Similar findings were observed among 45,757 people in the U.S. and 27,373 people in Sweden, although only in the U.K. were the reductions driven by benefits only among women. There was no association between zinc, vitamin C or garlic supplementation and COVID-19 risk (Louca, BMJ Nutrition, Prevention & Health, April 2021). 

Do take note that for optimal effectiveness, each supplement should not be considered on their own merit as most of the supplements are given as part of a combination protocol. Further, each nutrient will also have influence on another nutrient.  To illustrate this multi-approach thinking, vitamin K2 and magnesium are synergistic with vitamin D and act as co-nutrients that can improve your vitamin D level. On the other hand, vitamin C and zinc might cause copper deficiency, if given at the same time.

Note: This is a highly dynamic topic; therefore, we will be updating this article as new information emerges. 

Here are the handful of supplements under the most intense study for effectiveness against the coronavirus:

Nutraceutical Therapy by Mode of Action

Please check this page regularly for updates – new natural alternatives may be added and/or dose changes to existing alternatives may be made as further scientific studies emerge.

Vitamin D3 - Anti-inflammatory, Anti-coagulant and Immuno-modulator

At this point, there is simply no question that vitamin D optimization is a crucial component of COVID-19 prevention and treatment. In addition to the many studies published during 2020 and 2021, since December 2021, four large systematic meta-analyses (RRRR) have been published, looking at either vitamin D levels, supplementation or both.

In a peer-reviewed study published in Nature (Nov 2022), treatment with Vitamin D can actually reduce the risk of severe COVID-19 infection.

In all cases, the data consistently show that low vitamin D levels raises your risk of COVID while higher baseline levels and/or supplementation lowers all risks by 1.5 to three times.

Vitamin D, as an immuno-modulator, is a perfect candidate for countering the immune dysregulation common with COVID-19. Vitamin D deficiency affects the body’s susceptibility to infection and has been associated with influenza, hepatitis C, human immunodeficiency virus (HIV) and other viral diseases [Source]. Surveys indicate that most people in the United States consume less than recommended amounts of vitamin D. Sun exposure, which increases serum 25(OH)D levels, is one of the reasons serum 25(OH)D levels are usually higher than would be predicted on the basis of dietary vitamin D intakes alone. Vitamin D deficiency is also known to enhance a process known as the “cytokine storm” (Marik 2020).

According to a July 2022 paper:

“Each 10 ng/mL increase in vitamin D levels was associated with a 45 % and 26 % lower risk of 45-day mortality (HR: 0.55, 95 % CI: 0.40–0.74) and ICU mortality due to COVID-19 (HR: 0.74, 95 % CI: 0.60–0.92), respectively.”

Since the outbreak of the pandemic, many vitamin D sufficiency studies have been conducted. Almost all of them show that an adequate vitamin D level reduces the chance of (a) getting ill, (b) ending up in the hospital, and (c) dying.

Almost all studies consider a vitamin D blood serum level of >30 ng/mL ‘adequate/good’. Several studies have shown that people with a blood serum level of >50 ng/mL hardly get sick at all. Results of a systematic review and meta-analysis (Nutrients 2021) suggested that COVID-19 mortality risk correlates inversely with vitamin D3 status, and a mortality rate close to zero could theoretically be achieved at 50 ng/ml 25(OH)D3.

In a June 2022 review paper titled “The Role of Diet and Supplements in the Prevention and Progression of COVID-19: Current Knowledge and Open Issues”​ published in the journal Preventive Nutrition and Food Science, researchers said:

"The consumption of vitamin C and D supplements, in addition to a healthy diet, could be promoted as a co-adjuvant therapy for COVID-19..."

For more evidence, check out the evidence tracker on vitamin D and COVID-19 from c19vitamind.com (constantly updated), with more than 80 published treatment studies and more than 120 sufficiency studies by more than 900 scientists.


Trusted Source
Vitamin D has also been shown to have an anticoagulant effect. A decrease in 25-hydroxyvitamin D [25(OH)D] concentration has also been associated with an increased risk of venous thromboembolism (PubMed).

Safety: Daily intakes of up to 25–100 mcg (1,000 IU–4,000 IU) vitamin D in foods and dietary supplements are safe for children (depending on their age) and up to 100 mcg (4,000 IU) are safe for adults. These values, however, do not apply to individuals receiving vitamin D treatment under the care of a physician. Higher intakes (usually from supplements) can lead to nausea, vomiting, muscle weakness, confusion, pain, loss of appetite, dehydration, excessive urination and thirst, and kidney stones. In extreme cases, vitamin D toxicity causes renal failure, calcification of soft tissues throughout the body (including in coronary vessels and heart valves), cardiac arrhythmias, and even death.

Vitamin D and Omicron

Will Vitamin D Work Against Omicron BA4 and BA 5? Vitamin D is not variant specific because it's primary mode of action is to support the body’s immune system which reacts in a variety of ways against viral attack, not just in a specific antibody reaction to a specific spike protein. 

Quercetin - Anti-inflammatory, anti-coagulant, zinc ionophore and anti-viral

As of January 2023, evidence from 9 studies on the topic of quercetin and COVID-19 have been published (c19quercetin.com). 


Quercetin is also no. 1 in this prevention studies league table:


Quercetin is a pigment that is found in plants, vegetables, and fruits, and serves as an immune nutrient offering many health benefits. Elderberry, red onions, white onions and cranberries are the richest sources of quercetin. It is a flavonoid and antioxidant that may help to reduce inflammatory cytokines, infections, allergies and anti-blood clot property. Research has found that quercetin may be particularly beneficial for viral respiratory infections.

Quercetin as a Zinc Ionophore

Quercetin is a zinc ionophore (J Agric Food Chem. 2014). A 2015 study found that that Quercetin shows inhibitory activity in the early stages of a wide range of influenza viruses, including H1N1 and H5N1 (Viruses 2016). Although influenza is not in the same family of viruses as the coronavirus, it’s plausible that a similar mechanism could apply here. There is actually some evidence that Quercetin has already proven effective at treating Ebola and Zika viruses.

Quercetin and Vitamin C

Incidentally, ascorbic acid (vitamin C) and the bioflavonoid quercetin (originally labeled vitamin P) were both discovered by the same scientist — Nobel prize winner Albert Szent-Györgyi. Quercetin and vitamin C also act as an antiviral drug, effectively inactivating viruses. 

Quercetin Dosage

The FLCCC I-MASK+ protocol recommends 250 mg daily for prevention and 250 mg twice daily for early treatment.

Quercetin works best when taken with vitamin C and Bromelain, as vitamin C helps activate it and bromelain helps with the absorption.

Precaution: Quercetin should be used with caution in patients with hypothyroidism (low thyroid hormone) and relevant thyroid hormone levels should be monitored.
Quercetin and ivermectin interactions? According to Drugs.com: "No interactions were found between ivermectin and Quercetin. This does not necessarily mean no interactions exist. Always consult your healthcare provider."
Quercetin and COVID-19

For an updated list of studies, check out c19quercetin.com

A word about quercetin: Some physicians are recommending this supplement to reduce viral illnesses because quercetin acts as a zinc ionophore to improve zinc uptake into cells. It is much less potent than HCQ (hydroxychloroquine) as a zinc transporter, and it does not reach high concentrations in lung cells that HCQ does. Quercetin may help reduce risk of viral illness if you are basically healthy. But it is not potent enough to replace HCQ for treatment of COVID once you have symptoms, and it does not adequately get into lung tissue unless you take massive doses (3-5 grams a day), which cause significant GI (gastrointestinal) side effects such as diarrhea.


Zinc - Anti-viral 

Zinc is another powerful immune nutrient known for its benefits for providing immune health support and inflammation reduction as well as for improving cold and respiratory symptoms, wound healing, acne reduction, and lowering the risk of age-related diseases. This trace element is essential to to cell function and involved in over 100 enzymes. Research on atherosclerosis and diabetes mellitus suggests that zinc deficiency may contribute to low-grade systemic inflammation.

Aging is associated with compromised immunity, that just means that your immune response to pathogens and infections starts to slow and is less robust, including a reduced vaccine immune response/efficacy. ⁣

Improving zinc intake/zinc status improves/modulates/enhances immune function. The flip side is, while some aspects of immunity slow, others increase. Uncontrolled immune responses drive excess inflammation. Zinc helps to balance all of this. ⁣

The National Institutes of Health (NIH) states:

“Zinc is involved in numerous aspects of cellular metabolism. It is required for the catalytic activity of approximately 100 enzymes and it plays a role in immune function, protein synthesis, wound healing, DNA synthesis, and cell division. Zinc also supports normal growth and development during pregnancy, childhood, and adolescence and is required for proper sense of taste and smell.”

Zinc and COVID-19

In this Singapore peer reviewed study, published in the journal Clinical and Translational Medicine (Feb 2022), those who received zinc (80 mg daily) and vitamin C (500 mg daily) were found to subsequently mount a greater antibody response.

For more evidence, check out the evidence tracker on zinc and COVID-19 from c19zinc.com (constantly updated). 


Foods that are high in zinc include oysters, crab, lobster, mussels, red meat, and poultry. Cereals are often fortified with zinc. Most multivitamin and nutritional supplements contain zinc.

Zinc has been shown in a lab study to inhibit regular coronavirus (not the current SARS-CoV-2) since 2010, in a 2010 publication.

Safety: Taking zinc long term is typically safe for healthy adults, as long as the daily dose is under the set upper limit of 40 mg of elemental zinc (PubMed). Be aware that typical daily doses of zinc provided by zinc lozenges generally exceed tolerable upper limits for zinc, and for this reason, they should not be used for longer than about a week

Excessive doses may interfere with copper absorption, which could negatively affect your immune system as it can cause copper deficiencies, blood disorders and potentially permanent nerve damage. Zinc can also impair the absorption of antibiotics, and use of zinc nasal gels or swabs has been linked to temporary or permanent loss of smell.

Zinc Form and Dosage

There are several types of zinc supplements. Supplements contain several forms of zinc, including zinc gluconate, zinc citrate and zinc picolinate. The percentage of elemental zinc varies by form. To find out the percentage of elemental zinc in each form, check out elemental zinc percentage.
Chelated zinc is a general form of supplementary zinc in which the zinc is chelated — or bound — to a compound to make it easier for the body to absorb. Zinc picolinate or zinc gluconate are formed when zinc is chelated to picolinic acid or gluconic acid, so the main difference between zinc gluconate and picolinate is what compound it is bound to.

To find out which zinc supplement to consider, check out best zinc supplement.

Most people do not lack an intake of zinc, but in disease state, there might be an increase in demand by the body. The FLCCC I-MASK+ protocol recommends 30 mg a day for prevention and 100 mg a day for early treatment of COVID-19. This should not be taken long term without evaluation of your zinc/copper ratios.
The ideal dose for prevention while the COVID-19 risk is high is 40-100 mg/d, a portion of which comes from zinc lozenges to spread the zinc through the tissues of the nose, mouth and throat. It should be accompanied by at least 1 mg copper from food and supplements for every 15 mg zinc.

Do take note that you should keep the dosage back to within 40 mg/d once the exposure risk is back to normal.

Vitamin C - Anti-inflammatory

Vitamin C, which most of us reach for with any cold or flu, was used in high doses to great effect by COVID-19 early treatment doctors.

Vitamin C may be one of the most well-known immune nutrients that protect against immune deficiencies and which supports the prevention and recovery from the common cold and upper-respiratory issues, and also protects your cardiovascular system, eyes, skin, and other parts of your body. Research has found that vitamin C may help to optimize the immune system.

Do take note that the vitamin C dosages given in the hospitals intravenously are different from those over the counter vitamin C supplements. Therefore, when you come across studies on vitamin C, you need to differentiate those that are given intravenously vs oral vitamin C.

Vitamin C and COVID-19
 
Check out the evidence tracker on vitamin C and COVID-19 from c19vitaminc.com (constantly updated).

Safety: The U.S. Recommended Dietary Allowance (RDA) for vitamin C is 75 to 120 milligrams per day. Taking large doses of vitamin C (ascorbic acid) on a regular basis lowers your level of copper, so if you are already deficient in copper and take high doses of vitamin C, you can compromise your immune system.

While generally considered safe even in high doses, way too much vitamin C — anything above 2,000 milligrams daily—can cause headaches, insomnia, diarrhea, heartburn, and other issues.

Temporarily taking megadoses of vitamin C supplements to combat a case of the cold or flu is likely not going to cause a problem. 

Many vitamin C supplements that are above the US RDA are sold in the market. It’s important to seek a physician’s advice if you intend to take high dose vitamin C on a long term basis. To be on the safe side, you may also request for your kidney functions to be monitored.

For long-term, daily use, your best bet is to eat a diet that is full of high quality organic vegetables and fruits that are minimally processed. Not only will you get vitamin C, but you will get all the other accessory nutrients and micronutrients that are needed to optimize it. 

Vitamin C, Omicron and Deltacron

Will Vitamin C Work Against Omicron or Deltracron? Vitamin C is not variant specific because it's primary mode of action is to support the body’s immune system which reacts in a variety of ways against viral attack, not just in a specific antibody reaction to a specific spike protein. 


Conclusion and Key Takeaways

Do take note that the dosages for micronutrients or vitamins are higher for treatment as opposed to maintenance or preventive. This is probably due to higher demand of the body or the deficiency of the micronutrients are worse during a complicated viral infection. However, for prevention or maintenance, the dosages for most of the micronutrients should be much lower.

Keeping your immune system strong is the best way to protect yourself against all infections, and there are many effective ways to do that, including the following:
  • Optimize your vitamin D level — Higher levels of vitamin D have been shown to decrease your risk of developing a severe case of, and dying from, COVID-19. 
  • Immune-boosting nutraceuticals such as vitamin C, quercetin with zinc and N-acetylcysteine (NAC) — Zinc is a potent broad-spectrum antiviral and quercetin helps transport it into the cell, where it’s needed. Vitamin C is also a premiere treatment for many infections and helps boost overall immune function.
  • NAC, meanwhile, is a precursor to reduced glutathione, which appears to play a crucial role in COVID-19 specifically. Benefits of NAC include inhibiting expression of proinflammatory cytokines, improving T cell response and inhibiting the hypercoagulation that can result in stroke and/or blood clots that impair the ability to exchange oxygen in the lungs.
Aside from supplements, there are other ways that may help improve immune response and to prevent you from catching the coronavirus.
  • Abundant evidence suggests that eating whole in fruits, vegetables and whole grains—all rich in networks of naturally occurring antioxidants and their helper molecules—provides protection against free radicals.
  • Vaccination
  • Time-restricted eating (TRE) and intermittent fasting
  • Getting Enough Sleep
  • Avoid sugar, red meat and processed foods.
  • Don't smoke.
  • Take steps to avoid infection, such as washing your hands frequently, using hand sanitizer and cooking meats thoroughly.
  • Try to minimize stress.
  • Drink enough water to keep your body hydrated.
  • Avoid excess alcohol.
  • Avoid crowded areas.
  • Regular physical activity. Those with active lifestyle has lower risk if hospitalised as compared to those with sedentary lifestyle (Infectious Diseases and Therapy, 2021)
  • Consult your nearest local healthcare provider if you have any doubt.

More COVID-19 related topics > COVID-19


Be aware that most of the supplement dosages are above the recommended dietary allowance (RDA) and therefore such dosages should not be maintained on a long term basis. 

Disclaimer: The information on this website is not intended to replace a one-on-one relationship with a qualified health care professional and is not intended as medical advice. It is intended as a sharing of knowledge and information from the research and experience of third party sites. If you are pregnant, nursing, taking medication, or have a medical condition, consult your health care professional before using products or supplements based on this content.


Related: Natural Treatment for COVID Long-Haulers

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