Disclaimer: I am not a medical professional – I am just some dude with a blog. You are responsible for your own medical care, treatment, and oversight. The contents of this document are for informational and educational purposes only and DO NOT CONSTITUTE THE PROVIDING OF PROFESSIONAL MEDICAL ADVICE.  This document is not intended as a substitute for independent professional medical judgment, advice, diagnosis, or treatment. The content is not intended to establish a standard of care for you to follow. You understand and acknowledge that you should always seek the advice of your physician or other qualified health provider with any questions or concerns you may have regarding your health.

In this article, I discuss the scientific evidence that has led me to take a daily Vitamin D supplement as a possible protective measure against the worst effects of COVID-19.

This article presents evidence that vitamin D helps protect against respiratory infections, including viruses such influenza and the common cold (caused by rhinoviruses and coronaviruses.) I also look into the specific mechanisms by which vitamin D offers these protections. Then I discuss the prevalence of vitamin D deficiency in adults, how to get vitamin D and whether there are any dangers from vitamin D. Finally, I list the conclusions I’ve drawn from the evidence and explain why I, personally, am taking a vitamin D supplement.

A complete list of reference sources appears at the end of this post, so you can examine the evidence for yourself and draw your own conclusions.

The Case for Vitamin D Reducing Respiratory Infections 

A 2017 study conducted by the Queen Mary University of London looked at 25 randomized, double blind, placebo controlled trials of supplementation with Vitamin D. The authors concluded the following (bold highlighting by me):

Vitamin D supplementation reduced the risk of acute respiratory tract infection among all participants. Vitamin D supplementation was safe and it protected against acute respiratory tract infection overall.

This meta-analysis looked at the effects of Vitamin D on over 10,000 test subjects. The researchers found that people with very low vitamin D levels experienced the most benefit, but even people with higher levels benefited from supplements. Here is another statement from the same paper:

Among those receiving daily or weekly vitamin D, protective effects were strongest in those with profound vitamin D deficiency at baseline, although those with higher baseline 25-hydroxyvitamin D concentrations also experienced benefit.

According to an article in the Harvard Gazette about this study:

The investigators found that daily or weekly supplementation had the greatest benefit for individuals with the most significant vitamin D deficiency (blood levels below 10 mg/dl) — cutting their risk of respiratory infection in half — and that all participants experienced some beneficial effects from regular vitamin D supplementation.

A similar study published in 2012 by the US National Laboratory of Medicine performed a meta-analysis of five randomized, placebo controlled clinical trials exploring the effect of vitamin D supplementation on prevention of respiratory tract infections like influenza, pneumonia, common cold, etc. Those researchers wrote the following:

Events of respiratory tract infections were significantly lower in vitamin D group as compared to control group. On the basis of this study, we can conclude that vitamin D is useful in prevention of respiratory tract infections.

Similarly, a smaller 2010 study of Japanese schoolchildren (randomized, double-blind, placebo-controlled) found that Influenza A occurred in 10.8% children in the vitamin D3 group compared with 18.6% children in the placebo group. The children who took vitamin D had a 40% lower incidence of flu.

Another meta-analysis conducted in 2013 looked at 5,660 subjects. Those researchers concluded the following:

Results indicate that vitamin D has a protective effect against RTI [respiratory tract infections], and dosing once-daily seems most effective

Why Might Vitamin D Protect Against Respiratory Infections?

There are several proposed explanations for why Vitamin D might protect against respiratory infections. Most refer to vitamin D’s effects on, and use by, the human immune system. In a 2006 journal article published by the Cambridge University Press entitled “Epidemic Influenza and Vitamin D,” the authors wrote the following:

D acts as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the ‘oxidative burst’ potential of macrophages. Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection.

More details from the article:

Perhaps most importantly, three independent research groups have recently shown that [vitamin D] dramatically stimulates genetic expression of antimicrobial peptides (AMP) in human monocytes, neutrophils, and other human cell lines. 

These endogenous antibiotics, such as defensins and cathelicidins, directly destroy invading microorganisms. AMP display broad spectrum antimicrobial activity, including antiviral activity, and have been shown to inactivate the influenza virus. Not only do neutrophils, macrophages, and natural killer cells secrete AMP, but epithelial cells lining the upper and lower respiratory tract secrete them as well, where they play a major role in pulmonary defense.

According to the article, Vitamin D is performing at least two roles that relate to the body’s susceptibility to influenza: boosting anti-microbial and antiviral peptides (amino acids) while also preventing excessive expression of cytokines. From the same article:

Vitamin D has been found to modulate macrophages’ response, preventing them from releasing too many inflammatory cytokines and chemokines.

This latter point is important in relation to the current COVID-19 pandemic, because many of the patients who have “crashed” suddenly and died from the disease appear to have experienced so-called “cytokine storms” in which their immune system overreacts and attacks the patient’s own healthy tissue. Per this article from the New York Times:

There are many variations on the phenomenon, and they go by many names: systemic inflammatory response syndrome, cytokine release syndrome, macrophage activation syndrome, hemophagocytic lymphohistiocytosis. Broadly speaking, they are all marked by an unbridled surge in immune molecules, and may all result in the fatal shutdown of multiple organs.

And another explanation from this article at Vox:

Critically important studies emerging from China suggest that for many patients who die of Covid-19, it may be their own immune system, rather than the virus itself, that deals the fatal blow. This is called a cytokine storm. During a cytokine storm, an excessive immune response ravages healthy lung tissue, leading to acute respiratory distress and multi-organ failure. Untreated, cytokine storm syndrome is usually fatal.

A paper from The Scripps Research Institute written in 2014 also found that cytokine storms occur in and kill patients with other respiratory infections such as influenza.

During pathogenic influenza virus infection, robust cytokine production (cytokine storm), excessive inflammatory infiltrates, and virus-induced tissue destruction all contribute to morbidity and mortality.

Do We Get Enough Vitamin D? How Do We Get More?

Do we get enough vitamin D? The evidence shows that many of us do not.

Humans primarily get their vitamin D through the interaction of sunlight (specifically ultraviolet light) on skin. Adequate amounts of vitamin D can be produced with moderate sun exposure to the face, arms and legs, averaging 5–30 minutes twice per week, or approximately 25% of the time for minimal sunburn. The darker the skin, and the weaker the sunlight, the more minutes of exposure are needed.

A study published in Nutrition Research in 2011 looked at 4,400 participants and found that large numbers of people have a vitamin D deficiency:

The overall prevalence rate of vitamin D deficiency was 41.6%, with the highest rate seen in blacks (82.1%), followed by Hispanics (69.2%). Vitamin D deficiency was significantly more common among those who had no college education, were obese, with a poor health status, hypertension, low high-density lipoprotein cholesterol level, or not consuming milk daily.

So, based on this study, at least 40 percent of Americans are deficient in vitamin D. Greater deficiency is seen, unsurprisingly, in people with darker skin, which produces less vitamin D (because melanin absorbs UVB photons before the body can use them to synthesize vitamin D.)

People who spend a lot of time indoors or who cover up their skin when they go outside are more prone to vitamin D deficiency. So are people who live at higher latitudes where sunlight is weaker and hours of sunlight are fewer. This, by the way, is one of several possible explanations for why respiratory infections increase in the winter.

How Much Vitamin D Do We Need?

According to the National Institutes of Health, the recommended daily allowance (RDA) of vitamin D for adequate health is 600 IU (“International Units”) for people under 70. For persons over 70, the RDA is 800 IU

Food Sources of Vitamin D: Milk, Fatty Fish

In the 1900s, rickets, a childhood bone disorder caused by not getting enough vitamin D, impacted a good number of children. Around 80 percent of children in Boston had rickets at that point in history. In 1922, Dr. E. V. McCollum (who discovered or co-discovered vitamins A, B, and D) discovered that vitamin D could prevent rickets, because it was needed to help the calcium to get absorbed. There was therefore a scramble to get vitamin D into the diets of American children.

Because vitamin D is not prevalent in many foods, fortifying milk with additional vitamin D was a natural solution, since it also contained key minerals for bone development. So, to this day, the milk you buy in America is fortified with added vitamin D. Unfortunately, the USDA says that an 8 ounce serving (one cup) of 2% milk contains 110 IU of vitamin D, so an adult would need to drink five to six cups of milk a day to consume the RDA of 600 IU of vitamin D.

Fatty fishes, such as wild salmon, cod, bluefish, and others provide vitamin D, but a study published in 2009 found that only wild caught salmon provided an amount near the RDA in a 3.5-ounce serving. All other fatty fishes, from farmed salmon to cod, trout, and tuna provided only 20%-50% of the RDA per 3.5-ounce serving.

Fat extracted from fish contains high amounts of vitamin D. Cod liver oil, for example, contains 1,360 IUs per tablespoon, or about twice the recommended daily allowance.

Conclusions About Sources of Vitamin D

Based on the articles and studies I’ve examined (you can read them all for yourself via the links below,) there are not many good, reliable dietary sources of vitamin D. It seems from the literature that your options for getting the RDA of vitamin D are the following:

  • Expose your whole body (face, arms, and legs) to direct sunlight for 10-30 minutes twice a week, without protective clothing or sunscreen. If you live in a place with cold weather, indirect sunlight, or if you spend all day indoors due to work/school/infirmity, this may not be practical.
  • Drink 40-48 ounces of fortified milk every day. (That’s about one-third gallon of milk.)
  • Eat a 3.5-ounce serving of wild caught salmon every day.
  • Consume a tablespoon of cod liver oil every day.
  • Take a vitamin D supplement every day. (This is what I do.)

How Much is Too Much?
What Are the Dangers of Vitamin D?

First, you should know that the risks of receiving toxic levels of vitamin D from sunshine or food are extremely low. According to the National Institutes of Health:

Excessive sun exposure does not result in vitamin D toxicity because the sustained heat on the skin is thought to photodegrade previtamin D3 and vitamin D3 as it is formed. In addition, thermal activation of previtamin D3 in the skin gives rise to various non-vitamin D forms that limit formation of vitamin D3 itself. Some vitamin D3 is also converted to nonactive forms. Intakes of vitamin D from food that are high enough to cause toxicity are very unlikely. Toxicity is much more likely to occur from high intakes of dietary supplements containing vitamin D.

How much vitamin D is too much? From the same NIH article:

Most reports suggest a toxicity threshold for vitamin D of 10,000 to 40,000 IU/day.

There is also evidence that daily intake in the 5,000 IU range can cause adverse health effects over time. Again, from the same NIH article:

The FNB concluded that [blood] serum 25(OH)D levels above approximately 125–150 nmol/L (50–60 ng/mL) should be avoided, as even lower serum levels (approximately 75–120 nmol/L or 30–48 ng/mL) are associated with increases in all-cause mortality, greater risk of cancer at some sites like the pancreas, greater risk of cardiovascular events, and more falls and fractures among the elderly. The FNB committee cited research which found that vitamin D intakes of 5,000 IU/day achieved serum 25(OH)D concentrations between 100–150 nmol/L (40–60 ng/mL), but no greater.

Based on all this, the National Institutes of Health says that 4,000 IU is the upper safe limit for daily vitamin D consumption by an adult.

As we have already seen, the RDA for vitamin D is 600-800 IU per day, and even that level of intake is difficult for many people to achieve without a supplement. Exceeding the recommended upper safe limit of 4,000 IU would be extremely rare except in people taking too many, or too potent, vitamin D supplements.

What Are the Risks of Too Much Vitamin D?

The main consequence of vitamin D toxicity is a buildup of calcium in your blood (hypercalcemia), which can cause nausea and vomiting, weakness, and frequent urination. Symptoms might progress to bone pain and kidney problems, such as the formation of calcium stones. According to the National Institutes of Health:

Vitamin D toxicity can cause non-specific symptoms such as anorexia, weight loss, polyuria, and heart arrhythmias. More seriously, it can also raise blood levels of calcium which leads to vascular and tissue calcification, with subsequent damage to the heart, blood vessels, and kidneys. The use of supplements of both calcium (1,000 mg/day) and vitamin D (400 IU) by postmenopausal women was associated with a 17% increase in the risk of kidney stones over 7 years in the Women’s Health Initiative.

My Overall Conclusions

Your interpretations of all this information may differ from mine, and I encourage you to take time and examine the sources linked below for yourself. Based on my reading of all the information given above, here are my personal conclusions about vitamin D and the current coronavirus that causes COVID-19:

  1. There is strong evidence, from repeated studies over many years, that vitamin D helps reduce your risk of contracting a respiratory infection, including viral infections from influenza and the common cold. The common cold is caused by both rhinoviruses and coronaviruses.
  2. COVID-19 is a respiratory infection of epithelial cells caused by a coronavirus.
  3. Some people who die of COVID-19 are dying because of “cytokine storms” in which their own immune system damages tissue and ignores secondary infections like bacterial pneumonia.
  4. Vitamin D appears to perform two protective functions that have relevance for COVID-19. First, it stimulates the creation of antibacterial and antiviral peptides and killer cells that directly destroy invading microorganisms. Second, vitamin D suppresses the over-expression of cytokines by the immune system.
  5. Adults need 600-800 IU per day of vitamin D to maintain adequate health.
  6. At least 40% of Americans have insufficient intake of vitamin D. 
  7. The chances that you have insufficient vitamin D are greater if you have darker skin, live at higher latitudes, spend all day indoors, or cover your body when you go out in the sun.
  8. It is very difficult to get sufficient vitamin D from your diet, unless you drink 40 ounces of milk a day, eat wild salmon every day, or take a tablespoon of cod liver oil daily.
  9. While there are dangers from consuming too much vitamin D, there is a wide margin between the recommended daily allowance and the levels that are toxic or that cause long-term health problems.

There is strong evidence that (A) Vitamin D does protect against a wide variety of viral respiratory infections and cytokine storms, and (B) there is a high likelihood that I am deficient in vitamin D. Based on these conclusions, I am taking a daily vitamin D supplement that meets the RDA requirement. Whether you should do so is up to you, of course (see disclaimer at the top of this post – I am not a medical professional or nutritionist.)

Update: Study Finds Vitamin D deficiency raises COVID-19 infection risk by 77%

On September 3, 2020, The Journal of the American Medical Association published a study of 489 patients that found a 77% increase in the risk of COVID-19 infection among people who are deficient in Vitamin D. Here’s a quote from the study:

These findings appear to support a role of vitamin D status in COVID-19 risk; randomized clinical trials are needed to determine whether broad population interventions and interventions among groups at increased risk of vitamin D deficiency and COVID-19 could reduce COVID-19 incidence.


For further reading, here are the materials referenced or used by me in writing this document.

Vitamin D and Respiratory Tract Infections

Cytokine Storms in Patients with COVID-19 or Influenza

Vitamin D Deficiency and Sources of Vitamin D

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