Numerous studies have been done on vitamin D—it is a massive topic. The following is a quick look at one benefit that taking a vitamin D supplement has been found to provide to people with a vitamin D deficiency (which many of us are estimated to have)—the ability to reduce the risk of the flu and upper respiratory infections, including pneumonia. I’ve also provided some general information on vitamin D for background. There will be more posts in the future on Vitamin D’s other benefits.

As with all my “Health and Wellness” posts, this is for informational purposes only, and does not in any way constitute medical advice, particularly with regard to COVID-19.

Who couldn’t use some sunshine these days? Whether we’re referring to good weather, good news, or both—we all could.

Sunshine is important. In addition to affecting our mood, a shortage of it raises our risk of developing a vitamin D deficiency. And maintaining an optimal level of vitamin D is really important.

What is vitamin D? The short version—

Vitamin D, aka, “the Sunshine Vitamin,” is actually a hormone. While a vitamin is an essential nutrient that our body must obtain from food, our body produces vitamin D. Vitamin D is fat soluble, meaning that it dissolves in fats and oils. As such, it is absorbed along with fats in our diets, and can be stored in our bodies’ fatty tissue. If you are obese, vitamin D dilutes in your body fat.

Very few foods contain vitamin D. Dietary sources, however, do include fatty fish, such as tuna, mackerel, and salmon; foods fortified with vitamin D, such as dairy products, soy milk, and cereals; beef liver, cheese, and egg yolks.[i]

Vitamin D’s most widely used forms as supplements are vitamin D2 and vitamin D3. Vitamin D2 is a synthetic form of vitamin D and is derived from plants. Grandfathered by the US Food and Drug Administration (USFDA), it is the type that is typically prescribed by doctors. By contrast, most (but not all) Vitamin D3 supplements come from fish oils (the skin of fatty fish) or sheep lanolin (the waxy substance secreted by the glands in sheep skin).[ii]

The most natural way of getting vitamin D is from exposure to the sun’s ultraviolet B (UVB) rays—and the vitamin D produced by our skin can last at least twice as long as the vitamin D that is taken in through food and supplements.[iii] Cholesterol plays an important part in our bodies’ production of vitamin D. The process starts when the sun’s UVB rays are absorbed by the precursor of cholesterol in our skin. Solar radiation then converts the cholesterol precursor to pre-vitamin D3, which then rapidly converts to vitamin D.[iv]

Our skin makes cholesterol, however—blood cholesterol has nothing to do with the vitamin D synthesis in our skin, according to Dr. Michael F. Holick, MD, PhD of Boston University (BU).[v] Dr. Holick is a Professor of Medicine, Physiology and Biophysics at BU Medical School, in addition to having other responsibilities, including Director of the Vitamin D, Skin and Bone Research Laboratory, Endocrinology, Diabetes, Nutrition & Weight Management, Medicine, BU School of Medicine, Graduate Medical Sciences.[vi] He has authored or co-authored at least 13 books and 500+ medical articles, and is considered by many to be one of the world’s leading experts on vitamin D.[vii] Also according to Dr. Holick, people who take statin drugs are able to make this precursor of vitamin D and cholesterol without any problem, despite the statins inhibiting cholesterol biosynthesis. This is because skin is a bloodless tissue and the statins don’t get there. [viii]

Getting too much sun, though and its risk for skin cancer, is something that is on the minds of most of us, these days—more on that later.

Increased risks that are caused by deficiency

Vitamin D is often associated with good bone health—however, research has shown that it provides a wide range of health benefits. A vitamin D deficiency has been connected to increasing risk in a variety of illnesses and diseases, including osteoporosis; various cancers; infectious diseases including tuberculosis and influenza; schizophrenia; heart disease; autoimmune diseases including multiple sclerosis, type 1 diabetes, and rheumatoid arthritis; strokes;[ix] type 2 diabetes; preeclampsia; and Alzheimer’s disease and neurocognitive disfunction, according to Dr. Holick.

People with low levels of vitamin D also may be more likely to get acute respiratory tract infections (ARTIs).[x] McMaster University in Canada explored the idea of whether taking vitamin D supplements reduced the risk of ARTIs, studying a wide age range of both healthy people and those with respiratory conditions such as asthma or chronic obstructive pulmonary disease (COPD). Overall, the results were positive, showing that vitamin D3 supplements provided the greatest benefit to people whose vitamin D level had been low to start with, and who had taken supplements daily or weekly. People who received one or two large doses (once, monthly, or every two months) in addition to their daily supplement, did not experience these benefits. Dr. Mehmet C. Oz, MD, FACS, Director of Integrative Medicine Center at Columbia University’s Irving Medical Center—known to many of us as simply, “Dr. Oz”—also states that vitamin D is unlikely to help people who are not deficient in it. However, when it is given to people who are deficient in it, a vitamin D supplement can reduce the risk of the flu and upper respiratory infections, including pneumonia.[xi]

An updated analysis of a nearly-11,000-person study led by Queen Mary University of London and published in The BMJ (originally called The British Medical Journal) had similar findings to McMaster University. Participants came from 25 clinical trials conducted in 14 countries, including the UK, the US, Japan, India, Afghanistan, Belgium, Italy, Australia, and Canada. When reviewed individually, the studies were determined to have conflicting results—however, pooled results revealed definitive evidence that vitamin D protected against respiratory infections.[xii]

The pooled data showed that people with the lowest levels of vitamin D who took daily or weekly supplements saw their risk of ARTI decline the most after taking the supplements. People with higher baseline levels also saw a reduced risk of ARTI, however the effect was much more modest.[xiii]

Vitamin D is believed to protect against respiratory infections by boosting levels of antimicrobial peptides—natural antibiotic-like substances—in the lungs. The results fit with the observation that colds and the flu are most common in winter and spring, when vitamin D levels are at their lowest. The evaluators concluded that the low levels may also explain why vitamin D protects against asthma attacks, which are commonly triggered by respiratory viruses.[xiv]

Another study came out of New Haven, Connecticut, reporting that healthy adults who maintained blood levels of 25-hydroxy vitamin D of around 40 nanograms per mL (ng/mL) reduced their risk of developing upper respiratory tract infections by almost 50%.[xv] Additionally, Japanese research showed that school children who received 1200 units of vitamin D a day, just during the wintertime, reduced their risk of getting influenza A infection by more than 40%.[xvi]

Optimal vitamin D levels

How do we know if we have a vitamin D deficiency? We need to find out our vitamin D level, which is determined by a blood test. Often, a vitamin D test is completed as part of our routine blood work. That is not always the case, though. Not all doctors test for a vitamin D level, so if you’re interested in learning yours, you will have to request that specific test when your blood work is done.

The Endocrine Society, formerly chaired by Dr. Holick, recommended a minimum blood level for maximum bone health of 30 nanograms (ng) per milliliter (mL). In Dr. Holick’s opinion, though, people really should be in the range of at least 40-60 ng/mL. He believes that a level up to 100 is safe, and that we should have toxicity concerns only when that level is above 150 ng/mL.[xvii] Dr. Joseph Mercola, DO, would take that target range a little higher, noting that a compelling body of research suggests that 40 ng/mL would be a cutoff for sufficiency and that a range of 60-80 ng/mL is optimal for health and disease prevention.[xviii]

Based on a sufficiency level of 30 ng/mL, 75% of American adults and teens have a vitamin D deficiency, according to data published in the Archives of Internal Medicine. [xix]

Vitamin D, sun exposure, and skin cancer

So, how do we intelligently balance optimizing our vitamin D level through sun exposure without increasing our risk of developing skin cancer?

There are a lot of variables associated with the sun and our ability to produce vitamin D, including time of day, season, latitude, and degree of skin pigmentation.[xx] As many of us have heard, if we live north of Atlanta, GA, our bodies can’t make vitamin D from about November until around mid-March because of the angle of the sun.[xxi] However, time of day also counts. Our bodies make vitamin D between the hours of 10:00 am and approximately 3:00 pm, no matter where we live.[xxii] And because their skin pigment is a natural sunscreen, African-Americans need to be outside up to five-to-ten times longer than a Caucasian to make the same amount of vitamin D.[xxiii]

And btw, sunscreen with a 30 SPF reduces our skins’ ability to make vitamin D by about 95%.[xxiv] Therein lies the conundrum…


…That’s where supplements come in. Dr. Holick’s vitamin D3 supplement recommendations depend on age, and are listed below. Another variable, as noted earlier, is body fat because vitamin D dilutes in a person’s body fat. As a result, obese people may need two-to-three times more vitamin D to satisfy their requirement.[xxv]

Dr. Holick’s recommendations:[xxvi]

Children: 1,000 units/day

Teenagers and Adults: 2,000-3,000 units a day

In the interest of full disclosure, Dr. Holick’s supplement recommendations are considerably higher than the National Institutes of Health recommendations, seen below:[xxvii]

National Institutes of Health recommendations:

Age 0-12 months: 400 units/day

Age 1-70 years:     600 units/day; Women, ages 14-50, who are pregnant or lactating should increase their dosage by an additional 600 units/day

Age 70+ years:       800 units/day

Too much of a good thing?

Vitamin D toxicity is a serious condition but it’s rare, and it’s typically caused by taking megadoses of the supplement rather than by diet or sun exposure. Vitamin D toxicity can result in hypercalcemia—too much calcium in your blood. Its symptoms include nausea and vomiting, weakness, and frequent urination. The condition can also progress to bone pain and kidney problems, such as creating calcium stones.[xxviii] 

In a 2018 interview, Dr. Holick stated that vitamin D toxicity was one of the rarest medical conditions in the world.[xxix] The Mayo Clinic agrees that it is rare, and it seems, that it would take a lot to get to a toxic level. In a question posed to the Mayo Clinic about vitamin D toxicity, Katherine Zeratsky, RD, LD responded that taking 60,000 units (international units) per day for several months has been shown to cause toxicity.[xxx] She may not have meant that as a base level, but Ms. Zeratsky’s response seems to indicate that reaching a toxic level would take an unusually high dosage over a period of time.

The first step in knowing whether you are vitamin D deficient, and whether a vitamin D supplement can benefit you is determined by knowing your vitamin D level—and that needs to be done through a blood test ordered by your doctor.

As always, check with your doctor before taking any supplements—and stay healthy, safe, and happy.

Until next time,

[i] Vitamin D: Finding a Balance; Harvard Health Publishing; Harvard Medical School; 25 July 2017;


[iii] Vitamin D: Finding a Balance; Harvard Health Publishing; Harvard Medical School; 25 July 2017;

[iv]Dr. Michael Holick on How Vitamin D Benefits Your Health;

[v] Ibid

[vi] Boston University School of Medicine, Michael F. Holick, MD, PhD, Professor of Medicine, 17 March, 2020;

[vii] Dr. Michael Holick on How Vitamin D Benefits Your Health;

[viii] Ibid

[ix] Ibid

[x] Can vitamin D ward off acute respiratory tract infections?; McMaster University; 14 May 2019;

[xi] 3 Top Supplements for Colds and Flu, Dr. Oz;

[xii] Vitamin D protects against colds and flu, finds major global study; Science Daly; 16 February 2017;

[xiii] Ibid

[xiv] Ibid

[xv] Dr. Michael Holick on How Vitamin D Benefits Your Health;

[xvi] Ibid

[xvii] Dr. Michael Holick on How Vitamin D Benefits Your Health;

[xviii] For Optimal Health, Make Sure You Have a Vitamin D Level of 60 ng/mL; Dr. Mercola; 4 July 2018;

[xix] Ibid

[xx] Dr. Michael Holick on How Vitamin D Benefits Your Health;

[xxi] Ibid

[xxii] Ibid

[xxiii] Ibid

[xxiv] Ibid

[xxv] Ibid

[xxvi] The IOM “D-bate;” Dr. Holick responds to a recent study by the Institute of Medicine about vitamin D intake; Blog 25 March 2020;

[xxvii] Vitamin D Fact Sheet for Health Professionals; Updated 24 March 2020;

[xxviii] Vitamin D toxicity: What if you get too much? The Mayo Clinic; 7 February 2018;

[xxix] Dr. Michael Holick on How Vitamin D Benefits Your Health;

[xxx] Ibid

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