Have you ever wondered why you are much more likely to have a cold in the winter than in the summer? It turns out that researchers have puzzled over the seasonality of respiratory viruses for decades. In 1926, one scientist proposed that the seasonality was due to the vitamin found in dairy products and produced by sunlight exposure on the skin.[ref] This idea that vitamin D levels influence the seasonality of respiratory viruses had been tossed around ever since, in different variations.
This article will cover the more recent research on vitamin D levels, supplementation, and respiratory infections.
Can Vitamin D reduce the risk of respiratory infections?
First, a little context.
By respiratory infection, researchers are generally referring to colds, flu, and what the CDC calls ‘influenza-like illness’ or ILI. These are the viruses that go around every winter causing coughing, sneezing, wheezing, and runny noses.
- The virus families that cause the common cold include rhinovirus, respiratory syncytial virus (RSV), adenovirus, and coronavirus.
- The flu is caused by many different strains of influenza A or influenza B.
- The CDC reports each year on ILI- influenza-like illness. This is a catch-all for influenza virus, RSV, rhinovirus, adenovirus, parainfluenza virus, coronavirus, and metapneumovirus.[ref]
Why do we care about respiratory viruses? Excluding data from the current COVID-19 pandemic (which is measured differently than previously), about 20% of the US population will get a respiratory infection each year. Several hundred thousand will end up in the hospital, and of those who are hospitalized with a respiratory infection, over 10% will die.[ref]
Additionally, respiratory tract infections statistically have links to an increased likelihood of heart attacks. A study looking at English hospital admission for people with myocardial infarction found that up to 5% of MI admissions may be due to respiratory virus infection in the previous week or two. Influenza was the least likely to be linked to MI, while the common cold viruses were most likely.[ref]
Vitamin D: the sunshine hormone
Vitamin D is produced in the skin as a reaction between UVB radiation from sunlight and cholesterol in the epithelial cells.
Once synthesized (or ingested), the liver transforms it into 25-(OH)D (25-hydroxyvitamin D). This is what you usually see on lab tests.
25-(OH)D is the main form available in the body, and it has a half-life of two to three weeks, so it can hang around for a while.
The kidneys transform 25-(OH)D into the active form of 1,25-(OH)2D. Certain other cells, such as immune system cells, can also transform the storage form of D into the active form. The active vitamin D hormone is transported throughout the body by vitamin D binding protein.
What does vitamin D do in a cell? The active form of vitamin D binds to a vitamin D receptor in the nucleus of certain cell types and then turns on genes for transcription. There are several different types of cells with vitamin D receptors. For example, in macrophages, active vitamin D increases the production of anti-microbial peptides. In T-cells, vitamin D causes a shift towards Th2.[ref] In muscle cells, the binding of vitamin D to the receptor increases calcium and phosphate transport and increases muscle cell proliferation.[ref]
Regulating the immune response:
When challenged by a pathogen, the body needs to respond with enough immune response to vanquish the invader — but not going overboard and killing healthy cells. Many times, death from a respiratory virus is due to ARDS (acute respiratory distress syndrome) which is, at least in part, the body’s over-activation of the immune system in the lungs.
Vitamin D is important in activating immune system cells in a way that keeps the immune response from going overboard. Specifically, it skews the T-cells to mature as a regulatory T-cell phenotype rather than an inflammatory Th1/Th17 phenotype.[ref]
Clinical trials on Vitamin D and respiratory tract infections:
Research clearly shows that low vitamin D levels are associated with an increased risk of respiratory infections in both young and old[ref][ref]. But does that mean that taking a vitamin D supplement will help to prevent illness?
This can be a harder question to answer than you would think. There are a ton of trials on vitamin D supplementation, but many of the trials are small and use low doses of vitamin D in the trial.
A 2019 meta-analysis combined the data from 25 randomized controlled trials to try to answer the question of whether any vitamin D supplementation would reduce respiratory infections. The results showed that overall a daily or weekly vitamin D supplement decreased the average risk of respiratory infections by about 20%. Large doses via a shot of vitamin D did not have a statistical effect. Unsurprisingly, the benefit of supplementation was much greater in people who were vitamin D deficient (< 25 nmol/l). For vitamin D deficient individuals, supplemental vitamin D reduced respiratory tract infections by 70%.[ref]
What about COVID-19 vitamin D trials?
At the very beginning of the COVID-19 pandemic researchers associated vitamin D levels with a 20-fold increased risk of severe COVID-19.[ref]
But many argued that the association didn’t prove causation, nor did it tell us whether vitamin D supplementation would be beneficial for prevention. Years of previous research on the mechanism of action of vitamin D on the immune system didn’t seem to be enough to ‘prove causation’. Of course, even without clinical trials showing a benefit, there isn’t much of a risk to boosting vitamin D levels via a daily or weekly supplement.
A year down the road, the clinical trial results are rolling in, showing a clear benefit to supplementing with vitamin D, especially for people with lower baseline levels.
- A Dec. 2020 study showed a significant reduction in mortality for COVID-19 hospitalized patients given vitamin D in the hospital. The patients who had received a shot of vitamin D (280,000IU) were 87% less likely to die from COVID-19.[ref]
- A small trial of vitamin D (1,000 IU), magnesium, and vitamin B12 in hospitalized COVID patients showed a >80% reduction in the need for oxygen or ICU admission.[ref]
- In another small trial of COIVD hospitalized patients, 50 of the patients received a large dose of oral vitamin D on days 1, 3, and 7 of hospitalization. All patients (control and vitamin D arm) also received other medications such as hydroxychloroquine and azithromycin as part of the normal hospital protocol. There was a significant difference in the patients needing admittance to the ICU — 1 / 50 for the vitamin D participants vs. 50% of the control group who didn’t receive vitamin D.[ref]
Should you take vitamin D before a vaccine?
If vitamin D modulates the immune response, should you stop taking it before a vaccine? Let’s take a look at the research studies on the topic.
Several studies have looked at the response to vaccines stratified by vitamin D levels. Most studies show that there is no difference in the response to the flu vaccine based on vitamin D levels. There is a difference in response to the rubella vaccine based on vitamin D receptor gene variants, suggesting that vitamin D does play a role in that vaccine. With the hepatitis B vaccine, though, there is a clear association between lack of response to the vaccine and low vitamin D levels. In children, vitamin A and D supplementation increase the response to the tuberculosis vaccine.[ref]
While no overall benefit shows for higher vitamin D levels for the flu vaccine, a benefit for certain strains of influenza might exist.[ref]
What does seem to matter for vaccine response is age, genetics, stress, BMI, season, and time of day of vaccination.[ref] For example, morning vaccinations are more likely to elicit a stronger immune response for certain flu vaccines than later in the afternoon (3-5 pm).[ref]
Vitamin D supplementation reduces viral respiratory illnesses:
Vitamin D deficiency clearly impacts the immune response to viral respiratory illnesses. This has been the theory for over a century and has repeatedly been shown for the past couple of decades in research studies.
With respiratory illnesses either directly or indirectly causing many deaths in people over 65 each year, it seems like a ‘no brainer’ to keep your vitamin D levels up in the normal range. Younger people also benefit from reduced respiratory infections, and even if a viral isn’t likely to kill you, reducing the number of times you are sick in a year is a benefit for everyone.
Sunlight on your skin is a great option for increasing your vitamin D levels. Of course, this is not such a great option in the fall and winter months when UVB rays are scarce and it is too cold to expose much skin.
Supplemental vitamin D is cheap and readily available. Check the label on your vitamin D supplement, though, since a lot comes with cheap soybean or cottonseed oil. A better option is made with coconut or olive oil.
Check your vitamin D levels:
Get your doctor to order a vitamin D test the next time you go. OR – order a test on your own. Ulta Lab Tests offers it for $39, and there are other places online to order it as well.