Early on in the pandemic, many touted taking vitamin D supplements as the way to prevent and to treat COVID-19 without good evidence. What does the evidence say now?
Vitamin D has been identified as important to our overall health and to our immune health, in particular. Vitamin D deficiency is relatively common. The two major sources for vitamin D are sunlight exposure (which stimulates the chemical reaction in our skin to synthesize vitamin D) and diet. Vitamin D does have a role to play in modulating our immune system and in decreasing inflammation.
There were some early observational studies (I have previously written about clinical trials and a bit about how to understand their designs, strengths and weaknesses if you want more information, but suffice it to say for purposes of this blog post that observational studies, though they are important in generating ideas and hypotheses to be tested in more rigorous studies, are among the weakest) that demonstrated a correlation (correlation is not the same as causation) between having deficient levels of vitamin D and more severe outcomes from COVID-19.
Therefore, claims that vitamin D supplementation might help prevent or treat COVID-19 did not seem far-fetched. On the other hand, physicians and researchers know well that many things that seem like they should be protective or effective treatments, including those that show promise in test tubes, or even in laboratory animals, don’t pan out in humans and under real-life conditions.
Even though vitamin D supplementation sounded reasonable, and even though when studies were performed using it showing that it reduced SARS-CoV-2-induced inflammation, we saw no difference in outcomes of disease (how severely ill someone became, whether hospitalization was necessary, whether they died or not) with vitamin D supplementation. A confounding factor is the fact that SARS-CoV-2 infection itself can lower one’s vitamin D level, so by the time someone is ill and the vitamin D level is first checked, it may not be possible to know whether the patient had preexisting low vitamin D levels unless a vitamin D level happened to have been checked at some point in the past. Regardless, a number of trials of supplementing vitamin D when patients got infected with SARS-CoV-2 have not demonstrated any benefit from this intervention.
Over the course of the pandemic, we now have 11 randomized control trials (these are the strongest type of clinical trial design, but of course, the overall quality of a trial depends upon more than just design type – e.g., the number of participants, the choice of controls, how well matched the participants and control groups are, the randomization method used, and whether the participants and treaters are aware of whether they are receiving the study intervention or are part of the control group). Still the majority of these trials showed no consistent or sustained improvement from vitamin D administration relative to length of hospitalization, the need for intensive care or mortality rates.
There are few randomized control trials examining the question of whether vitamin D supplementation prior to infection can prevent COVID-19 or at least reduce the severity of infection. Unfortunately, the study showing benefit showed the benefit in a relatively short time period that would not have been expected to have materially changed vitamin D levels in those who took it, and no vitamin D levels were measured or monitored. Other studies have failed to demonstrate protection. These studies are difficult to perform because one can’t easily control for other sources of vitamin D related to sun exposure, diet and vitamins or supplements. And, further, even if vitamin D supplementation was effective, we would have to answer the question as to whether it is just helpful when there is pre-existing vitamin D deficiency, or does it also benefit those with normal vitamin D levels. In fact, there is not even universal agreement as to what normal levels or optimal levels of vitamin D should be.
At this point, the conclusions that I come to are:
- It is reasonable to have your vitamin D levels checked and to supplement with vitamin D if your doctor determines your levels to be low. Keep in mind that vitamin D deficiency may be more likely during winter months when sun exposure is less.
- Don’t count on vitamin D supplements to prevent infection or to ensure that you have a mild case of COVID-19. While having normal levels of vitamin D are good for your overall health, and being vitamin D deficient may be associated with worse COVID-19 outcomes, there are no convincing data to date that vitamin D will prevent or be helpful in treating COVID-19. You are much better off using proven strategies to reduce your risk of getting COVID-19. COVID vaccination, including with the new updated booster is far more proven to prevent you from getting severely ill with COVID-19. And, if you are older or at higher risk and do get infected, check with your doctor promptly to determine whether you should receive an antiviral to help prevent severe disease and whether metformin may be a good option for you to help prevent Long COVID. Vaccination, early antiviral treatment and a short course of metformin are the best proven options to prevent Long COVID other than preventing infection in the first place.
Reference: https://www.nature.com/articles/s41577-022-00765-6.
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