Recent headlines have suggested vitamin D deficiency could increase the risk of dying from COVID-19, and in turn, that we should consider taking vitamin D supplements to protect ourselves.
Is this all just hype, or could vitamin D really help in the fight against COVID-19?
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Vitamin D and the immune system
At least in theory, there may be something to these claims.
Nearly all immune cells have vitamin D receptors, showing vitamin D interacts with the immune system.
The active vitamin D hormone, calcitriol, helps regulate both the innate and adaptive immune systems, our first and second lines of defence against pathogens.
And vitamin D deficiency is associated with immune dysregulation, a breakdown or change in the control of immune system processes.
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Many of the ways calcitriol affects the immune system are directly relevant to our ability to defend against viruses.
For example, calcitriol triggers the production of cathelicidin and other defensins – natural antivirals capable of preventing the virus from replicating and entering a cell.
Calcitriol can also increase the number of a particular type of immune cell (CD8+ T cells), which play a critical role in clearing acute viral infections (such as influenza) in the lungs.
Calcitriol also suppresses pro-inflammatory cytokines, molecules secreted from immune cells which, as their name suggests, promote inflammation. Some scientists have suggested vitamin D might help to alleviate the “cytokine storm” described in the most severe COVID-19 cases.
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Evidence from randomised controlled trials suggests regular vitamin D supplementation may help protect against acute respiratory infections.
A recent meta-analysis brought together results from 25 trials with more than 10,000 participants who were randomised to receive vitamin D or a placebo.
It found vitamin D supplementation reduced the risk of acute respiratory infections, but only when it was given daily or weekly, rather than in a large single dose.
The benefits of regular supplementation were greatest among participants who were severely vitamin D deficient to begin with, for whom the risk of respiratory infection went down by 70%. In others the risk decreased by 25%.
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My vitamin D levels are low, should I take a supplement?
Large one-off (or “bolus”) doses are often used as a quick way to achieve vitamin D repletion. But in the context of respiratory infections, there were no benefits if