Recent headlines have suggested that vitamin D deficiency may increase risk of dying of COVID-19, and in turn, that we should consider taking vitamin D supplements to protect us.
Is this all just hype, or could vitamin D really help in the fight against COVID-19?
Vitamin D and the immune system
At least in theory, there may be something to these claims.
Almost all immune cells have vitamin D receptorsshowing that vitamin D interacts with the immune system.
The active vitamin D hormone, calcitriol, helps regulate both innate and adaptive immune systemsour first and second lines of defense against pathogens.
And vitamin D deficiency is associated with immune dysregulationa breakdown or change in the control of immune system processes.
Many of the ways calcitriol affects the immune system are directly related to our ability to defend ourselves against viruses.
For example, calcitriol triggers the production of cathelicidin and other defensins – natural antivirals capable of prevent the virus to replicate and enter a cell.
Calcitriol may also increase the number of a particular type of immune cell (CD8+ T cells), which play a vital role in eliminate acute viral infections (like the flu) in the lungs.
Calcitriol also suppresses pro-inflammatory cytokines, molecules secreted by immune cells that, as their name suggests, promote inflammation. Some scientists have suggested that vitamin D may help alleviate the “cytokine stormdescribed in the most severe cases of COVID-19.
Evidence from randomized controlled trials suggests that regular vitamin D supplementation may help protect against acute respiratory infections.
A recent meta-analysis collated the results of 25 trials with more than 10,000 participants who were randomized to receive vitamin D or a placebo.
He found that vitamin D supplementation reduced the risk of acute respiratory infections, but only when given daily or weekly, rather than in a large single dose.
The benefits of regular supplementation were greatest in participants who were initially severely deficient in vitamin D, for whom the risk of respiratory infection was reduced by 70%. In others, the risk has decreased by 25%.
Large single doses (or “boluses”) are often used as a quick way to achieve vitamin D repletion. But in the context of respiratory infections, there was no benefit if participants received high single doses.
In reality, monthly Where annual vitamin D supplementation has sometimes had unexpected side effects, such as an increased risk of falls and fractures, when vitamin D was given to protect against these results.
It is possible that intermittent administration of large doses interfere with the synthesis and breakdown of enzymes regulating the activity of vitamin D in the body.
Vitamin D and COVID-19
We still have relatively little direct evidence on the role of vitamin D in COVID-19. And while early research is interesting, much of it can be circumstantial.
For instance, a small study from the United States and another study from Asia found a strong correlation between low vitamin D status and severe COVID-19 infection.
But none of the studies took confounding factors into account.
In addition to the elderly, COVID-19 generally has the greatest consequences for people with pre-existing conditions.
It is important to note that people with existing health conditions are often also deficient in vitamin D. Studies evaluating Intensive care patients reported high rates of deficiency even before COVID-19.
We would therefore expect to see relatively high rates of vitamin D deficiency in critically ill patients with COVID-19 – whether vitamin D plays a role or not.
Some researchers have noted high rates of COVID-19 infections in minority ethnic groups in the UK and US to suggest a role for vitamin D, as minority ethnic groups tend to have lower levels of vitamin D.
However, analyzes of the UK Biobank did not support a link between vitamin D concentrations and risk of COVID-19 infection, nor that vitamin D concentration could explain ethnic differences in COVID-19 infection.
Although this research adjusted for confounders, vitamin D levels were measured ten years earlier, which is a downside.
The researchers also suggested vitamin D play a role looking at the average vitamin D levels of different countries alongside their COVID-19 infections. But in the hierarchy of scientific evidence these types of studies are weak.
Should we try to get more vitamin D?
There are several registered trials on vitamin D and COVID-19 in their early days. So hopefully over time we will have more clarity on the potential effects of vitamin D on COVID-19 infection, especially from studies using more robust designs.
In the meantime, while we don’t know if vitamin D can help mitigate the risk or consequences of COVID-19, we do know that a vitamin D deficiency will not help.
It’s hard to get enough vitamin D from food alone. A generous portion of oily fish can cover a large part of our needs, but it is neither healthy nor pleasant to eat it every day.
In Australia, we get most of our vitamin D from the sun, but around 70% of us have insufficient levels during the winter. The exposure quantity we need to get enough vitamin D is usually low, only a few minutes in summer, while in winter it can take a few hours of exposure in the middle of the day.
If you think you are not getting enough vitamin D, talk to your GP. They may recommend incorporating daily supplements in your routine this winter.
