Is it 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.
Almost all immune cells have vitamin D receptors, which shows that vitamin D interacts with the immune system.
The active vitamin D hormone calcitriol helps regulate the innate and adaptive immune systems, our first and second lines of defense against pathogens.
And vitamin D deficiency is associated with immune deregulation, degradation, or a change in the control of immune system processes.
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Many of the effects of calcitriol on 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 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 an essential role in clearing acute viral infections (such as the flu) from the lungs.
Calcitriol also suppresses pro-inflammatory cytokines, molecules secreted by immune cells which, as their name suggests, promote inflammation. Some scientists have suggested that vitamin D may help alleviate the “cytokine storm” described in the most severe COVID-19 cases.
Evidence from randomized controlled trials suggests that regular vitamin D supplementation may help protect against acute respiratory infections.
A recent meta-analysis pooled the results of 25 trials with more than 10,000 participants 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 single large dose.
The benefits of regular supplementation were greater in participants who were initially severely deficient in vitamin D, for whom the risk of respiratory infection decreased by 70%. In others, the risk has decreased by 25%.
My vitamin D levels are low, should I take a supplement?
Large single doses (or “boluses”) are often used as a rapid means of replenishing vitamin D. But in the context of respiratory infections, there was no benefit if participants received high single doses.
In fact, monthly or yearly vitamin D supplementation has sometimes had unexpected side effects, such as an increased risk of falls and fractures, where vitamin D was administered to protect against these results.
The possible intermittent administration of large doses may 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 although the initial research is interesting, a large part can be circumstantial.
For example, a small study from the United States and another from Asia found a strong correlation between low vitamin D levels and serious COVID-19 infection.
But neither of the two studies considered confounding factors.
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 also often deficient in vitamin D. Studies evaluating patients in intensive care have reported high rates of deficiency before COVID-19.
We therefore expect to see relatively high rates of vitamin D deficiency in critically ill COVID-19 patients – whether vitamin D has a role or not.
Some researchers have noted high rates of COVID-19 infections in minority ethnic groups in the United Kingdom and the United States to suggest a role for vitamin D, since minority ethnic groups tend to have lower levels of vitamin D.
However, analyzes by the British Biobank did not establish a link between the concentrations of vitamin D and the risk of infection with COVID-19, nor that the concentration of vitamin D could explain the ethnic differences in the COVID-19 infection.
Although this research adjusted the confounding factors, vitamin D levels were measured ten years earlier, which is a drawback.
The researchers also suggested that vitamin D plays a role in examining average vitamin D levels in different countries alongside their COVID-19 infections. But in the hierarchy of scientific evidence, these types of studies are weak.
Should we be trying to get more vitamin D?
There are several registered trials of Vitamin D and COVID-19 in their early stages. We therefore hope that over time we will gain more clarity on the potential effects of vitamin D on COVID-19 infection, in particular through studies using more robust models.
In the meantime, while we don’t know if vitamin D can help reduce the risk or results of COVID-19, we know that vitamin D deficiency won’t help.
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It is difficult to get enough vitamin D from food alone. A generous portion of oily fish can cover a lot of our needs, but it’s not healthy or appetizing to eat it every day.
In Australia, we get most of our vitamin D from the sun, but about 70% of us have insufficient levels during the winter. The amount of exposure we need to get enough vitamin D is usually low, just a few minutes in the summer, while in the winter it can take a few hours of exposure in the middle of the day.
If you do not think you are getting enough vitamin D, talk to your doctor. They may recommend incorporating daily supplements into your routine this winter.