Vetagro Journal Club, #3

It is a fact. We are in a pandemic. And while we are facing extra-ordinary times and challenges one subtle question is slowly emerging: is vitamin D deficiency a pandemic within a pandemic?

Vitamin D (vitD) is a fat-soluble steroid essential for animals. It is produced in the skin by direct contact with sunlight and can also be ingested with animal-based food. VitD is essential for healthy bones but its benefits seem to go way beyond that.

Vitamin D sufficiency is defined as serum 25(OH) vitamin D (the major circulating form of vitD) level equal to or greater than 30 ng/mL measured at the end of the winter season. Insufficiency is defined as levels ranging between 20-29 ng/mL and deficiency with levels below 20 ng/mL. 

Recent estimates report vitamin D deficiency as a major healthcare problem involving almost 1 billion people globally with the highest prevalence in Europe, included the “sunny countries”. Paradoxically, Northern European countries, which one would expect to be more deficient because of the scarcity of sunlight, have much better VitD status probably due to higher consumption of fish-derived products and fortified foods compared to Mediterranean countries.

But why is vitD so important? While VitD deficiency involvement in the pathogenesis of osteoporosis doesn’t come as a surprise, little is communicated about the association between VitD and other biological functions such as those connected to the immune system. There is in fact growing scientific evidence that vitD deficiency can increase the severity of infectious diseases, especially respiratory tract infections.

Vitamin D is in fact a key player in the development of the immune innate and acquired response following bacterial and viral infections. More specifically, it is involved in the process of regulation and differentiation of monocytes into macrophages or dendritic cells, which are both considered as “first responders” during infection and they both contribute to the mechanism of pathogen elimination.
Evidence about the role of vitD in the development of mature monocytes and macrophages is supported by the presence of vitD receptors on these cells’ surface. Human B cells and T cells alike do express vitD receptors as well. Moreover, vitD is presumed to have a role in promoting a regulatory response of inflammation via inhibition of interleukin 12 synthesis. Other mechanisms of action might instead involve the production of antimicrobial peptides (AMP) via promotion of the AMP gene by vitD via vitD receptor. All of these mechanisms would suggest a role for vitD in the host defense against both bacterial and viral infections. 

The National Health Institute recommends daily intake vitD supplement during the fall/winter season and is strongly recommending to do so especially this year due to the pandemic and the stay at home/lockdown policies currently in place. While there isn’t still enough evidence and observational studies to claim an increased severity of COVID-19 in vitD deficient subjects, high dose vitD supplementation for the groups at risk is necessary to maintain optimal serum 25 (OH) vitD levels in the range of 40-60 ng/mL.

Food for our thoughts: despite in the moment we are writing this article there is not a general consensus on the presumed efficacy of vitD in preventing acute symptoms of respiratory infections, the number of scientific records connecting vitD and COVID-19 is growing at a fast pace, and there is absolute evidence of vitD worldwide deficiency as well as of the role of vitD itself as a player in the development of the immune response. While we wait for the Science to take its course, clinicians and caretakers should evaluate the risk/benefits of vitamin D intake from supplements in deficient people, as vitD supplementation might be carrying more benefits than risks if taken under medical supervision. 

Vitamin D and Infectious Diseases: Simple Bystander or contributing factor? Click here

Does vitamin D deficiency increase the severity of COVID-19? Click here

Vitamin D and COVID-19: It is time to act Click here

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