The biological actions of vitamin D are carried out via the binding of vitamin D3 to the vitamin D receptor (VDR) (Ashcroft et al 2020). VDR is expressed in most cells, and may activate between 200 and 500 genes, many related to the immune system (Bergman 2021), including B and T lymphocytes, monocytes, macrophages, and dendritic cells (Sassi et al 2018). In addition, immune cells are able to locally convert intermediate form of vitamin D into its active form, calcitriol, indicating that vitamin D has a significant role in immune response against the invading pathogens (Sassi et al 2018). Furthermore, as explained by Liu et al (2013), vitamin D can also help increase white blood cell count by decreasing the rate at which the white blood cells are destroyed and removed from the body.
The anti-inflammatory properties of vitamin D are also well established; as vitamin D can stall hyper-inflammatory responses in the lung tissue, it is effective against various upper respiratory infections, contributing to an accelerated healing process of the affected areas (Mohan et al 2020).
Sources of vitamin D
According to NHS (2018), sunlight is the most important factor responsible for the replenishment of vitamin D pools. During the ‘cold’ season, between October and early March, lack of sunlight exposure contributes to vitamin D deficiency as the sunlight does not contain enough UVB radiation for our skin to be able to make vitamin D. Therefore, we rely on getting our vitamin D from supplements and food sources such as fish and shellfish, eggs, butter milk, yogurt & cheese (Vaes et al 2017). However, from early April to the end of September, short periods of sun exposure (5 – 30 mins at least twice a week, between 10am – 2pm), with the forearms, hands or lower legs uncovered, and without sunscreens, most people should get enough vitamin D.
The timing of sunlight exposure plays an important role in vitamin D synthesis as after 2pm skin damaging, and carcinogenic UVA amount increases in sunrays (Qureshi et al 2015).
Vitamin D & COVID-19
As noted in the review by Xu et al (2020), COVID-19 is caused by a highly infectious novel coronavirus leading to a plethora of mild-to-severe clinical symptoms such as flu-like symptoms, dry cough, fever, loss of smell and taste, headache, muscle pain, increased shortness of breath, etc. However, there is hope on the horizon; apart from vaccination, research shows that vitamin D supplementation may be able to prevent and possibly treat COVID-19 (Alexander et al 2020 Ali 2020 Mitchell 2020 Mohan 2020 Bergman 2021).
As explained by Mitchell (2020), vitamin D can make infection with the virus and development of COVID-19 symptoms less likely, by supporting production of antimicrobial peptides in the respiratory epithelium. In addition, vitamin D, due to its anti-inflammatory properties, might help to reduce the inflammatory response to severe acute respiratory syndrome (SARS), caused by COVID-19.
According to a review by Zdrenghea et al (2017), patients with respiratory disease are frequently deficient in vitamin D due to its role in local “respiratory homeostasis”, either by stimulating the exhibition of antimicrobial peptides or by directly interfering with the replication of respiratory viruses.
Vitamin D supplementation
Ali (2020) suggested that people who are at higher risk of vitamin D deficiency during this global pandemic should consider taking vitamin D supplements to maintain the circulating 25(OH)D in the optimal levels (75-125nmol/L).
The Scientific Advisory Committee on Nutrition (2016), the committee of independent experts that advises Government on matters relating to diet, nutrition and health, recommend that from October to March everyone over the age of 5 should consider taking a daily supplement containing 10 micrograms (400iu) of vitamin D. Since vitamin D is found only in a small number of foods, it might be difficult to get enough from foods that naturally contain vitamin D and/or fortified foods alone.
Vitamin D supplementation is particularly vital for individual with a risk of vitamin D deficiency, e.g. obesity, old age, dark skin, wearing covering clothes or no sunshine exposure (Zemb et al 2020). Individuals with darker skin pigmentation, due to melanin blocking UV-B rays, require longer time in the sun to produce equivalent amounts of Vitamin D3 than individuals with less pigmented skin (Taksler et al 2013). Obese individuals, with the higher the fat mass may also have lower blood concentration of vitamin D as the result of sequestration of this vitamin into the adipose tissue (Earthman et al 2012).
However, in certain rare situations daily vitamin D supplementation may not be safe – e.g. kidney stones, sarcoidosis, etc. (Zemb et al 2020).
It is also important that you consult your GP if taking any medications to avoid any drug-nutrient interactions.
References are available on request
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