Vitamin D is a nutrient that is both endogenously synthesized in the skin and consumed dietarily.  Its biologically active form [1,25-(OH)2D] is a known negative regulator of the Renin-Angiotensin System (RAS).  1,25-(OH)2D has been shown to increase ACE2 expression and suppress the inflammatory outcomes caused by angiotensin II (Ang II).  Additionally, vitamin D can modulate the immune system by inhibiting B-cell and T-cell proliferation as well as inducing T-regulatory cells, which suppress pro-inflammatory cytokines and increase anti-inflammatory cytokines.  These processes may make vitamin D a useful molecule in treating or preventing a SARS-CoV-2 infection.

Using Vitamin D to Mitigate COVID-19

Clinically, vitamin D deficiency is defined as having circulating 25(OH)D concentrations < 20 ng/mL.  Approximately one billion people worldwide fit into this category.  Vitamin D deficiency is associated with geographic location, as people who live further from the equator have less sunlight exposure.  Darker complected skin has been shown to reduce the ability of UV-B radiation to synthesize vitamin D in the skin.  Additionally, advanced age decreases the ability of skin to synthesize the vitamin.  

Vitamin D Deficiency

Vitamin D insufficiency affects almost 50% of the population worldwide. An estimated 1 billion people worldwide, across all ethnicities and age groups, have a vitamin D deficiency (VDD). VDD results in abnormalities in calcium, phosphorus, and bone metabolism. the major cause of VDD is inadequate exposure to sunlight. VDD causes a decrease in the absorption of dietary calcium and phosphorus, resulting in an increase in PTH levels.

Vitamin D Insufficiency

Vitamin D is a nutrient that is synthesized endogenously from 7-dehydrocholesterol by the skin after UV-B exposure from sunlight and is also found in some foods.  It is processed in the liver by the enzyme 25-hydroxylase to form 25-hydroxyvitamin D [25(OH)D].  It is then further processed by the enzyme 1-ɑ-hydroxylase (CYP27B1) to form the biologically active molecule 1,25-dihydroxyvitamin D [1,25(OH)2D].  This molecule can then go on to interact with vitamin D receptors in a variety of tissues which are involved in physiological processes such as the endocrine regulation of minerals and some cell signaling mechanisms related to immune responses.  

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These vitamins are able to be stored in the body in fatty tissue and i the liver. Vitamins A, D, E, and K are fat-soluble. 

Fat-Soluble Vitamins

Zwart, S. R., & Smith, S. M. (2020). Vitamin D and COVID-19: Lessons from Spaceflight Analogs. The Journal of Nutrition.
https://academic.oup.com/jn/article/doi/10.1093/jn/nxaa233/5876210
doi: 10.1093/jn/nxaa233

Vitamin D and COVID-19: Lessons from Spaceflight Analogs

Vitamin A is a fat-soluble vitamin involved in:
- Immune function
- Bone health
- Vision
- Cell production and communication
- Growth and development

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