Vitamin D is both a paracrine (cell-to-cell) and autocrine (self) signaling molecule.  All cells in the immune system either express vitamin D receptors or have the ability to.  Additionally, they are all sensitive to the activity of its biologically active counterpart 1,25(OH)2D.  Vitamin D deficiency may weaken the body's control of immune responses and impair other physiological processes that involve vitamin D, including regulation of the renin-angiotensin system (RAS).  Therefore, these phenomena may make the body more susceptible to a SARS-CoV-2 infection.  

Vitamin D Deficiency May Exacerbate SARS-CoV-2 Infections

It is linked to Restless Leg Syndrome symptoms including low-quality sleep, increased pain perception, and nocturnal restlessness. 

VDD in children has been associated with decreased sleep time, sleep efficacy, and delayed bedtimes. It also leads to pain hypersensitivity by modulating the opiate signaling system, which is important for children with ASD.

There is also a link to depression and autism.

Vitamin D Depletion, Sleep, and Related Disorders

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.  

Michigan State University

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.  

Vitamin D

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 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

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.

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