Vitamin D supplementation has previously been shown (via RCT) to improve outcomes for acute respiratory tract infections:
Martineau, Adrian R., David A. Jolliffe, Richard L. Hooper, Lauren Greenberg, John F. Aloia, Peter Bergman, Gal Dubnov-Raz, et al. “Vitamin D Supplementation to Prevent Acute Respiratory Tract Infections: Systematic Review and Meta-Analysis of Individual Participant Data.” BMJ 356 (February 15, 2017). https://doi.org/10.1136/bmj.i6583.
So of course it's no surprise that there's been interest/emerging evidence of it's role w/ COVID-19:
Daneshkhah, Ali, Adam Eshein, Hariharan Subramanian, Hemant Kumar Roy, and Vadim Backman. “The Role of Vitamin D in Suppressing Cytokine Storm in COVID-19 Patients and Associated Mortality.” Preprint. Infectious Diseases (except HIV/AIDS), April 10, 2020. https://doi.org/10.1101/2020.04.08.20058578.
Grant, William B., Henry Lahore, Sharon L. McDonnell, Carole A. Baggerly, Christine B. French, Jennifer L. Aliano, and Harjit P. Bhattoa. “Evidence That Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths.” Nutrients 12, no. 4 (April 2020): 988. https://doi.org/10.3390/nu12040988.
Meltzer, David O., Thomas J. Best, Hui Zhang, Tamara Vokes, Vineet Arora, and Julian Solway. “Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results.” JAMA Network Open 3, no. 9 (September 1, 2020): e2019722–e2019722. https://doi.org/10.1001/jamanetworkopen.2020.19722.
As for Vitamin D deficiency, it's pretty common. About 40%: "The National Health and Nutrition Examination Survey 2005 to 2006 data were analyzed for vitamin D levels in adult participants (N = 4495). Vitamin D deficiency was defined as a serum 25-hydroxyvitamin D concentrations ≤20 ng/mL (50 nmol/L). The overall prevalence rate of vitamin D deficiency was 41.6%, with the highest rate seen in blacks (82.1%), followed by Hispanics (69.2%)."
Forrest, Kimberly Y. Z., and Wendy L. Stuhldreher. “Prevalence and Correlates of Vitamin D Deficiency in US Adults.” Nutrition Research (New York, N.Y.) 31, no. 1 (January 2011): 48–54. https://doi.org/10.1016/j.nutres.2010.12.001.
That's about the same in the US as the current adult obesity rate (BMI >30) and estimated amount of NAFLD. Of course there's a fair bit of overlap between poor metabolic health and low Vitamin D levels, but the exact relationship is still not completely understood. Here's a recent discussion that's pretty readable:
Vranić, Luka, Ivana Mikolašević, and Sandra Milić. “Vitamin D Deficiency: Consequence or Cause of Obesity?” Medicina 55, no. 9 (August 28, 2019). https://doi.org/10.3390/medicina55090541.
Hewison, Martin. “An Update on Vitamin D and Human Immunity.” Clinical Endocrinology 76, no. 3 (March 2012): 315–25. https://doi.org/10.1111/j.1365-2265.2011.04261.x.
Vitamin D supplementation has previously been shown (via RCT) to improve outcomes for acute respiratory tract infections:
Martineau, Adrian R., David A. Jolliffe, Richard L. Hooper, Lauren Greenberg, John F. Aloia, Peter Bergman, Gal Dubnov-Raz, et al. “Vitamin D Supplementation to Prevent Acute Respiratory Tract Infections: Systematic Review and Meta-Analysis of Individual Participant Data.” BMJ 356 (February 15, 2017). https://doi.org/10.1136/bmj.i6583.
So of course it's no surprise that there's been interest/emerging evidence of it's role w/ COVID-19:
Daneshkhah, Ali, Adam Eshein, Hariharan Subramanian, Hemant Kumar Roy, and Vadim Backman. “The Role of Vitamin D in Suppressing Cytokine Storm in COVID-19 Patients and Associated Mortality.” Preprint. Infectious Diseases (except HIV/AIDS), April 10, 2020. https://doi.org/10.1101/2020.04.08.20058578.
Grant, William B., Henry Lahore, Sharon L. McDonnell, Carole A. Baggerly, Christine B. French, Jennifer L. Aliano, and Harjit P. Bhattoa. “Evidence That Vitamin D Supplementation Could Reduce Risk of Influenza and COVID-19 Infections and Deaths.” Nutrients 12, no. 4 (April 2020): 988. https://doi.org/10.3390/nu12040988.
Meltzer, David O., Thomas J. Best, Hui Zhang, Tamara Vokes, Vineet Arora, and Julian Solway. “Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results.” JAMA Network Open 3, no. 9 (September 1, 2020): e2019722–e2019722. https://doi.org/10.1001/jamanetworkopen.2020.19722.
As for Vitamin D deficiency, it's pretty common. About 40%: "The National Health and Nutrition Examination Survey 2005 to 2006 data were analyzed for vitamin D levels in adult participants (N = 4495). Vitamin D deficiency was defined as a serum 25-hydroxyvitamin D concentrations ≤20 ng/mL (50 nmol/L). The overall prevalence rate of vitamin D deficiency was 41.6%, with the highest rate seen in blacks (82.1%), followed by Hispanics (69.2%)."
Forrest, Kimberly Y. Z., and Wendy L. Stuhldreher. “Prevalence and Correlates of Vitamin D Deficiency in US Adults.” Nutrition Research (New York, N.Y.) 31, no. 1 (January 2011): 48–54. https://doi.org/10.1016/j.nutres.2010.12.001.
That's about the same in the US as the current adult obesity rate (BMI >30) and estimated amount of NAFLD. Of course there's a fair bit of overlap between poor metabolic health and low Vitamin D levels, but the exact relationship is still not completely understood. Here's a recent discussion that's pretty readable:
Vranić, Luka, Ivana Mikolašević, and Sandra Milić. “Vitamin D Deficiency: Consequence or Cause of Obesity?” Medicina 55, no. 9 (August 28, 2019). https://doi.org/10.3390/medicina55090541.