Important: take Vitamin K2 and Vitamin A when supplementing with Vitamin D3 (cholecalciferol). See Health and Vitality Start with getting Key Nutrients.
The science in this particular article doesn’t even matter: Vitamin D has long been known to have influence on a positive immune system response. Every cell in the body has receptors for Vitamin D, and most people are deficient in Vitamin D. Those with dark skin are particularly at risk in winter and at northern latitudes or just from working inside.
The human body can produce 10000 (ten thousand) IU of Vitamin D via mid-day sun exposure to UV-B rays over a good portion of the body, for fair skin** (expose the body and do NOT use sunblock except on sensitive areas like face)*.
Remember, correlation is NOT causation. But this is all about risk assessment. If you are even little Vitamin D deficient, it’s just insanely bad risk management to not address an actual or probable or unkknown Vitamin D deficiency in the face of COVID-19.
* The ultra-conservative medical advice that more than 1000 IU may be harmful seems out of touch in the context of the body being able to produce 10000 IU in 20 minutes with fair skin, but see the Vitamin K2 discussion below.
** Dark-skinned and well-tanned people need much longer sun/UV-B exposure. Very dark skin might require all day, with some studies showing a 99% attenuation of UV-B rays. Such people are likely to be severely Vitamin D deficient in northern latitudes and/or when working inside most days. Thus if your skin is very dark, deficiency should be a prime concern.
The authors propose that, whereas optimising vitamin D levels will certainly benefit bone and muscle health, the data suggests that it is also likely to reduce serious COVID-19 complications. This may be because vitamin D is important in regulation and suppression of the inflammatory cytokine response, which causes the severe consequences of COVID-19 and 'acute respiratory distress syndrome' associated with ventilation and death.
If you don’t know, then assume you are Vitamin D deficient: just make sure you get 1000 IU of Vitamin D3 every day, or at least get out in mid-day sun for 20 minutes every day (more for darker skin).
When I supplement, I go with 5000 IU Vitamin D3 as cholecalciferol (such as YounGlo Research Vitamin D3 @AMAZON). In years past, I used a lot more (up to 50000 IU), and I think I was both Vitamin K2 deficient and magnesium deficient and the excess calcium may have been harmful to me by contributing to calcium deposits in my heart. So take Vitamin D3 with Vitamin K2 and magnesium. Vitamin K2 activates (carboxylates) key proteins that send that calcium where it belongs—into bones and magnesium is crucial to maintain the calcium/magnesium balance in cells (I use both emu oil and MenaQ7 for K2).
I have fair skin, and I don’t supplement in summer since I get a lot of sun—one bike ride and my body maxes-out on all it needs. But if you work indoors all day, you are almost certainly deficient, though perhaps weekend sun exposure can mitigate that. Also, those with dark skin can need hours of mid-day exposure for the body to produced enough Vitamin D.
Note that Vitamin D is produced naturally by the body from sun exposure using cholesterol, one of the most important life-sustaining substance in the body . It makes me wonder if the millions steadily and inexorably poisoning themselves with statins for no change in mortality are at higher risk of Vitamin D deficiency (not to mention far higher risks of diabetes and some kinds of strokes).