Back in 2019, I tested low on Vitamin D3. Because vitamin D3 helps regulate mood, I believe my deficiency was a factor in the depression that I was faced with then, and which I wrote about here: www.countryripples1.blog/2019/06/05-depression-a-heavy-spirit.
Vitamin D3 not only regulates mood, it also helps with processes in the muscles and nerves. It is needed to absorb calcium, which makes it important for healthy bones.
Because vitamin D3 boosts the immune system, low levels of it can promote COVID-19 infections.
Sunshine is a well-known source of Vitamin D3, but if you live in the Northern Hemisphere in the winter, (as I do), it can be hard to get enough of it. Another deterrent to getting enough D is that vitamins you take may not be easily absorbed by the body and may simply be eliminated without the body getting benefit.
Your doctor can do a simple blood test to show whether you have a deficiency or an insufficiency in the vitamin, and he or she can tell you the amount you need to take.