For further information about vitamin D, I always recommend one particular episode [1] of the TWiT Security Now podcast where the host Steve Gibson talks for about an hour on his research into the effects of vitamin D and how he thinks that taking it is important.

I can't vouch for the medical accuracy of what he is saying, but he does tend to research things pretty thoroughly before broadcasting them, and I generally trust his conclusions.

[1]https://www.grc.com/sn/sn-209.htm