I am reading your link. But for sure some people who get enough sun are not making enough D3 on their skin. They have a poor conversion process. This is why all Americans should take vitamin D3 supplements. 5000 units daily at least.
I take lots of D3 to boost my immune system. But D3 does many other good things. It acts like a super hormone that stimulates certain body systems into action. It is also a mood elevator. Makes me feel more positive.