I go by the fact that for a millenia or so, we ran around basically naked under a tropical sun. When we moved closer to the poles, we had to eat lots of cod liver oil and other natural sources of D, and also got paler so we could still manufacture it despite being covered in clothing... Germans and Scandinavians became nudists.

On the advice of a knowledgeable friend, I have switched to topical D with magnesium; seems to work better, at least at the moment when I am dealing with a low point of my health journey.