Just a quick update, since I was hypothesizing, when this topic was first started, that it just didn't make sense for cells not be able to generate 25-D directly from the precursors, such as ergosterol and 7-dehydrocholesterol, without any sunlight exposure being necessary.
Since then our knowledge of Vit D metabolism has been turned on its head. There are a lot more enzymes producing 25-D than just CYP27A1, although exactly how everything works just gets more confusing with every discovery.
