This morning I read this: http://ajcn.nutrition.org/content/84/4/694.full. And just recently En Bloc mentioned on here that she had a argument with her doctor about D3 versus D2, where she says her doctor claimed D3 may raise the serum levels but D2 works and maintains itself on a cellular level. I'm not sure how to interpret this: how would he measure that? Or more generally, what's the basis for such a claim? Are there studies supporting such a claim?