Re: Natural versus synthetic
That one was a test on "healthy volunteers",
This http://www.ncbi.nlm.nih.gov/pubmed/23386645 is interesting because "Of the adults, 82% were vitamin D insufficient (....) at the start of the study"
The conclusion of the paper is that " ingestion of 1000 IU vitamin D2 or vitamin D3 for 11 weeks was effective in raising total serum concentrations of 25(OH)D"