Vitamin D3 (cholecalciferol) is the preferred form for supplementation. It is the kind naturally found in humans and easiest to absorb (from Dr. Weil http://www.drweil.com/drw/u/ART02812/vitamin-d ).   Adequate vitamin D is necessary to good health. It is a pro-hormone for various hormones.   While research is limited regarding which form is best, it seems vitamin D3 in oil is better absorbed than that in tablet form http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3033429/ Sublingual may be the best