Vitamin D3 (cholecalciferol) is the preferred form for supplementation. It is the kind naturally found in humans and easiest to absorb (from Dr. Weil ).   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 Sublingual may be the best