Dietary Vitamin D and Its Metabolites Non-Genomically Stabilize the Endothelium
This is probably the most important paper on vitamin D3 that has ever been published. It changes our understanding of how cholecalciferol is used in the body and we must change the advice we are providing about vitamin D3 to take account of the fact that in it's basic form it is ACTIVE.

Because in it's basic form Cholecalciferol only has a half life of 19-25hrs we have to take it, or create it from UVB exposure, DAILY.
And because every cell has a vitamin D receptor and will bind to free cholecalciferol if it is spare, we have to raise 25(OH)d to the point that there is spare free unbound cholecalciferol bioavailable in tissue.
Work by Hollis shows measurable amounts of cholecalciferol are available at or above 25(OH)D levels of 50ng/ml 125nmol/l