Is Ostelin metabolized the same way as Rocaltrol? Why are kidney patients taken off Ostelin and told to take Rocaltrol instead?

The form of Vitamin D in Ostelin is cholecalciferol (Vit D3) which is actually a prohormone (a hormone precursor). In the body it is transformed (as needed) by the liver into 25-hydroxyvitamin D3 and then by the kidney into the active form which is 1,25-dihydroxyvitamin D3 (also called calcitriol – the form of Vitamin D in Rocaltrol). People with kidney disease would have a reduced capacity to produce the active form from the precursor and so they would be given the directly-active form. This form has a short life in the body (several hours) and needs to be given regularly.