 Postmenopausal women with vitamin D insufficiency will participate in this one-year study. We will study the change in intestinal calcium absorption from baseline vitamin D insufficiency to follow up vitamin D repletion and whether increased absorption results in subsequent increased retention of calcium within bone over the one-year interval as measured by bone densitometry. We will also study the effect of vitamin D repletion upon whole body muscle mass, quality of life and physical function. 