All eligible subjects will have the initial study entry evaluation performed at the SLRHC Pediatric Body Composition Unit by virtue of their participation in the cross-sectional study (Section D.2.f). As the qualifying serum 25(OH)D level will be known prior to the cross-sectional study visit, study participants eligible for the randomized clinical trial can be randomly assigned (see Methods) during the cross-sectional study visit to one of the two treatment arms; in such cases, the cross-sectional visit will also function as the entry/baseline visit. Each subject will be given the first dose of vitamin D3/placebo and the first 30 day supply of calcium/placebo will be dispensed. females of child bearing capacity