This study is double blind, parallel design, randomized clinical trial that will assess genetic and metabolic characteristics that modify the response to cholecalciferol treatment. . Eligible participants will be randomly assigned to receive cholecalciferol treatment (2,000 international units of cholecalciferol daily by mouth) or placebo in a 3:1 ratio for a total duration of 16-weeks. The planned sample size is 1,600. The primary aim of this study is to identify genetic polymorphisms, clinical characteristics, and biomarkers that modify the biologic response to vitamin D3 treatment, assessed by changes in serum concentrations of parathyroid hormone (PTH) and 1,25(OH)2D and urine calcium excretion.