Blood samples were obtained at baseline during the run-in period from all willing participants — 16,956 of the 25,871 persons who underwent randomization (65.5%). At no cost to the trial, Quest Diagnostics donated and performed serum 25-hydroxyvitamin D assays with the use of liquid chromatography–tandem mass spectrometry on all samples that could be analyzed. Quest had no role in the design of the trial, accrual of the data (other than the assays), analysis of the data (other than assay standards), or preparation of the manuscript. Our trial participated in the vitamin D standardization program of the Centers for Disease Control and Prevention.