In the parent study (Personalized Prevention of Colorectal Cancer Trial, NCT01105169), the investigators proposed to measure blood concentration of total 25(OH)D as a secondary aim using Elisa approach. However, following the novel finding of Mg-vitamin D interaction published by the investigators in 2013, they submitted a separate grant application to NCI which was funded in 2014. In the new study, the investigators proposed to use a LC-MS approach, which is more accurate and specific than an Elisa method, to measure 5 vitamin D metabolites. This new ancillary study allows the investigators to evaluate whether Mg supplementation differentially affects vitamin D synthesis and metabolism dependent on baseline serum 25(OH)D levels using existing biospecimens collected in our double-blind placebo-controlled randomized chemoprevention trial.