The investigators will conduct an intention-to-treat analysis to analyze differences in the serum level of vitamin D metabolites and vitamin D metabolite ratios within supplement and placebo group pairs at times 0, 1 month and 2 months. Differences in the pairwise log concentration of metabolites and log metabolite ratios will be tested by Wilcoxon Signed-Rank Tests and linear mixed-effect models (SAS Proc Mixed) using restricted maximum likelihood estimation to account for one time point as well as repeated measures of vitamin D metabolites. Potential confounding and effect modification by dietary intake of vitamin D will be assessed.