The study is mainly based on the results of this meta-analysis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4103214/ https://www.ncbi.nlm.nih.gov/pmc/articles/

The study is mainly based on the results of this meta-analysis: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4103214/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4103214/ which showed an increase mortality risk for patients < 30ng/L They seem to say the confounders were corrected for in most studies but drawing a causal link between vit D and mortality purely from observational data is shaky. They say: >> Differences in the amount of adjustment may account for some of the heterogeneity that was present among studies. All studies adjusted for age, and most adjusted for other covariates. Even with extensive adjustment, residual confounding is always possible. Confounding is also possible by factors that were not measured, such as genetic predisposition. It is inherently difficult to completely exclude any possibility of confounding in observational studies, so caution in interpretation of the findings is appropriate. >> Supplements of 2000 to 4000 international units (IU) per day of vitamin D3 would produce an approximately 20 to 40 ng/mL increase in serum 25(OH)D.65,66 But the conclusion about the usefulness of Vit D supplementation can only be drawn from Randomised controlled trials: >> Two randomized controlled clinical trials using 2000 IU per day of vitamin D3 are under way in the United States, a study designed to replicate the study by Lappe et al.,8 and a new randomized controlled trial by Manson et al.67 These studies will not have results available for several years.