Yes, the very same paper. > Although results of an observational study, such as this one, need to be interpreted with caution, as done by the authors [1], due

Yes, the very same paper. > Although results of an observational study, such as this one, need to be interpreted with caution, as done by the authors [1], due to the potential of residual confounding or reverse causality (i.e., vitamin D insufficiency resulting from poor health status at baseline rather than vice versa), You need a control group with vitamin D deficiency but no other health issues to prove the hypothesis. They are just stating there's a correlation worth looking into.