To the Editor In their analysis of data from a prospective cohort study, Miller and colleagues1 found that cross-sectional differences in cognitive ability were only weakly associated with vitamin D concentrations at baseline, whereas they found small statistically significant differences in trajectories of cognitive performance during follow-up according to baseline vitamin D status. Preclinical studies certainly provide a plausible mechanism linking vitamin D deficiency to cognitive impairment, but, to my knowledge, these mechanisms have not been investigated or demonstrated in humans. Moreover, causality cannot be established from observational studies. Statistically significant, but small or weak, associations found in observational studies, such as those reported in this cohort study, are likely to be false and attributable to various unmeasured variables and other sources of bias. A particular concern in this study is the absence of vitamin D data prior to baseline and during the course of follow-up.
Howland RH. Association Between Vitamin D Status and Cognitive Decline. JAMA Neurol. 2016;73(6):762. doi:10.1001/jamaneurol.2016.0553