Smoking inhibits the expression of CYP27B1 (51) (the key enzyme required for activation of VD), reduces serum PTH level, increases the expression of CYP24A1 (52) (the key enzyme required for breakdown of VD), thereby reducing serum VD levels and cognitive impairment (53) (Figure 3). This evidence concerns the gene CYP24A1 and Cognitive impairment.