APOE and vitamin D deficiency: In the subgroup analyses, a more significant association between vitamin D deficiency and AD was observed in the subgroup of studies that considered the serum 25(OH)D APOE gene compared to studies that did not consider the APOE gene (pooled HRs: 1.34, 95%CI: 1.13, 1.60, I2 = 72.9%, n = 5 vs. 1.27, 95%CI:1.05, 1.54, I2 = 28.1, n = 4, respectively).