In the subgroup analyses, a more significant association between vitamin D deficiency and dementia 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: 1.47, 95%CI: 1.10, 1.98, I2 = 65.9%, n = 6 studies vs. 1.28, 95%CI: 1.10, 1.49, I2 = 29.1%, n = 10 studies, respectively, Additional file 1: Fig. S6). Here, APOE is linked to vitamin D deficiency.