Beta amyloid clearance. Significant elevation of β-secretase amyloid cleaving enzyme (BACE1) level (A) led to increased production of Aβ oligomers (B) in the aged rats when compared to the young animals. Moreover, the significantly reduced levels of neprilysin (NEP), classical amyloid-degrading enzyme (C), in the aged control rats when compared to the aged vitamin D group possibly decreased amyloid clearance. Nonetheless, the development of this age-related neuropathology was abated by vitamin D supplementation. *P <0.05. Legend: A, aged; CON, control; vit, vitamin; Y, young.