However, it is unclear whether low vitamin D exposure contributed to the
onset of diabetes or whether low vitamin D status was a consequence of diabetes. The major circulating form of vitamin
D, 25-hydroxycholecalciferol (25D3), circulates bound to vitamin D–binding protein (DBP) until the 25D3-DBP complex is internalized through endocytosis and activated to
1,25-dihydroxycholecalciferol by the renal proximal tubule
cells or reabsorbed into the blood as 25D3