VDR and chronic kidney disease: Causes and risk factors for 25(OH) D deficiency or insufficiency observed in CKD and dialysis patients are involved, among them, one can consider: (1) age, female sex, adiposity, (2) proteinuria, (3) low physical activity, (4) peritoneal dialysis, (5) diabetes mellitus, (6) reduced VDR, reduced skin synthesis of vitamin D; (7) impaired 25(OH) D tubular reabsorption, (8) calcineurin inhibitor prescription, and (9) reduction of the liver CYP450 isoform in secondary hyperparathyroidism [219].