Fig. 3.

Changes in Vitamin D metabolism and the renal-bone axis with CKD. Arrows indicate direction of changes with CKD. With chronic kidney disease the combination of limited vitamin D intake and reduced renal capacity to activate 25(OH)D to 1,25(OH)₂D results to a chain reaction of changes in metabolism. A decrease in 1,25(OH)₂D results in a decrease in intestinal absorption of calcium. This stimulates PTH secretion which in turn increases bone resorption. Phosphate retention due to reduced kidney filtration capacity further stimulates PTH and the production and release of FGF23 from bone cells to increase renal phosphate excretion