Effect of Vitamin D3 Supplementation on Arterial and Bone Remodeling in Chronic Kidney Disease Patients

Structural changes to large arteries and abnormalities in mineral and bone metabolism are frequent manifestations in patients with chronic kidney disease (CKD). Together, they contribute in a large part to the heightened morbidity and mortality observed in this population. Epidemiological data in end stage renal disease and in the general population suggest the existence of a bone-vascular axis. Metabolic bone disease (MBD) in CKD encompasses altered bone remodeling and the propensity for vascular calcification. These pathological processes are driven by the multiple disorders of mineral metabolism in CKD, among them, abnormalities of vitamin D metabolism. Vitamin D deficiency \[25(OH)D\] is widely observed in CKD patients and has been associated with an increased rate of cardiovascular events in both the general population and in CKD patients. The mechanisms involved are not clearly established. Vitamin D influences blood pressure through effects on the renin-angiotensin system (via a vitamin D response element in the renin gene), vascular smooth muscle cells and cardiomyocyte proliferation and hypertrophy, vascular inflammation and calcification. Vitamin D deficiency has been associated with large arterial stiffness in end-stage renal disease patients. Aortic and carotid stiffness are independent predictors of cardiovascular and overall mortality in end-stage renal disease patients. Large arterial remodeling and stiffening could be the missing pathogenic link between vitamin D deficiency and increased cardiovascular event rate. In terms of mineral metabolism, many CKD patients develop secondary hyperparathyroidism. This results from a combination of hyperphosphatemia, hypocalcemia and low levels of active Vitamin D \[1,25(OH)D2\]. Since several observational studies have shown that parathyroid hormone (PTH) levels are inversely correlated with blood 25(OH)D levels, it is possible that 25(OH)D deficiency may also be contributing to the hyperparathyroid state. Secondary hyperparathyroidism contributes to cardiovascular risk and to bone disease. Elevated PTH has been associated with large arterial stiffness and remodeling. Elevated PTH is also associated with high bone turnover and participates in the development of bone disease of CKD-MBD. Bone disease in CKD-MBD comprises abnormalities in bone turnover, mineralization, linear growth and strength. Bone biopsy remains the gold standard for evaluation of bone disease in CKD but its invasive nature limits its practice. Serum biomarkers of bone remodeling allow direct estimation of bone remodeling but lack evaluation and precision. Among them, guidelines issued by Kidney Disease Improving Global Outcomes (KDIGO) recommend PTH (1-84) and bone specific alkaline phosphatase (BSALP). Other biomarkers exist including osteocalcin, osteoprotegerin, tartrate-resistant acid phosphate-5b (TRAP-5b), pyridinoline and deoxypyridinoline, procollagen type 1 amino-terminal extension peptides, C terminal cross-link (CTX) , FGF-23 and fetuin-A. The major limitation of the use of these biomarkers is their kidney-dependent elimination that affects their measured levels depending on the degree of kidney dysfunction. We have chosen to study, in addition to PTH (1-84) and BSALP, CTX, osteoprotegerin, osteocalcin, fetuin-A and fibroblast growth factor-23 (FGF-23) because a relationship between these biomarkers and arterial disease has never been demonstrated. This study seeks to compare the impact of standard versus aggressive Vitamin D3 supplementation (in Vitamin D deficient CKD patients) on important vascular and bone health endpoints.