Kidney disease and vitamin D levels: 25-hydroxyvitamin D, 1,25-dihydroxyvitamin D, and VDR activation

A normal vitamin D status is essential for human health. Vitamin D deficiency is a recognized risk factor for all-cause mortality in normal individuals and in chronic kidney disease (CKD) patients. The link between vitamin D deficiency and death is a defective activation of the vitamin D receptor (VDR) by 1,25-dihydroxyvitamin D (calcitriol, the vitamin D hormone) to induce/repress genes that maintain mineral homeostasis and skeletal integrity, and prevent secondary hyperparathyroidism, hypertension, immune disorders, and renal and cardiovascular (CV) damage. The kidney is the main site for the conversion of 25-hydroxyvitamin D (25D) to circulating calcitriol, and therefore essential for the health benefits of endocrine VDR activation. The kidney is also essential for the uptake of 25D from the glomerular ultrafiltrate for its recycling to the circulation to maintain serum 25D levels, extrarenal calcitriol synthesis, and the prosurvival benefits of autocrine/paracrine VDR activation. Indeed, both calcitriol and vitamin D deficiency increase progressively in the course of CKD, and associate directly with accelerated disease progression and death. Therefore, the safe correction of calcitriol and vitamin D deficiency/insufficiency is becoming a high priority among nephrologists. This review updates the pathophysiology behind 25D and calcitriol deficiency and impaired VDR activation in CKD, the adequacy of current recommendations for vitamin D supplementation, and potential markers of the efficacy of therapy to prevent or slow the development of renal and CV lesions unrelated to parathyroid hormone suppression, a knowledge required for the design of trials to obtain evidence-based recommendations for vitamin D and calcitriol replacement at all stages of CKD.