Vitamin D Binding Protein Is Not Involved in Vitamin D Deficiency in Patients with Chronic Kidney Disease

Objective. This study was designed to evaluate vitamin D status with separate determination of 25-OH D-2 and 25-OH D-3 and its relationship to vitamin D binding protein (VDBP) in patients with chronic kidney disease (CKD) and long-term haemodialysis patients (HD). Methods. 45 CKD patients, 103 HD patients, and 25 controls (C) were included. Plasma vitamin D concentrations were determined using chromatography and VDBP in serum and urine in CKD using enzyme immunoassay. Results. Plasma vitamin D levels were lower in CKD (30.16 +/- 16.74 ng/mL) and HD(18.85 +/- 15.85 ng/mL) versus C(48.72 +/- 18.35 ng/mL), P < 0.0001. 25-OH D-3 was the dominant form of vitamin D. Serum VDBP was higher in CKD (273.2 +/- 93.8 ug/mL) versus C (222 +/- 87.6 ug/mL) and HD (213.8 +/- 70.9 ug/mL), P = 0.0003. Vitamin D/VDBP ratio was the highest in C and the lowest in HD; however, there was no correlation between vitamin D and VDBP. Urinary concentration of VDBP in CKD (0.25 +/- 0.13 ug/mL) correlated with proteinuria (r = 0.43, P = 0.003). Conclusions. Plasma levels of vitamin D are decreased in CKD patients and especially in HD patients. 25-OH D-3 was the major form of vitamin D. Despite urinary losses of VDBP, CKD patients had higher serum VDBP concentrations, indicating compensatory enhanced production. Vitamin D binding protein is not involved in vitamin D deficiency.