Objective: To determine vitamin D status in a subtropical climate among an unselected, referred predialysis chronic kidney disease (CKD) population; assess risks and correlates; and review whether higher 25-hydroxyvitamin D (25-OHD) concentration can mitigate the decrement in circulating 1,25-dihydroxyvitamin D (1,25-OHD) normally encountered with advancing CKD.
Design: Prospective cross-sectional cohort study. Setting: Renal unit in Brisbane, Australia (27°28' S).
Subjects: Five hundred ninety-three consecutive CKD patients (stage 1 to 5). Main Outcome Measure: 25-OHD insufficiency (concentrations: 15 to 30 ng/mL) and deficiency (<15 ng/mL), bone-mineral parameters, including 1,25-OHD, calcium, and phosphate. Results: Despite potentially higher environmental ultraviolet (UV) exposure, only 48% of patients with CKD were 25-OHD sufficient. Traditional risks for hypovitaminosis D were maintained, and sufficiency was independently predicted by testing in the summer/autumn period (odds ratio [OR]: 2.77, 95% confidence interval [CI]: 1.88 to 4.08, P < .001), male gender (OR: 2.18, 95%CI: 1.46 to 3.24, P < .001), Caucasian race (OR: 2.28, 95%CI: 1.37 to 3.78, P = .001), hypoalbuminemia (OR: 0.47, 95%CI: 0.25 to 0.85, P = .01), macroalbuminuria (OR: 0.60, 95%CI: 0.39 to 0.92, P = .02), and normal body mass index (OR: 1.94, 95%CI: 1.22 to 3.07, P = .005). Vitamin D sufficiency was also associated with higher corrected calcium (0.4 mg/dL increments; OR: 1.29, 95%CI: 1.08 to 1.55, P = .005). Although circulating 25-OHD concentrations were relatively maintained across the range of renal function observed, 1,25-OHD concentrations decreased with advancing CKD.
Conclusion: 25-OHD insufficiency is mitigated but still highly prevalent in patients with CKD in a high ambient UV environment. Despite the maintenance of relatively higher 25-OHD concentrations with advancing CKD, substrate availability does not appear to be a major determinant of circulating 1,25-OHD.