Estimated GFR and Circulating 24,25-Dihydroxyvitamin D3 Concentration: A Participant-Level Analysis of 5 Cohort Studies and Clinical Trials

Background: Decreased glomerular filtration rate (GFR) leads to reduced production of 1,25-dihydroxyvitamin D-3 from 25-hydroxyvitamin D-3 (25[OH] D-3). Effects of low GFR on vitamin D catabolism are less well understood. We tested associations of estimated GFR (eGFR) with the circulating concentration of 24,25-dihydroxyvitamin D-3 (24,25[OH](2)D-3), the most abundant product of 25(OH) D-3 catabolism, across populations with a wide range of GFRs. Study Design: Cross-sectional study. Setting & Participants: 9,596 participants in 5 cohort studies and clinical trials: the Diabetes Control and Complications Trial (N = 1,193), Multi-Ethnic Study of Atherosclerosis (N = 6,470), Cardiovascular Health Study (N = 932), Seattle Kidney Study (N = 289), and Hemodialysis Study (N = 712). Predictor: eGFR. Outcome: Circulating 24,25(OH)(2)D-3 concentration. Measurements: GFR was estimated from serum creatinine using the Chronic Kidney Disease Epidemiology Collaboration equation. Vitamin D metabolites were measured by mass spectrometry. Results: Circulating 24,25(OH)(2)D-3 concentration was correlated with circulating 25(OH)D-3 concentration (Pearson r range, 0.64-0.88). This correlation was weaker with lower eGFRs. Moreover, the increment in 24,25(OH)(2)D-3 concentration associated with higher 25(OH)D-3 concentration (slope) was lower with lower eGFRs: 2.06 (95% CI, 2.01-2.10), 1.77 (95% CI, 1.74-1.81), 1.55 (95% CI, 1.48-1.62), 1.17 (95% CI, 1.05-1.29), 0.92 (95% CI, 0.74-1.10), 0.61 (95% CI, 0.22-1.00), and 0.37 (95% CI, 0.35-0.39) ng/mL of 24,25(OH)(2)D-3 per 10 ng/mL of 25(OH)D-3 for eGFRs >= 90, 60-89, 45-59, 30-44, 15-29, and <15 mL/min/1.73 m(2) and end-stage renal disease treated with hemodialysis, respectively. As a result, at a 25(OH)D-3 concentration of 20 ng/mL, mean 24,25(OH)(2)D-3 concentrations were 2.92 (95% CI, 2.87-2.96), 2.68 (95% CI, 2.64-2.72), 2.35 (95% CI, 2.26-2.45), 1.92 (95% CI, 1.74-2.10), 1.69 (95% CI, 1.43-1.95), 1.14 (95% CI, 0.62-1.66), and 1.04 (95% CI, 1.02-1.07) ng/mL for each category, respectively. This interaction was independent of other relevant clinical characteristics. Race, diabetes, urine albumin excretion, and circulating parathyroid hormone and fibroblast growth factor 23 concentrations more modestly modified the association of 24,25(OH)(2)D-3 with 25(OH)D-3. Limitations: Lack of direct pharmacokinetic measurements of vitamin D catabolism. Conclusions: Lower eGFR is associated strongly with reduced vitamin D catabolism, as measured by circulating 24,25(OH)(2)D-3 concentration. (C) 2014 by the National Kidney Foundation, Inc.