Total 25(OH) vitamin D, free 25(OH) vitamin D and markers of bone turnover in cirrhotics with and without synthetic dysfunction

Abstract

Current clinical assays for total 25-hydroxy (OH) vitamin D measure vitamin D bound to vitamin D-binding protein (DBP) and albumin plus unbound ('free') D. We investigated the relationship between total and free 25(OH)D with bone metabolism markers in normal (>3.5 g/dl) vs. low (≤3.5 g/dl) albumin cirrhotics. Eighty-two cirrhotics underwent measurement of free and total 25(OH)D by immunoassay, DBP and markers of bone metabolism [intact parathyroid hormone (iPTH), C-telopeptide (CTX), bone-specific alkaline phosphatase (BSAP), osteocalcin, amino-terminal pro-peptide of type 1-collagen (P1NP)]. Pearson's coefficients assessed relevant associations. Cirrhotics with low (n = 54) vs. normal (n = 28) albumin had lower total 25(OH)D (12.1 vs. 21.7 ng/ml), free 25(OH)D (6.2vs.8.6 pg/ml) and DBP(91.4 vs. 140.3 μg/ml) [P < 0.01 for each]. iPTH was similar in low and normal albumin groups (33 vs. 28 pg/ml; P = 0.38), although serum CTX(0.46vs.0.28 ng/ml) and BSAP(31.7 vs. 24.8 μg/L) were increased (P < 0.01). An inverse relationship was observed between total 25(OH)D and iPTH in normal (r = -0.47, P = 0.01) but not low albumin cirrhotics (r = 0.07, P = 0.62). Similar associations were seen between free 25(OH)D and iPTH(Normal: r = -0.46, P = 0.01; Low: r = -0.03, P = 0.84). BSAP, osteocalcin and P1NP were elevated above the normal range in all cirrhotics but not consistently associated with total or free 25(OH)D. Cirrhotics with low vs. normal albumin have lower levels of DBP, total and free 25(OH)D. The expected relationship between total or free 25(OH)D with iPTH was observed in normal but not in low albumin cirrhotics, demonstrating that total 25(OH)D is not an accurate marker of bioactive vitamin D status in cirrhotics with synthetic dysfunction. Additional investigation into the role of vitamin D supplementation and its impact on bone mineral homoeostasis in this population is needed.