Bioavailable vitamin D levels are reduced and correlate with bone mineral density and markers of mineral metabolism in adults with nephrotic syndrome

Aim: Blood levels of 25-hydroxyvitamin D [25(OH) D] are reduced in patients with nephrotic syndrome (NS). The lowering is thought to be due to urinary loss of vitamin D binding protein (DBP). A link between vitamin D deficiency and bone disease or markers of mineral metabolismhas not yet been shown in NS. We hypothesized that alterations in bioavailable vitamin D levels might be linked to these abnormalities in NS. Methods: We measured circulating levels of 25(OH) D, 1,25-dihydroxyvitamin D [1,25(OH) 2D], DBP, serum albumin and intact parathyroid hormone (iPTH) in 106 adults with sporadic idiopathic NS and 40 healthy controls. Bioavailable vitamin D was calculated from previously validated formulae. Bone mineral density (BMD) was measured at left hip (neck of femur) by DEXA. Results: Compared to healthy controls, total and bioavailable 25(OH) D levels were significantly reduced in patients with NS as compared to healthy controls. Among the nephrotic patients, BMD was positively correlated with bioavailable 25(OH) D (r = 0.358; P = 0.0002) but not with total 25(OH) D (r = 0.174; P = 0.079). Total 1,25(OH) 2D and bioavailable 1,25(OH) 2D did not correlate with BMD (r = 0.131; P = 0.206 and r = 0.107, P = 0.295). Bioavailable 25(OH) D levels showed a strong inverse correlation with iPTH on univariate (r = -0.457; P < 0.0001) and multivariate (beta = similar to 0.453, P<0.0001) analyses. Conclusions: We conclude that bioavailable 25(OH) D is a better measure of vitamin D status with respect of BMD and mineral metabolism in patients of nephrotic syndrome.