Bioavailable Vitamin D Levels in Children With First Episode Nephrotic Syndrome: A Longitudinal Study.

OBJECTIVE: To estimate the levels of serum bioavailable vitamin D in children presenting with first episode nephrotic syndrome (FENS) at diagnosis and after 4 weeks of standard steroid therapy while the child is in remission, and compare the same with age-sex matched healthy controls.

METHODS: We included children aged 1 month to 12 years presenting as FENS and estimated the serum calcium, phosphorus, alkaline phosphatase, 25-hydroxy vitamin D (25(OH)D), parathormone, serum and urine Vitamin D binding protein (VDBP) at diagnosis and after 4 weeks of standard steroid therapy while the child is in remission. We also included age-sex matched healthy controls for comparison. Bioavailable and free 25(OH)D were estimated at enrolment and at 4 weeks of therapy.

RESULTS: The mean 25(OH)D level (ng/mL) in children with FENS was 11.3 (6.1) at diagnosis and 13.6 (6.2) at 4 weeks follow-up, while corresponding values in healthy controls was 16 (7) ng/mL. The median (IQR) serum VDBP level in FENS at enrolment was 223.0 (144, 305.5) μg/mL. There was significant correlation of serum VDBP with serum albumin levels (P = 0.04). At 4 weeks (remission), the VDBP levels increased to 554.5 (383, 644.75) μg/mL (P < 0.001). The median (IQR) free 25(OH)D levels (pg/mL) in children with FENS was 1.07 (0.8, 1.6) at enrolment and 0.53 (0.37, 0.86) at 4 weeks follow-up. The median (IQR) bioavailable vitamin D in FENS during proteinuria was 0.58 (0.4, 0.83) ng/ml, much lower as compared to controls 0.97(0.85, 1.08) ng/ml (P < 0.001). On follow-up at 4 weeks of remission the median (IQR) bioavailable vitamin D levels increased to 0.87 (0.59, 1.42) ng/ml (P = 0.015). There was a very strong positive correlation between free vitamin D and bioavailable vitamin D (r = 0.9, P < 0.001); a strong negative correlation between serum VDBP and bioavailable vitamin D (r = - 0.69, P < 0.001). There was a positive correlation between 25-hroxy vitamin D and bioavailable vitamin D (r = 0.63, P < 0.001). A positive correlation was seen between urine UP/UC and urine VDBP (r = 0.51, P = 0.004). Serum VDBP and serum albumin showed statistically significant positive correlation (r = 0.37, P < 0.05).

CONCLUSION: Children with FENS are deficient in vitamin D. The free and bioavailable vitamin D levels are reduced in children with FENS during the proteinuric phase. Further studies to assess the association between bioavailable vitamin D and 25(OH)D with bone mineral density are needed in children with nephrotic syndrome to validate the utility of bioavailable vitamin D in clinical practice.