Impact of seasonality on the dynamics of native Vitamin D repletion in long-term renal transplant patients

Background: Renal transplant recipients (RTRs) are often Vitamin D (VitD) depleted as a result of both chronic kidney disease and mandated sun avoidance behaviours. Repleting VitD may be warranted, but how, and for how long, is unknown, as is the impact of seasonality on the success of repletion. We investigated the impact of seasonality on VitD status following VitD repletion in a large cohort of stable, long-term RTRs. Methods: Serum 25-hydroxyvitamin D [25(OH)D] concentrations and bone biochemistry parameters were analysed from 102 VitD repletion courses in 98 RTRs that had undergone VitD repletion. Repletion was delivered over 6 months with either 240 000 IU colecalciferol if pre-repletion serum VitD was between 20 and 50 nmol/L, or with 360 000 IU if VitD was <20 nmol/L. Twelve months post-repletion 25(OH)D and parathyroid hormone (PTH) were available for 75 patients. Results: At baseline, 25(OH)D was 20.1 ± 1.0 nmol/L, increasing to 65.4 ± 1.8 nmol/L following repletion (+7.55 nmol/L/month, P < 0.0001). Twelve months post-repletion and after no further VitD administration, 25(OH)D fell to 35.4 ± 1.8 nmol/L (14.2 ± 0.7 ng/mL; -2.50 nmol/L/month, P < 0.0001). PTH followed the opposite trend with baseline, repletion-end and post-repletion values being 144.2 ± 12.0, 109.6 ± 7.5 and 129.2 ± 11.4 ng/L, respectively. VitD repletion during the summer was associated with significantly higher at repletion-end 25(OH)D compared with any other time of year [summer 80.9 ± 4.0, autumn 64.1 ± 3.0 (P = 0.002), winter 48.9 ± 3.0 (P <0.001), spring 63.8 ± 2.5 nmol/L (P <0.001)]. There was no hypercalcaemia during repletion and renal transplant function remained stable without any evidence of allograft rejection. Conclusions: VitD repletion can safely and effectively be achieved in the majority of chronic stable RTRs using a 6-month bolus intermediate-dose schedule. Winter repletion is associated with an inadequate response in 25(OH)D; however, all patients experience a post-repletion fall towards deficiency in the absence of maintenance supplementation, irrespective of the season of repletion.