Seasonal Variation of Vitamin D Status in Long-Term Kidney Transplant Recipients

Background. There is little information about circannual rhythm of vitamin D level in kidney transplantation (KTx) patients. Material and Methods. In 71 patients (27 females; 44 males) in the long term after KTx (5.5 +/- 2.6 years) during the winter and summer months plasma concentration of 25-hydroxyvitamin D (25(OH)D), 1,25-hydroxyvitamin D, parathormone (PTH), fibroblast growth factor 23 (FGF-23), calcium, and phosphorus were assessed. Vitamin D status was classified according to 25(OH)D level (ie, insufficiency, <= 30 ng/mL; deficiency, < 15 ng/mL). Results. In this study, 96% of KTx patients had vitamin D insufficiency including 37% deficiency during winter and 89% of KTx patients had vitamin D insufficiency and 24% had vitamin D deficiency, respectively, during summer. Mean 25(OH)D level during winter was lower than in summer (17.4 +/- 7.1 vs 20.2 +/- 7.2 ng/mL; P = .02), similar to calcitriol (163.6 +/- 37.4 vs 284.5 +/- 77.8 pmol/L; P = .001). There were no significant differences in winter and summer levels of calcium, phosphorus, and PTH. The 25(OH)D level was significantly higher in patients with estimated glomerular filtration rate (eGFR) >= 45 mL/min/1.73 m(2) compared with those with lower eGFR (21.6 +/- 7.5 vs 17.6 +/- 6.0; P = .02) only in the summer time. Conclusions. Most of the KTx patients have vitamin D insufficiency during both winter and summer with higher concentration of vitamin D metabolites in summer. Other factors than graft function may have an impact on vitamin D levels in KTx patients.