25-hydroxyvitamin D levels and bone histomorphometry in hemodialysis renal osteodystrophy

The importance of 25-hydroxyvitamin D (25-OHD) serum levels in hemodialysis chronic renal failure has not been so far histologically evaluated. Information still lacking relate to the effect of 25-OHD deficiency on serum parathyroid hormone (PTH) levels and on bone and its relationship with calcitriol levels.

This retrospective study has been performed on a cohort of 104 patients on hemodialysis from more than 12 months, subjected to transiliac bone biopsy for histologic, histomorphometric, and histodynamic evaluation. The patients, 61 males and 43 females, mean age 52.9 +/- 11.7 years, hemodialysis length 97.4 +/- 61.4 months, were treated with standard hemodialysis and did not receive any vitamin D supplementation. Treatment with calcitriol was not underway at the time of the biopsy. Transiliac bone biopsies were performed after double tetracycline labels. In addition, serum intact PTH (iPTH), alkaline phosphatase, and 25-OHD were measured. Calcitriol serum levels was also measured in a subset of patients (N= 53). The patients were divided according to serum 25-OHD levels in three groups: (1) 0 to 15 (15 patients), (2) 15 to 30 (38 patients), and (3) >30 ng/mL (51 patients).

There was no significant difference in average age, hemodialysis age, serum PTH [490 +/- 494, 670 +/- 627, and 489 +/- 436 pg/mL, respectively (mean +/- SD)], alkaline phosphatase, and calcitriol between the three groups. The parameters double-labeled surface, trabecular mineralizing surface, and bone formation rate were significantly lower in group 1 than in the other groups (P < 0.03, < 0.03, and < 0.02, respectively). Osteoblast surface and adjusted apposition rate were borderline significantly lower in group 1 (P < 0.06 and < 0.10). There was no statistical difference in the biochemical and bone parameters between groups 2 and 3. A positive significant correlation was found between several bone static and dynamic parameters and 25-OHD levels in the range 0 to 30 ng/mL, showing a vitamin D dependence of bone turnover at these serum levels. However, actual evidence of an effect on bone of 25-OHD deficiency was found at serum levels below 20 ng/mL. With increasing 25-OHD levels beyond 40 ng/mL, a downslope of parameters of bone turnover was also observed.

Since PTH serum levels are equally elevated in low and high 25-OHD patients, while calcitriol levels are constantly low, an effect of 25-OHD deficiency (group 1) on bone, consisting of a mineralization and bone formation defect, can be hypothesized. The effect of vitamin D deficiency or bone turnover is found below 20 ng/mL. The optimal level of 25-OHD appears to be in the order of 20 to 40 ng/mL. Levels of the D metabolite higher than 40 ng/mL are accompanied by a reduction of bone turnover.