Losses of 25-hydroxyvitamin D in peritoneal fluid: possible mechanism for bone disease in uremic patients treated with chronic ambulatory peritoneal dialysis.

Despite a good control of serum phosphate levels, bone disease still occurs in patients treated with chronic ambulatory peritoneal dialysis (CAPD). The purpose of this study was to determine if 25-hydroxyvitamin D (25-OH-D) penetrates through the peritoneal membrane and whether chronic peritoneal dialysis may thereby decrease the serum 25-OH-D levels. In 10 CAPD-treated patients the serum 25-OH-D was 8.8 +/- 1.1 ng/ml, which is significantly lower than in the hemodialysis-treated group (15.5 +/- 2.3 ng/ml) and lower than in healthy controls (30.3 +/- 3.6 ng/ml). The mean total daily loss of 25-OH-D in the peritoneal fluid was 1,491 +/- 260 ng/day. The serum 25-OH-D binding capacity was significantly lower in the CAPD group than in normal controls. The mean daily loss of 25-OH-D binding capacity in the dialysate was 153 +/- 28 nmol/day. We conclude that permeability of the peritoneal membrane to middle-size proteins leads to loss of 25-OH-D binding protein, and this increases loss of 25-OH-D. Low levels of 25-OH-D in plasma may aggravate the symptoms of osteodystrophy of renal failure in patients treated with CAPD.