EFFECTS OF ASCORBIC-ACID AND PYRIDOXINE SUPPLEMENTATION ON OXALATE METABOLISM IN PERITONEAL-DIALYSIS PATIENTS

We studied the effect of vitamin C and B6 supplementation on oxalate metabolism in seven patients receiving chronic peritoneal dialysis therapy. The study was divided into three phases, each lasting 4 weeks. Plasma oxalate, total ascorbic acid, and pyridoxal-5'-phosphate (PLP) were measured at.the end of each phase. Twenty-four-hour urinary excretion and dialysate removal rates of oxalate were also obtained. At the end of phase I (supplement-free period), plasma oxalate levels were markedly elevated at 47.6 ± 7.1 μmol/L (437 ± 66 μg/dL) (normal, 3.4 ± 0.4 μmol/ L [30.3 ± 1.6 μg/dL]). Plasma total ascorbic acid levels were 62 ± 6 μmol/L (1.0 ± 0.1 mg/dL) (normal, 45 to 57 μmol/L [0.8 to 1.0 mg/dL]), while plasma PLP levels were markedly reduced to 24 ± 5 nmol/L (normal, 40 to 80 nmol/L). Daily supplements of 0.57 mmol (100 mg) ascorbic acid orally (phase II) resulted in a 19% increase in the plasma oxalate levels to 57.8 ± 6.1 μmol/L (520 ± 55 μg/dL) (P < 0.03), with a concomitant 60% increase in the plasma ascorbate levels (91 ± 6 mol/L [1.6 ± 0.1 mg/dl], P < 0.01). Plasma PLP values remained low. Finally, during phase III (0.57 mmol or 100 mg ascorbic acid plus 59.6 μmol or 10 mg pyridoxine HCI orally daily), plasma oxalate levels declined by 17% to 47.9 ± 5.2 μmol/L (431 ± 47 μg/dl) (P > 0.05 v phase II). The plasma ascorbate and PLP concentrations were 102 ± 6 μmol/L (1.8 ± 0.2 mg/dL) (P < 0.01 v phase I), and 109 ± 18 nmmol/L (P < 0.001 v phases I and II), respectively. Urinary oxalate excretion and dialysate removal rate remained unchanged. The dietary intake of ascorbic acid, vitamin B6, oxalate, and protein were similar. We conclude that (1) peritoneal dialysis patients have marked hyperoxalemia, (2) routine administration of the ascorbic acid supplements (0.57 mmol or 100 mg/d) results in a modest increase in the plasma oxalate level, and (3) vitamin B6 supplements (59.6 μmol or 10 mg/d) are ineffective in reducing the markedly elevated plasma oxalate levels. © 1992, National Kidney Foundation. All rights reserved. All rights reserved.