Secondary hyperoxalemia caused by vitamin C supplementation in regular hemodialysis patients.

Oxalosis can be a problem in renal failure. As vitamin C is a precursor of oxalate in patients on regular hemodialysis, we have measured plasma levels of vitamin C, oxalate, pyridoxine, thiamine and creatinine twice before and 4 weeks after a change of vitamin C dosage in 49 dialysis patients who had been receiving 500 mg of oral vitamin C daily for more than 6 months. Ten unsupplemented dialysis patients served as controls. The mean plasma levels of vitamin C and oxalate were 3.3 +/- 0.3 mg/dl and 50.4 +/- 8.2 mumol/l respectively. Four weeks after the vitamin C dosage was changed from 500 to 100, 50 and 0 mg, plasma oxalate levels were 34.1 +/- 1.4, 33.3 +/- 3.7, and 25.7 +/- 3.9 mumol/l respectively. There was a strong correlation between plasma vitamin C and oxalate levels (r = 0.755, p less than 0.01) but none between pyridoxine and oxalate. A significant correlation was also noted between the duration of hemodialysis and plasma oxalate levels (r = 0.582, p less than 0.01). Our results suggest that hyperoxalemia in regular hemodialysis patients is aggravated by routine vitamin C supplementation. The administration of vitamin C should be restricted to a dose necessary to correct vitamin C deficiency.