Effect of high dose vitamin C on urinary oxalate levels

The effect of high dose ascorbate on urinary oxalate levels in healthy adults was investigated using a modified ion chromatography method. Subjects ingested 1, 5 and 10 gm. supplemental ascorbate per day for 5 days, separated by 5 days of no supplementation. Urine ascorbate levels demonstrated variable increases with ascorbate supplementation. Ascorbate added directly to urine in vitro resulted in statistically significant but modest increases in measured oxalate. Addition of 5.68 mmol./l. ascorbate increased measured urinary oxalate by 36 mumol./l., implying conversion of ascorbate to oxalate during analysis. Measurement of 24-hour urinary oxalate levels with 5 and 10 gm. ascorbate per day showed similar, modest increases, which could be entirely accounted for by oxalate production during analytical procedures. Thus, no genuine increase in urinary oxalate was demonstrable despite a greatly increased ascorbate intake.