Stone disease

Abstract

Background: The contribution of ascorbate to urinary oxalate is controversial. The present study aimed to determine whether urinary oxalate and pH may be affected by vitamin C supplementation in calcium stoneforming patients.Methods: Forty-seven adult calcium stone-forming patients received either 1 g (N=23) or 2 g (N=24) of vitamin C supplement for 3 days and 20 healthy subjects received 1 g. A 24-hour urine sample was obtained both before and after vitamin C for calcium, oxalate, magnesium, citrate, sodium, potassium, and creatinine determination. The Tiselius index was used as a calcium oxalate crystallization index. A spot fasting morning urine sample was also obtained to determine the urinary pH before and after vitamin C.Results: Fasting urinary pH did not change after 1 g (5.8 +/-0.6 vs. 5.8 +/-0.7) or 2 g vitamin C (5.8 +/-0.8 vs. 5.8 +/-0.7). A significant increase in mean urinary oxalate was observed in calcium stone-forming patients receiving either 1 g (50 +/-16 vs. 31 +/-12 mg/24 hours) or 2 g (48 +/-21 vs. 34 +/-12 mg/24 hours) of vitamin C and in healthy subjects (25 +/-12 vs. 39 +/-13 mg/24 hours). A significant increase in mean Tiselius index was observed in calcium stone-forming patients after 1 g (1.43 +/-0.70 vs. 0.92 +/-0.65) or 2 g vitamin C (1.61 +/-1.05 vs. 0.99 +/-0.55) and in healthy subjects (1.50 +/-0.69 vs. 0.91 +/-0.46). Ancillary analyses of spot urine obtained after vitamin C were performed in 15 control subjects in vessels with or without ethylenediaminetetraacetic acid (EDTA) with no difference in urinary oxalate between them (28 +/-23 vs. 26 +/-21 mg/L), suggesting that the in vitro conversion of ascorbate to oxalate did not occur.Conclusion: These data suggest that vitamin C supplementation may increase urinary oxalate excretion and the risk of calcium oxalate crystallization in calcium stone-forming patients. Editorial CommentAscorbic acid has been implicated in calcium stone formation based on its conversion to oxalate and its potential urinary acidifying properties. Numerous studies have evaluated the effect of ascorbic acid consumption on urinary oxalate and pH in normal subjects and in stone-formers. However, results have been conflicting because of inaccuracies in measuring oxalate in the presence of ascorbate, which is readily oxidized to oxalate in vitro. Historical assays involving heating or alkaline conditions favored oxidation of unmetabolized ascorbate to oxalate thereby confounding results.These authors evaluated the effect of vitamin C supplementation on urinary oxalate and pH in healthy subjects and stone formers given 1 g (healthy subjects and stone formers) or 2 g (stone formers) of ascorbic acid daily for 3 days. Urine collected over a 24-hour period was analyzed for stone risk factors before and after the administration of vitamin C, and urine samples were acidified before processing to prevent in vitro oxidation of ascorbate. Stone formers had higher levels of urinary oxalate than healthy subjects both at baseline and after vitamin C supplementation. Howev...