Effect of vitamin C supplements on urinary oxalate and pH in calcium stone-forming patients.
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 stone-forming 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.


CONCLUSIONS
These data suggest that vitamin C supplementation may increase urinary oxalate excretion and the risk of calcium oxalate crystallization in calcium stone-forming patients.
Baxmann
Alessandra Calábria
AC
Nephrology Division, Universidade Federal de São Paulo, UNIFESP, Brazil.
De O G  Mendonça
Claudia
C
Heilberg
Ita Pfeferman
IP
D000328:Adult; D000975:Antioxidants / Q000009:adverse effects*; D001205:Ascorbic Acid / Q000009:adverse effects*; D002118:Calcium / Q000378:metabolism*; D002129:Calcium Oxalate / Q000737:chemistry / Q000652:urine*; D003460:Crystallization; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D007669:Kidney Calculi / Q000453:epidemiology / Q000209:etiology*; D008297:Male; D008875:Middle Aged; D012307:Risk Factors
D000975:Antioxidants; D002129:Calcium Oxalate; D001205:Ascorbic Acid; D002118:Calcium