DALLAS, TEXAS. Ascorbic acid (vitamin C) is metabolized to oxalate in the body. Most kidney stones are formed from calcium oxalate, so not surprisingly warnings have been issued to the effect that high intakes of vitamin C may promote kidney stones. Several studies have questioned the validity of these warnings. Now researchers at the University of Texas Southwestern Medical Center weigh in with new evidence to the effect that daily vitamin C intakes of 2000 mg or less do not increase the risk of forming kidney stones. Their randomized, placebo-controlled clinical trial involved 12 participants with no history of kidney stones and 12 who were known to form calcium oxalate stones. The participants were given 1000 mg of ascorbic acid with breakfast and dinner or matching placebo for two 6-day study periods.

The researchers conclude that supplementing with 2000 mg/day of vitamin-C does not change urinary pH (a key factor in stone formation) in neither normal subjects nor in known stone formers. Both stone formers and non-stone formers did, however, show a moderate increase in urinary oxalate excretion. The researchers recommend that daily vitamin C supplementation be limited to 2000 mg among people known to have a tendency to form kidney stones. They also point out that no correlation linking vitamin C consumption to risk of kidney stone formation has ever been reported in published studies involving vitamin C supplementation.
Traxer, Olivier, et al. Effect of ascorbic acid consumption on urinary stone risk factors. Journal of Urology, Vol. 170, August 2003, pp. 397-401