Urinary oxalate excretion plays a critical role in calcium oxalate stone formation. The majority of urinary oxalate is derived from endogenous synthesis and dietary sources. There has been an ongoing debate regarding the impact of vitamin C supplementation on urinary oxalate excretion. The 2 articles discussed here pro-vide further information on this subject.
Effect of Vitamin C Supplements on Urinary Oxalate and pH in Calcium Stone-Forming Patients
Baxmann AC, De O G Mendonca C, Heilberg IP.
Kidney Int. 2003;63:1066–1071.
Adult calcium oxalate stone-formers were administered either 1 gram (group 1) or 2 grams (group 2) of vitamin C daily for 3 days while healthy, non-stone-forming adults (group 3) were given the 1-gram dose daily during the same time. The participants were not placed on a metabolic diet, but were instructed to refrain from consuming foods rich in vitamin C and oxalate, as well as dairy products. Twenty-four-hour urine collections were obtained at baseline and after vitamin C administration.
Urinary pH did not change after vitamin C supplementation in any of the groups. However, there were statistically significant increases in oxalate excretion in all 3 groups: 61% in group 1, 41% in group 2, and 56% in group 3. A significant increase in calcium oxalate urinary supersaturation (by Tiselius index) occurred in all groups after supplemental vitamin C intake.
Effect of Ascorbic Acid Consumption on Urinary Stone Risk Factors
Traxer O, Huet B, Poindexter J, et al.
J Urol. 2003;170:397–40112853784.
In this double-blind, randomized, crossover study, adult calcium oxalate stone-formers and non-stone-forming adults were placed on a controlled metabolic diet for 2 6-day phases. During one of the phases, they were given 1 gram of vitamin C twice a day, and they received a placebo during the other phase. Twenty-four-hour urine specimens were collected on days 5 and 6 of each phase.
There were no changes in urinary pH after vitamin C ingestion in either group, although there was a moderate and statistically significant increase in urinary oxalate excretion in controls (20%) and stone-formers (33%).
Conclusions
Both of these articles clearly demonstrate that 1 to 2 grams of ascorbic acid administered daily to both normal subjects and calcium oxalate stone-formers result in no urinary pH changes but in increased oxalate excretion. Therefore, this practice should not be used in managing patients with struvite calculi, because it does not reduce urinary pH, or in calcium oxalate stone-formers, as it might promote stone activity.