Effect of High Dose Vitamin C on Urinary Oxalate Levels

Abstract

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.

Vitamin C pharmacokinetics in healthy volunteers: evidence for a recommended dietary allowance.

“…When patients ingested .1000 mg of vitamin C daily, unexpected calculi were not reported (51,(54)(55)(56). Based on our new data, recent urinary excretion findings (36,37) applied to women because it is possible that they achieve higher plasma concentrations at lower doses compared to men (14,15). The data here are also relevant to subjects over a wide age range, although it is possible that the elderly will require more ascorbate than younger subjects (19,20,22).…”

“…The recommended dietary allowance (RDA) for vitamin C is 60 mg daily, based on threshold urinary excretion of the vitamin and on preventing the vitamin C deficiency disease scurvy with a margin of safety (1,2).. Ingestion of 60 mg daily was proposed to prevent scurvy for [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] days if vitamin C intake ceased (1)(2)(3)(4)(5)(6)(7). Threshold urinary excretion of vitamin C was reported at the 60-mg daily dose (3,4,7,8).…”

“…At the lowest and highest vitamin C doses, there were no differences in psychometric testing for each volunteer (data not shown). At (35)(36)(37). If high doses were administered longer, it is unclear whether the elevations would remain and whether renal calculi would result.…”

A short overview of vitamin C and selected cells of the immune system

“…These results appear to be due to in vitro conversion of ascorbate to oxalate during the analytical procedure rather than in vivo conversion [88]. Later studies confirmed these observations, showing that large ascorbic acid doses did not produce kidney stones but, also, reduced the risk of kidney stone formation, concluding the ascorbic acid restriction, due to possibility of kidney stones formation, is unwarranted [86][87][88]. Pro-oxidative effects have been described in vitro for vitamin C in the presence of transition metal such as iron and copper [92].…”

“…Namely, earlier report has demonstrated that 8 g/day of ascorbic acid for 8 consecutive days, could cause harmful calcium oxalate crystalluria in persons who have a predisposition for increased crystal aggregation, indicating that these individual's response to ascorbic acid ingestion is probably rare and concluded that ingestion of these doses did not affect the principal risk factors associated with calcium oxalate kidney stone formation [86]. Another report observed a modest increase in urinary oxalate after administration 5 and 10 g/day ascorbic acid for 5 consecutive days [87]. These results appear to be due to in vitro conversion of ascorbate to oxalate during the analytical procedure rather than in vivo conversion [88].…”