This article describes a novel method to analyze oxalic acid accurately in the presence of very high concentrations of ascorbic acid and its use to determine the urinary oxalic acid excretion profiles of patients administered very large intravenous doses of ascorbic acid.
Accurate oxalic acid analysis in the presence of ascorbic acid requires a strongly acid environment and sample storage at low temperature to prevent in vitro oxidation of ascorbic acid to oxalic acid. Acid conditions are also necessary during sample preparation for GC-MS analysis. In particular, alkaline extraction oxidizes large amounts of ascorbic acid to oxalic acid. When appropriate precautions are taken, and unlike other methods, GC-MS/SIM permits sensitive and accurate oxalic acid analysis even in the presence of massive amounts of ascorbic acid. Thus, in this study, oxalic acid in amounts of 27 to 81 mg was accurately detected in the presence of 11 to 79 g ascorbic acid.
It appears from this study that intravenous ascorbic acid increases urinary oxalic acid excretion in a dose-dependent fashion. For doses of 0.2 and 0.6 g/kg, less than 40 mg oxalic acid was excreted during and over the 6 hours after the ascorbic acid infusion, at which time oxalic acid excretion was continuing at the rate of 4 to 5 mg/h. At the 1.5-g/kg dose, approximately 80 mg of oxalic acid was excreted 6 hours after the infusion ended (approximately 20 mg of which was already present in the pharmaceutical product before the infusion began); and excretion was continuing at approximately 10 mg/h. By that time, the plasma ascorbic acid concentration had fallen from a peak value of 26.2 to 5.3 mmol/L, and approximately 80% of the ascorbic acid dose had been excreted. Because the circulatory half-life of oxalic acid is approximately 3.6 hours [26
], it may be predicted that the oxalic acid excretion rate would fall to normal after a further 4 half-lives or approximately 14 hours. Oxalic acid is absorbed from the diet and synthesized from endogenous sources other than ascorbic acid, but samples were unavailable to measure endogenous oxalic acid excretion rates in the participants in this study. The oxalic acid excretions reported here therefore overestimate the extent to which the infused ascorbic acid was oxidized to oxalic acid. In the absence of ascorbic acid administration, urinary oxalic acid excretion normally ranges from 10 to 60 mg/24 h [2
], with higher rates occurring during the day [24
Contrary to the findings in an early human study in which 44% of the radioactivity in an intravenous dose of 14
C-ascorbic acid was recovered in the urine as oxalic acid [7
], we found that only approximately 0.2% (mol/mol) of large doses of ascorbic acid appeared in the urine as oxalic acid 6 hours postinfusion, and as explained above, this is an overestimation. The situation would be different for people with a reduced glomerular filtration rate, which would increase the circulatory dwell time of ascorbic acid and any oxalic acid resulting from its oxidation. People with end-state renal disease have hyperoxalemia that increases after the administration of even conventional amounts of ascorbic acid [28
]; the appropriate dose and route of ascorbic acid administration for people with end-stage renal disease are controversial [29
]. It is important, therefore, to reiterate that only patients whose serum creatinine did not exceed 175 μ
mol/L participated in this study.
Much smaller doses of ascorbic acid than administered in this study have been reported to increase urinary oxalic acid far more than observed here. Several factors could explain this apparent anomaly. First, despite ample documentation that ascorbic acid in urine samples will lead to an overestimation of oxalic acid when the sample is not appropriately handled, stored, and analyzed [2
], sufficient attention is not always paid to this issue. Thus, in 2 articles that report a considerable percentage increase in oxalic acid excretion when the amount of ascorbic acid included in home parenteral nutrition solutions was increased [11
], urine samples were acidified only after their delivery to the analytical laboratory, by which time considerable artifactual oxalic acid formation can be assumed to have taken place. It should also be noted that ascorbic acid solutions for clinical infusion may be unstable over time [32
]. In our study, ascorbic acid was rapidly administered shortly after the infusate was prepared, whereas parenteral nutrition solutions are commonly infused over 12 to 24 hours, during which time considerable ascorbic acid degradation is known to occur [32
]. It is unclear how much ascorbic acid is converted to oxalic acid in parenteral nutrition mixtures [35
Finally, it is conceivable that intravenous ascorbic acid is less prone to oxidation to oxalic acid than high-dose oral ascorbic acid. Because its intestinal absorption is limited, much of a large oral dose of ascorbic acid could remain for a long time in the alkaline medium of the small and large intestines, favoring its oxidation to oxalic acid with absorption of the resulting sodium oxalate into the bloodstream [20
The findings in this study should be considered in light of case reports describing renal abnormalities or acute renal failure in patients who ingested or received large doses of ascorbic acid orally or intravenously [8
]. Careful study of those cases suggests that, in some cases, they occurred in people with preexisting advanced renal failure and, in others, they occurred in people who were consuming or being administered high-dose ascorbic acid when acute renal failure developed for other reasons and precipitated oxalate nephropathy.
Does the excretion of 80 mg of oxalic acid over the 6 hours after a 100-g intravenous dose of ascorbic acid substantially increase the risk of calcium oxalate stone formation? Oxalate nephrocalcinosis and calcium oxalate stones develop over months to years in primary hyperoxaluria, a disease in which oxalic acid excretion exceeds 100 mg/d and can reach 400 mg/d [20
]. The relatively slow natural history of primary hyperoxaluria suggests that a time-limited course of ascorbic acid infusions 3 times per week would not create a severe or immediate risk of oxalate stone accumulation. Nevertheless, oxalate stones occur commonly in the general population; for people already at high risk of oxalate stone, one may assume that even intermittent high-dose ascorbic acid infusions could further increase this risk. The present data are important because they indicate a remarkable lack of severe hyperoxaluria after massive intravenous doses of ascorbic acid in people with normal renal function.