Hyperoxaluria ascorbic acid

Ascorbic acid 4 g/day increases uric acid clearance in volunteers (23), although it does not reduce protein-bound uric acid in blood. Ascorbic acid 4-12 g/day causes acidification of the urine, which can cause precipitation of urate and cystine and consequently formation of urate stones or cystinuria. Ascorbic acid is excreted largely as oxalate, and hyperoxaluria results when large doses are... 

Urinary oxalate is derived from three sources (i) hepatic synthesis, (ii) breakdown of ascorbic acid and (hi) diet. Because of an overestimation of the ascorbic acid fraction, it has long been thought that the dietary fraction was responsible for only 10 to 20% [8]. A more recent study identified a dietary fraction of around 50%, making it an important determining factor in the urinary oxalate concentration [9]. Another study demonstrated that a dietary oxalate load resulted in transiently increasing plasma and urine oxalate levels peaking in the 2 to 4 hour post-load period [10], implying that an oxalate rich meal is able to induce a temporally state of hyperoxaluria. The fact that oxalate is an unavoidable component of the human diet since it... 

Glyoxalate can be transaminated to glycine, reduced to glycolate, converted to a-hydroxy-/3-ketoadipate by reaction with a-ketoglutarate, or oxidized to oxalate and excreted in urine. The first three reactions require pyridoxal phosphate, NADH, and thiamine pyrophosphate, respectively. In humans, ascorbic acid (vitamin C) is a precursor of urinary oxalate (Chapter 38). Since calcium oxalate is poorly soluble in water, it can cause nephrolithiasis and nephrocalcinosis due to hyperoxaluria. 

Hyperoxaluria can be induced by vitamin C. Investigations in healthy women have shown that less than 20% of ingested doses of ascorbic acid are excreted in the urine in the first day while by the second day after supplementation both plasma and urine levels are indistinguishable from those of the controls. In some healthy women ascorbate excretion did not rise even with 6 g. A dose of 8 g ascorbic acid, however, resulted in a rise of excretion from 30 mg to 250 mg/24 hours. The results suggest that in healthy women the vitamin C is excreted as oxalate or dehydroascorbic acid (28 ). 

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