Both ascorbate and dehydroascorbate are filtered at the glomerulus, then reabsorbed by facilitated diffusion. When glomerular filtration exceeds the capacity of the transport systems, at a plasma concentration of ascorbate above about 85 mmol/l, the vitamin is excreted in the urine in amounts proportional to intake.

It has been reported that approximately 25% of the dietary intake of ascorbate is excreted as oxalate; this would account for about 40% of the total urinary excretion of oxalate. However, there is no known metabolic pathway for the synthesis of oxalate from ascorbate, and it is likely that all or most of the oxalate found in urine after loading doses of ascor-bate is formed nonenzymically, after the urine has been collected. Even in people at risk of forming oxalate renal stones it is unlikely that normal or high intakes of ascorbate pose any additional hazard.

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