Some choose to take supplemental vitamin C (ascorbic acid). Such supplements are usually in the gram range which is considered more than can be obtained from a high fruit diet (~600 mg from such a diet is possible). Many ‘experts’ claim that such doses are firstly necessary, and secondly dangerous. The first point is easily dismissed because studies show that saturation of plasma with vitamin C does not occur until around 2.5 grams of the vitamin are consumed, and further the benefits of gram doses of vitamin C on the symptoms and duration of the common cold are well reported. Turning to the second point, many ‘experts’ claim that mega doses of vitamin C may increase the excretion of oxalate, a metabolite of ascorbate metabolism. It is thought that oxalate plays a role in the formation of calcium urolithiasis which can lead to kidney stones. However the formation of kidney stones is highly complex and involves other factors such as pH, urinary calcium, acid mucopolysaccharides, uric acid, pyrophosphate and citrate.
For example, 9 grams of ascorbic acid taken orally has been shown to increase oxalate excretion by 100 mg per day. However, 4 grams of vitamin C had no effects in another study. Some of the earlier studies that are often used as proof that ascorbic acid can increase oxalate levels were rather unspecific in their measurements with regards the analysis of oxalate in urine. It cannot therefore be said that these studies were reliable. As techniques have developed and new more specific methods have been adopted, a clearer picture has emerged of the effects of ascorbic acid on oxalate excretion. For example. one study1 used isotachophoresis to measure the oxalate excretion in 5 healthy male subjects in response to intake of 10 grams of vitamin C in 5 divided doses of 2 grams. The results showed that about 25 % of the vitamin C was absorbed and excreted in the urine. The mean oxalate excretion was increased from about 50 mg to 87 mg (with a range from 60 to 126 mg) per day with the vitamin C supplements.
A more detailed analysis of the results showed that following the 3rd dose of vitamin C when 6 grams had been consumed, oxalate excretion plateaued at 0.6 μg/mL per min. Addition of subsequent vitamin C supplements was not able to increase this excretion rate further. When vitamin C supplements were stopped, oxalate excretion remained at the elevated 0.6 μg/mL per min rate for a further 6 hours before returning to baseline levels. Calcium excretion was slightly reduced with vitamin C supplements and there was no change in the creatinine, uric acid or inorganic phosphate excretion rates. Only around 1 to 2 % of the ascorbic acid that was absorbed (25 % absorption rate) was excreted as oxalate, and therefore only around 0.5 % of the ascorbic acid was metabolised to oxalate. The increase in the oxalate excretion rate with these vitamin C doses were not greater than could be expected from dietary changes (which may cause changes in oxalate excretion which ranges from about 70 and 980 mg per day through dietary changes).
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