The fear of megadoses of vitamin C would lead to excessive oxalate excretion in the urine, thereby increasing the possibility of calcium oxalate kidney stones, has abated. Although massive doses of vitamin C do increase somewhat the amount of oxalate excreted, the amount usually remains within a normal (and safe) range. Only those persons pre disposed to urolithiasis are likely to be troubled by oxalate stones.
Large doses of ascorbic acid (approximately 4g) will increase the amount of uric acid excreted in the urine. The resulting urine acidification along with the excessive amount of urate being excreted could cause precipitation of urate crystals. However, the actual clinical importance of uricosuria with regard to urolithiasis is unknown.
Because ascorbic acid when ingested with sources of nonheme iron increases iron absorption, chronic high doses of vitamin C may be unsafe for those persons unable to regulate absorption of iron: individuals with hemochromatosis, thalassemia and sideroblastic anemia.
Excessive doses of vitamin C can cause diarrhea but the effect is not long lasting. The effect of excessive ascorbate excretion in the urine and feces, however can interfere with a variety of clinical laboratory tests: false negative tests for fecal occult blood may generated, occult blood in the urine may not be detected; tests for glucose in the urine can be rendered invalid.
Based upon the ratio of toxic reactions recorded to the number of persons mega-dosing with vitamin C, one must conclude that, despite all the postulated potential problems, a high intake of ascorbate is relatively harmless.
Ascorbic acid toxicity
