Vitamin C is a water soluble compound, which can be easily flushed from the body. Synthetic vitamin C though is a relatively unstable compound and a portion of excess ingested vitamin C breaks down into oxalic acid in the body.
Oxalic acid is beneficial to the body as well as detrimental. As vitamin C breaks down in to oxalic acid the oxalic acid serves as an antioxidant to the vitamin C helping to prevent oxidative destruction of the vitamin C. On the other hand, oxalic acid can bind with minerals forming insoluble oxalates. Of particular importance is calcium oxalate, which can form kidney stones. Studies have shown that oxalic stones, which make up 80% of kidney stones only formed in people with kidney diseases but not in healthy individuals at doses of 200mg daily. At 1,500mg daily intake there was only a tiny rise in the incidence of oxalic stone formation. It is believed that the insignificant rise is due to the fact that vitamin C is poorly absorbed by the body. The higher levels of vitamin C are not being absorbed and therefore are not converted in to oxalic acid.
Oxalic acid also binds with the electrolytes sodium and potassium and the mineral magnesium. Among other functions of sodium and potassium is the regulation of heart rate. Magnesium serves a multitude of important functions including maintaining normal blood pressure, proper muscle function; including the heart, preventing muscle cramping and insulin production and sensitivity.
Oxalic acid is an irritant to the urinary tract. Irritation of the urinary tract from oxalic acid can lead to urinary tract infections in sensitive individuals.
There is also concern that vitamin C may cause uric acid stones to form from excess excretion of uric acid. Acidification of the urine with vitamin C increases the ratio of uric acid to the more soluble sodium urate. For this reason treatment of uric acid stones includes alkalizing the urine with sodium bicarbonate (baking soda) or calcium citrate to increase sodium urate formation.
Excessive levels of vitamin C are contradicted in people suffering from kidney stones, gout, cirrhosis, kidney diseases and certain other disorders.
Safety studies at doses of 200 to 1,500mg daily are conflicting. Safety studies of extremely high doses, up to 20,000 have not been done. Therefore I recommend not exceeding 2,000mg daily for healthy individuals. Normally I recommend 500mg 3 times daily for most individuals. Slightly higher levels are recommended for smokers, individuals under a lot of stress, stimulant users; including caffeine (coffee, tea, guarana, kola nut, etc.) and those taking medications known to deplete vitamin C such as Prednisone.