Massive doses of vitamin C have been shown to reduce serum vitamin B12 levels. In part, this may be due to an effect of ascorbic acid on vitamin B12 in food.
Ascorbic acid destroys B12 in food. Ascorbic acid also inhibits the utilization of beta-carotene.
Some individuals, exhibit accelerated conversion of ascorbic acid to oxalate, probably through greater inducibility of enzymes in the ascorbate-oxalate pathway.
The hyperoxaluria of these individuals, following large doses of vitamin C, increases the risk of urinary stone formation.
Excessive doses of vitamin C can cause diarrhea but the effect is not long lasting. The large excess of vitamin C is promptly excreted in the urine but may cause uricosuria and increased absorption of iron, with the potential for iron reloaded.
Although toxicity is rare, the tolerable upper intake level established by the food and Nutrition Board (FNB) of the Institute of Medicine for vitamin C in adults was set at 2000 mg.
The effects of ascorbic acid toxicity
