Concern has been expressed, particularly among opponents to vitamin supplementation, that high levels of vitamin C (ascorbic acid) intake may have potential adverse effects such as vitamin B12 deficiency, iron overload, and kidney stones. Researchers at the Veterans Affairs Medical Center have just concluded a major study aimed at investigating these concerns. Their study involved over 10,000 participants aged 20 to 74 years. The researchers found no correlation between vitamin B12 deficiency and blood levels of vitamin C. As a matter of fact people with high vitamin C levels also had high vitamin B12 levels. There also was no correlation between serum ferritin (iron) levels in men and their vitamin C levels. However, among women there was some correlation between high vitamin C levels and high iron levels prompting the researchers to warn that women with a genetic susceptibility to iron overload should consider moderating their vitamin C intake. No association was found between high vitamin C levels and the risk of developing kidney stones. As a matter of fact among men a higher vitamin C level was highly correlated with a significantly lower risk of kidney stones. NOTE: This study was supported in part by Hoffmann-LaRoche, a major manufacturer of pharmaceuticals and vitamins.
Simon, Joel A. and Hudes, Esther S. Relation of serum ascorbic acid to serum vitamin B12, serum ferritin, and kidney stones in US adults. Archives of Internal Medicine, Vol. 159, March 22, 1999, pp. 619-24