Because oxalate is a metabolite of vitamin C, there is some concern that high vitamin C intake could increase the risk of oxalate kidney stones. Some (49-51) but not all (52-54) studies have reported that supplemental vitamin C increases urinary oxalate levels. Whether any increase in oxalate levels would translate to an elevation in risk for kidney stones has been examined in epidemiological studies. Two large prospective studies, one following 45,251 men for six years and the other following 85,557 women for 14 years, reported that consumption of ≥ 1,500 mg of vitamin C daily did not increase the risk of kidney stone formation compared to those consuming < 250 mg daily. However, a more recent prospective study that followed 45,619 men for 14 years found that those who consumed ≥ 1,000 mg/day of vitamin C had a 41% higher risk of kidney stones compared to men consuming < 90 mg of vitamin C daily—the current recommended dietary allowance (See RDA; (55)). In this study, low intakes (90-249 mg/day) of vitamin C (primarily from the diet), those thought to generally benefit health (See Disease Prevention), were also associated with a significantly elevated risk. Supplemental vitamin C intake was only weakly associated with increased risk of kidney stones in this study (55). Despite conflicting results, it may be prudent for individuals predisposed to oxalate kidney stone formation to avoid high-dose vitamin C supplementation.
