Ascorbic Acid Supplements and Kidney Stone Incidence Among Men: A Prospective Study, JAMA Internal Medicine, February 2013.
"Ascorbic acid use was associated with a statistically significant 2-fold increased risk [of kidney stones]."It was a large, prospective study involving 23,355 participants from the Cohort of Swedish Men.
The risk remained at 2-fold even after adjustment for possible confounders (e.g. age, education, BMI, alcohol, other nutrients including dietary intake of vitamin C, tea and coffee consumption, smoking, diabetes, high blood pressure). The risk was even higher if men took more than 7 vitamin C tablets per week. Vitamin C in food, and interestingly vitamin C in multivitamins did not increase risk.
The mechanism? Vitamin C or ascorbic acid can contribute to the development of kidney stones when some of it is metabolized to oxalate and excreted in urine via kidneys. The authors: "urinary oxalate is an important determinant of calcium oxalate kidney stone formation."
In a commentary that accompanied the study (The Risk of Taking Ascorbic Acid), Robert H. Fletcher, MD, of Harvard Medical School said:
"Randomized controlled trials have established that ascorbic acid does not prevent mortality; that is, there is strong, consistent evidence against its effectiveness, not just the absence of evidence supporting it. Similarly, there is consistent evidence from randomized trials that ascorbic acid does not prevent the onset of or death from cardiovascular disease or cancer — nor occurrence of the common cold, for that matter."The RDA for vitamin C is ~90 mg/day.
"This [2-fold increase] is not an insignificant risk. But more to the point, is any additional risk worthwhile if high-dose ascorbic acid is not effective?"
The passing of a kidney stone is an acute condition. I can imagine that lower doses of vitamin C could lead to less overt chronic kidney problems, especially since there was a dose-response effect seen in this study.