The age old concern about vitamin C causing kidney stones stems from studies in the ‘90’s that reported raised oxalic acid levels in the urine of people supplementing vitamin C. Oxalic acid is a key component of calcium oxalate stones - 70% of all kidney stones contain this substance. Obviously, an elevated urinary oxalic acid level is undesirable. However, while metabolic conversion does indeed take place, it is insignificant.
The apparently higher levels of oxalic acid in the urine that have been previously reported arise from the fact that ascorbic acid which is excreted in the urine undergoes a chemical conversion to oxalic acid while it is in a test -tube prior to analysis. This was proven several years ago by research at the Kidney Stone Research Laboratory of the University of Cape Town where they conducted a controlled trial in which 10 volunteer subjects were required to ingest 4g of Vit C per day for 5 days. Urines (24h) were collected before, during and after the ingestion period. Unlike the earlier studies they simply put a preservative in our urine collection bottles to prevent this conversion in the test tube. Previous studies failed to take this precaution and hence reported erroneously high oxalic acid levels in their urine. These were rigorously analysed for a host of independent physicochemical risk factors, all of which are regarded as powerful indicators of the risk of kidney stone formation.
The results showed that these risk factors were not significantly altered. They concluded that ingestion of large doses of Vitamin C does not increase the risk of forming kidney stones. The results of this study were published internationally. I asked Professor Allen Rodgers from the University of Cape Town, one of the world’s leading experts on kidney stones, if vitamin C could cause kidney stones and he said “The answer is simply no.” On the basis of this research the official position of the Food Standards Agency in the Uk is that vitamin C doesn’t increase kidney stone risk, So why this apparent small increased risk in those just taking high dose vitamin C? In real terms the apparent increased risk was small – the difference in risk being three more cases in every two thousand people. The authors also point out that ‘we cannot rule out the possibility of residual confounding (factors)’ due to the anomaly that those taking high dose vitamin C with multivitamins showed no such risk. Also, according to the authors ‘our results may not be generalizable to women, who typically have a much lower kidney stone risk.’
The authors appear not to be fans if vitamin C saying “Currently there are no well-documented benefits of high-dose ascorbic acid supplement use” somehow ignoring the evidence that high dose vitamin C reduces severity and duration of infections, lowers cholesterol, as well as blood pressure, according to a meta-analysis of 29 studies published last year. There is certainly more reason to take high dose vitamin C, at a rate of 1 to 2 grams a day, than not to. Whether or not there is any real increased risk of kidney stones remains questionable.