New data from a large prospective study of Swedish men reveal a twofold increased risk of stone disease in association with vitamin C (ascorbic acid) supplementation.

Ingestion of vitamin C, whether dietary or supplemented, results in urinary excretion of oxalate, a key component of calculi. “It has long been suspected that high doses of vitamin C might increase the risk of kidney stones,” Laura Thomas, lead author of the study, told Nature Reviews Urology. “However, there is still considerable uncertainty regarding the kidney stone risk that may be associated with high-dose vitamin C supplement use.”

To address this issue, Thomas and colleagues analysed data from the Cohort of Swedish Men, a large population-based longitudinal study of 48,850 men born between 1918 and 1952. Researchers identified 23,355 men who, according to lifestyle and diet questionnaires, took only vitamin C supplements or no supplements at all and had no history of renal lithiasis. Over 11 years of follow-up, 436 of these men developed kidney stones.

Multivariate analysis revealed a dose-dependent increase in risk of kidney stone formation for men who took vitamin C. Compared with those who did not take any supplements, men who took fewer than seven ascorbic acid tablets per week had a relative risk (RR) of 1.66 (95% CI 0.99–2.79; P = 0.001) and those who took seven or more tablets had an RR of 2.23 (95% CI 1.28–3.88; P = 0.001). Accurate details of ascorbic acid dose were not available, but previous studies suggest that most vitamin C supplements on sale in Sweden contain 1,000 mg ascorbic acid per tablet. Researchers repeated the analysis on a cohort of men who took only a multivitamin supplement and found no association with the formation of kidney stones (RR 0.86, 95% CI 0.62–1.19).

The investigators are keen to emphasize that their findings relate to vitamin C supplementation only, not dietary ingestion, and cannot be translated to women. The results must be confirmed in other studies, but given the current lack of reported benefits for high-dose vitamin C supplementation, the authors suggest it should be avoided, especially by those with a history of kidney stones.