Kidney stone risk increased by high dose vitamin C

Kidney stone formation increases with higher amounts of oxalate in the urine because most kidney stones are made of calcium oxalate. Vitamin C can be metabolized into oxalate, so the authors of a study just published in JAMA Internal Medicine sought to clarify the long-standing concern that vitamin C in high doses could promote kidney stone formation:

"Urinary oxalate is an important determinant of calcium oxalate kidney stone formation. Vitamin C is excreted in urine both in its unmetabolized form and as oxalate; however, there remains considerable uncertainty over the kidney stone risk that may be associated with ascorbic acid supplement use...We examined whether ascorbic acid supplements (approximately 1000 mg) were associated with kidney stones in a population-based, prospective cohort of men."

They designed a strong study during which they examined data on supplement intake and kidney stone incidence over 11 years for 48,850 men aged 45 to 79 years at baseline. A strong association of kidney stone formation and daily doses of 1000 mg or more of vitamin C emerged:

"Ascorbic acid use was associated with a statistically significant 2-fold increased risk. In contrast, multivitamin use was not associated with kidney stone risk...Users of only ascorbic acid taking fewer than 7 (median) and 7 or more tablets per week showed increased risks of RR, 1.66 (95% CI, 0.99-2.79) and RR, 2.23 (95% CI, 1.28-3.88), respectively, compared with supplement nonusers in the full multivariate-adjusted model."

In other words, they found a 223% risk increase in men taking 7 or more 1000 mg tablets of vitamin C per week. Furthermore...

"...analysis of kidney stone material collected from 3176 men, treated with extracorporeal shockwave lithotripsy in Stockholm County, found calcium oxalate to be the dominant component in 92.6%. It could thus be assumed that at least 90% of the kidney stones in our study population were composed primarily of calcium oxalate."

Note that this is not necessarily the same for women:

"Our results may not be generalizable to women, who typically have a much lower kidney stone risk."

Regarding vitamin C in supplements versus vitamin C in foods:

"Because the risk associated with ascorbic acid may depend both on the dose and on the combination of nutrients with which the ascorbic acid is ingested, our findings should not be translated to dietary vitamin C."

Clinical note: Clearly high dose supplemental vitamin C should be avoided by those with a history of kidney stones. And caution should be observed otherwise not only due to concerns about kidney stone formation, but also because excessive antioxidant use can increase the risk for autoimmune inflammation as will be reported in a forthcoming post.

The authors conclude:

"...our results indicate that high-dose ascorbic acid supplements—one of the most commonly used vitamin preparations—are associated with a dose-dependent 2-fold increased risk of kidney stone formation among men."