Science Figures Interpreted and Analyzed by RealClearScience
Vitamin supplements are popular items these days. Vitamin C is particularly popular in the winter, when people pop tablets at the first sign of a cold. However, data indicates that vitamin C doesn't actually prevent colds, but long-term use may slightly reduce the severity or duration of a cold when you do catch one.
So, should we all start popping vitamin C tablets every day? Probably not. A new study in JAMA Internal Medicine showed that men who took 1,000-mg tablets of vitamin C were twice as likely to develop kidney stones as men who did not take vitamin C supplements. (See chart below.) The effect was not observed for men who took multivitamins.
To determine relative risk (RR), the authors compared the rate of kidney stones in men who did not take vitamin supplements (163 per 100,000 person-years) to the rate of kidney stones in men who took vitamin C supplements (310 per 100,000 person-years). Dividing 310 by 163 gives a crude RR of 1.90, which in this case, is the same as the age-adjusted RR.
Diving a little deeper into the data, the authors showed a dose-response relationship: Men who took fewer than 7 tablets per week had an increased risk of kidney stones (but it wasn't statistically significant), while men who took 7 or more tablets per week had a statistically significant doubling of risk. Thus, the authors conclude that high-dose vitamin C supplements should be avoided.
Why does vitamin C cause kidney stones? Vitamin C can be metabolized into oxalate. This ion combines with calcium in the body to form calcium oxalate -- a hard crystal that forms most kidney stones.
The authors indicate two important caveats: The study cannot be extended to women (who are not as prone to kidney stones as men) or to dietary intake of vitamin C (since other nutrients might affect vitamin C metabolism).
Still, this study should perhaps make people think twice before massively overdosing on vitamin C during cold season... or any other time of the year.
Source: Laura D. K. Thomas, Carl-Gustaf Elinder, Hans-Göran Tiselius, Alicja Wolk, Agneta Åkesson. "Ascorbic Acid Supplements and Kidney Stone Incidence Among Men: A Prospective Study" JAMA Intern Med 2013;():1-2. Published online February 4, 2013. doi:10.1001/jamainternmed.2013.2296