Although the finding doesn’t directly prove that vitamin C is at fault, and the actual occurrence of kidney stones — the tiny mass of crystals that can painfully clog the urinary tract — remained fairly rare, the findings raise questions about whether large doses of the vitamin are dangerous to the body.
“It is important that the public is aware that there may be risks associated with taking high doses of vitamin C,” said study co-author Agneta Akesson, an associate professor with the Karolinska Institute in Stockholm, Sweden. “Those with a history of kidney stones should consult their doctor before taking high-dose vitamin C supplements.”
Researchers have suspected that large amounts of vitamin C may increase the risk of kidney stones because the body breaks down the vitamin into a substance called oxelate, a component of the stones, Akesson said.
In the new study, which was published online Feb. 4 in the journal JAMA Internal Medicine, researchers tracked more than 23,000 Swedish men who were between 45 and 79 years old in 1997. The researchers followed the men, who hadn’t previously been diagnosed with kidney stones, until 2009.
About 900 of the men regularly took 1,000-milligram doses of vitamin C, and 31 of them (3 percent) developed kidney stones. Of the rest of the larger group, fewer than 2 percent developed kidney stones.
After the researchers adjusted their statistics so they wouldn’t be skewed by factors such as high or low numbers of men of certain ages, education levels and body weights, the researchers found that those who took the high-dose supplements had an increased kidney stone risk of between 1.7 and 2.2 times.
It’s easy to find vitamin C tablets in the 1,000-milligram size at American drugstores and health stores. The size provides much more vitamin C than the U.S. government’s recommended daily allowance, which is 75 milligrams to 90 milligrams for most adults.
Akesson said there are no well-documented reasons for anyone to take such high doses of vitamin C. The study doesn’t apply to vitamin C that people get from food, and the researchers did find that multivitamin supplements, which don’t contain mega doses of vitamin C, didn’t seem to boost the risk of kidney stones.
It’s not clear if the same possible risk from mega doses would apply to women. The study didn’t include women, and they typically face a much lower overall risk of kidney stones than men, Akesson said.
Akesson said more research is needed to confirm the findings, and another expert added that there are not many reasons to take such high doses of the vitamin.
For now, “there are no clear reasons to take supplemental vitamin C if adequate vitamin C is consumed in the diet,” said Dr. Gary Curhan, a professor of medicine at Harvard Medical School who has studied kidney problems. “Of note, we have found that higher vitamin C intake is associated with a reduced risk of gout, but this is not sufficient justification to take a supplement.”