Men who take vitamin C supplements are at higher-than-average risk of developing kidney stones, according to a Swedish study of more than 22,000 men.
"It has long been suspected that high doses of vitamin C may increase the risk of kidney stones as some of the vitamin C absorbed by the body is excreted in urine as oxalate, one of the key components of kidney stones," said lead researcher Laura Thomas at the Karolinska Institutet in Stockholm.
Thomas and her team, whose findings appeared in JAMA Internal medicine, used data from a large study of middle-aged and elderly Swedish men who answered a series of questions on their diet and lifestyle, then were tracked for an average of 11 years.
The current analysis included 907 men who said they took regular vitamin C tablets and more than 22,000 who didn't use any nutritional supplements.
Of the vitamin C users, 3.4 per cent developed kidney stones for the first time during the study, compared to 1.8 per cent of non-supplement users. Men who took vitamin C supplements at least once a day had the highest risk of kidney stones.
Stones are made up of tiny crystals, which can be formed by calcium combining with oxalate. They usually pass on their own, but can cause severe pain in the process, though larger stones occasionally require surgery.
The findings don't prove that the vitamin itself triggers stones to form. But researchers said that because there are no clear benefits tied to taking high-dose vitamin C, people who have had stones in the past might want to think before taking extra supplements.
Men are more likely to develop stones than women.
The findings don't mean people shouldn't get plenty of vitamin C through fruits and vegetables, since the antioxidant is important for bone and muscle health - and severe deficiency can cause scurvy.
"Vitamin C is an important part of a healthy diet," Thomas said. "Any effect of vitamin C on kidney stone risk is likely to depend both on the dose and on the combination of nutrients with which it is ingested.
Swedish supplements, like those the study participants would have taken, typically contain about 1,000 milligrams (mg) of vitamin C per tablet. Most vitamin C supplements sold in the US contain either 500 or 1,000 mg.
Brian Matlaga, a urologist who studies kidney stones at Johns Hopkins Medicine in Baltimore, said that more research is needed to determine for certain whether reasonable doses of vitamin C may increase the kidney stone risk.
For now, people who haven't had kidney stones before shouldn't worry about related risks tied to the vitamin.