Men who take vitamin C supplements are at higher-than-average risk of developing kidney stones, a new study from Sweden suggests.
The findings don't prove 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.
"I don't think I would hold this up and say, ‘You shouldn't take vitamin C, and here's the evidence,'" said Dr. Brian Matlaga, a urologist who studies kidney stones at Johns Hopkins Medicine in Baltimore.
But, "When you talk to patients, a lot of times you'll find patients are taking non-prescribed medications, like vitamin supplements… and there may not be great evidence that there's an actual health benefit associated with these," he told Reuters Health.
The new finding "suggests that stone formers who take regular vitamin C may actually place themselves at increased risk," said Matlaga, who wasn't involved in the study.
Researchers led by Laura Thomas of the Karolinska Institutet in Stockholm 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 of those 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 percent developed kidney stones for the first time during the study, compared to 1.8 percent of non-supplement users. Men who took vitamin C supplements at least once a day had the highest risk of kidney stones, researchers reported Monday in JAMA Internal Medicine.
"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," Thomas told Reuters Health by email.
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. Larger stones occasionally require surgery.
Men are more likely to form stones than women.
'An important part of a healthy diet'
The findings don't mean people shouldn't get plenty of vitamin C through fruits and vegetables, researchers said. 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, she noted. Most vitamin C supplements sold in the U.S. contain either 500 or 1,000 mg.
The U.S. Institute of Medicine recommends 90 mg per day for men - the amount in a small glass of orange juice or a cup of broccoli - and 75 mg for most women.
Matlaga said more research is needed to determine for certain whether reasonable doses of vitamin C may increase the risk of kidney stones. For now, he said people who haven't had kidney stones before shouldn't worry about any related risks tied to the vitamin.