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.
The new finding suggests that stone formers who take regular vitamin C may actually place themselves at increased risk.
Researchers from 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.
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.
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.
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.
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 North America 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. Other scientists advise at least 200 milligrams per day to prevent chronic disease.
More research is needed to determine for certain whether reasonable doses of vitamin C may increase the risk of kidney stones. For now, the researchers said people who haven't had kidney stones before shouldn't worry about any related risks tied to the vitamin.
SOURCE: JAMA Internal Medicine, online February 4, 2013.
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