To read this article in full you may need to log in, make a payment or gain access through a site license (see right).Nature Reviews Nephrology 9, 187 (April 2013) | doi:10.1038/nrneph.2013.29Ascorbic acid and risk of kidney stones Men who take ascorbic acid supplements (a form of vitamin C) are twice as likely to develop kidney stones as men who do not take any dietary supplements, say the authors of a research letter published in JAMA Internal Medicine. “High doses of vitamin C have long been suspected to increase the risk of kidney stones since some vitamin C is excreted as oxalate—one of the key components of kidney stones,” explains researcher Agneta Åkesson. To read this article in full you may need to log in, make a payment or gain access through a site license (see right).Vitamin C is a water-soluble substance, and therefore does not accumulate in the body. This accounts for the absence of any data on vitamin C toxicity per se. Vitamin C in itself is of low toxicity, and has only minor adverse effects, such as diarrhea, nausea and other digestive disturbances.
These upsets are due to the osmotic withdrawal of water from the intestinal contents by the unabsorbed vitamin C in the gastrointestinal tract. Other physiologic disruptions due to high vitamin C levels include metabolic acidosis and altered levels of some coagulation factors such as prothrombin. Vitamin C metabolism produces oxalate as a product. Some research suggests that vitamin C supplements may increase urinary oxalate concentrations, and double the risk of calcium oxalate kidney stones. Several drugs reduce vitamin C levels, such as the contraceptive pills that contain estrogen, barbiturates and tetracyclines. Vitamin C also raises estrogen levels. Regular aspirin intake above a certain frequency, such as 1000 mg every 6 hours, increases the urinary excretion of vitamin C and reduces its blood levels. Vitamin C causes higher serum levels of aspirin and other NSAIDs. Vitamin C can inhibit the action of warfarin, and thus the dosage of warfarin requires to be monitored by the prothrombin time.
It may also reduce the cardioprotective action of cholesterol-lowering drugs such as simvastatin and niacin, though the evidence is far from clear. Some large trials indicate that it does not affect the positive effects of these drugs. It has been suggested that vitamin C use during cancer cell chemotherapy could diminish the efficacy of treatment by protecting tumor cells from destruction. Other studies, though, indicate that vitamin C exerts a possible protective effect on normal cells against the damage induced by radiotherapy or chemotherapy. It has also been suggested that high doses of vitamin C could lead to oxidative damage by contributing to free radical production, but this has not been experimentally shown. Tests such as serum bilirubin, serum creatinine and the guaiac test for the presence of occult blood depend upon the presence of reducing substances, and thus high levels of vitamin C can interfere with their interpretation. A single study showed that vitamin C supplementation appeared to increase the cardiovascular disease mortality in post-menopausal diabetic women, but this has never been confirmed.
Vitamin C may theoretically cause excessive iron absorption, but this is likely to be of significance only in those who have hereditary hemochromatosis and consume high doses of vitamin C. When taken along with aluminium-containing antacids, vitamin C boosts aluminium absorption and therefore the risks of side-effects. Megadoses of vitamin C in pregnancy may cause rebound scurvy in the infant, due to the sudden drop in levels causing a relative deficiency of the vitamin in the baby’s blood after birth. Thus, doses above 1000 mg vitamin C should be avoided in pregnancy unless on medical advice. Other dubious effects, which have been raised over the years, but never been proven to exist, include: FNB-established upper levels are age-specific, and are intended to prevent side effects. Precautions to prevent adverse reactions include:NEW YORK (Reuters Health) - 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.