TORONTO (Canadian Press) -- Diabetics with impaired kidneys should avoid taking high doses of certain B vitamins because the supplements may do serious harm, researchers say.

In a study of 238 diabetics with failing kidneys, researchers found that those taking high doses of folic acid, B6 and B12 had increased renal damage and double the risk of heart attacks and strokes compared to those given dummy pills.

"People with kidney failure shouldn't be taking high doses of these water-soluble vitamins, like the B vitamins," said principal investigator Dr. David Spence of the University of Western Ontario's Robarts Research Institute.

More than three million Canadians have diabetes, and about 40 per cent will eventually develop kidney disease.

The 2001-2007 study, published in Wednesday's edition of the Journal of the American Medical Association, was designed to determine whether high-dose B vitamin therapy could improve kidney function in diabetics.

Previous research has suggested that B vitamins can lower levels of a blood-clotting substance called homocysteine. High concentrations of this amino acid are known to harm the lining of arteries, bumping up the risk of kidney damage, heart attack and stroke - all complications of poorly controlled diabetes.

"So we thought that lowering homocysteine with vitamins ought to slow down the rate of decline of kidney function in diabetics with kidney damage and maybe also reduce the risk of strokes and heart attacks," Spence said from London, Ont.

To conduct the study, subjects were randomly assigned to receive either the vitamins or a placebo. Neither they nor the researchers knew who was getting which treatment until after the data had been collected and analyzed, and the randomization revealed.

"The results were so surprising that when I first saw the results, I thought we must have the randomization code backwards," said Spence. "But then we looked and found that, in fact, the vitamin therapy had reduced the homocysteine levels. So clearly the randomization code was right."

He said discovering the vitamin-treated group's kidney function worsened more rapidly and that they had more strokes and heart attacks than the placebo group "was astonishing."

In their paper, the authors suggest a number of possible reasons. One strong possibility relates to the fact that water-soluble B vitamins are normally washed from the body via the urine after being filtered through the kidneys.

"It may be that people with kidney failure can't get rid of the vitamins (and they) build up toxic levels," Spence said, noting that B vitamin therapy in people without failing kidneys has been shown to reduce heart attack and stroke rates.

"So I think what that's telling us is that (these) vitamins are beneficial for people with normal kidney function and harmful for people with poor kidney function."

B.C. kidney specialist Dr. Adeera Levin said researchers have been trying to find ways to reduce the progression of kidney disease for some time. This "excellent, well-designed study" asked an important question -- and the answer "is certainly pretty definitive."

"And it's important because we need to make sure we're taking good care of patients and what works in non-kidney disease patients doesn't always work in kidney disease patient groups, and this is yet another example of that," Levin said Tuesday from Vancouver.

While the study specifically involved people with Type 1 or Type 2 diabetes, she said the results could likely be extrapolated to non-diabetics with kidney damage.

"If we're in an era where safety is our primary concern, then I would suggest that this study says that perhaps water-soluble vitamins in high doses are not so safe in a group of people who have problems with their kidneys," said Levin, a spokeswoman for the Kidney Foundation of Canada.

"This particularly applies to people with diabetes, but may well apply to others as well."

The doses used in the study were many times higher than the amounts of folic acid, B6 and B12 found in a typical multivitamin, said Spence, but elevated doses are "often" prescribed by doctors or "self-prescribed" by patients themselves.

He suggested the study's findings provide a cautionary note about vitamins in general, which can be harmful if taken in high quantities. Taking too much D, a fat-soluble vitamin, can lead to kidney stones, as can excess ingestion of water-soluble vitamin C; high doses of fat-soluble vitamin A can be toxic.

"Vitamins are really powerful chemicals, so just the fact that they're natural doesn't mean that high doses are always going to be OK."

Copyright The Canadian Press, 2010