TORONTO — 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.