Diabetics with kidney damage should not take high doses of certain B vitamins because the pills could do them serious harm, Canadian researchers advise in a new study.

The research, published in JAMA, the Journal of the American Medical Association, could have important implications for Canada's 3 million diabetics, since about 40 per cent will eventually develop kidney disease.

The study had been designed to determine whether high-dose B vitamin therapy could improve kidney function in diabetics. Instead, it found just the opposite.

Those diabetic patients who took high doses of folic acid, B6 and B12 had increased kidney damage and double the risk of heart attacks and strokes compared to those given placebo pills.

Dr. David Spence and Dr. Andrew A. House of the University of Western Ontario's Robarts Research Institute began the study after noting previous research which suggested that B vitamins can lower levels of a blood-clotting amino acid called homocysteine.

High blood levels of homocysteine have been linked to a higher risk of heart disease and stroke. Some studies have also suggested the substance can damage the inner lining of arteries and raise the risk of kidney damage.

Since diabetics are already at twice the risk of having heart disease or a stroke, the UWO team wanted to know whether B-vitamin therapy would slow the progression of kidney disease and prevent strokes and other vascular events in patients with type 1 or 2 diabetes.

They recruited 238 patients, half of whom took a daily tablet of B vitamins containing 2.5 mg of folic acid, 25 mg of vitamin B6, and 1 mg of vitamin B12; the rest took a placebo pill. Participants were then followed for an average of 32 months.

They found that the kidney function in vitamin-treated group actually worsened more rapidly than the placebo group. The group taking the vitamins also had twice the risk compared to the placebo group of having a heart attack, stroke, heart surgery or of dying within three years of the study start.

"Given the recent large-scale clinical trials showing no treatment benefit, and our trial demonstrating harm, it would be prudent to discourage the use of high-dose B vitamins as a homocysteine-lowering strategy outside the framework of properly conducted clinical research," the authors conclude.

The researchers say one reason for their findings could be that people who already have kidney damage may not be able to process high quantities of vitamins.

They note that water-soluble B vitamins are normally filtered through the kidneys. It may be that in people with kidney failure, the vitamins cannot be process adequately so that they build up to toxic levels.

Study co-author Dr. Spence noted in comments to The Canadian Press that many patients have been known to take the amount of vitamins used in his study. He suggested his team's findings should serve as a warning about vitamins in general, which can be harmful if taken in high quantities.

"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," he said.