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Diabetics with kidney damage who take B vitamins to offset their risk of heart attack and stroke may be doing just the opposite.

New research shows the vitamins seem to increase the risk of heart attack and stroke and cause further kidney damage among diabetics with an existing impairment of kidney function.

The surprising findings, published today in the Journal of the American Medical Association, send a warning to some people with diabetes about the potential dangers of taking vitamins and supplements.

"I think a lot of people take vitamins over the counter sort of assuming that vitamins are natural so they must be safe," said David Spence, lead author of the study, professor of neurology and clinical pharmacology at the University of Western Ontario and director of the Stroke Prevention & Atherosclerosis Research Centre at the Robarts Research Institute.

Diabetics typically have higher levels of the amino acid homocysteine than the average population, which has been associated with a higher risk of heart attack and stroke. B vitamins, including folic acid and vitamins B12 and B6, seem to lower homocysteine levels, so Dr. Spence and his colleagues set out to determine what role the vitamins play and how effective they are at reducing those risks.

The researchers studied 238 diabeticswho also suffered from impaired kidney function. Half of the group received high doses of vitamin B, and the other half received placebo and were monitored for 36 months.

But instead of finding the vitamins helped patients, researchers discovered it led to a significantly higher rate of heart attack and stroke, and even greater kidney problems.

Specifically, eight people who received vitamin B therapy suffered heart attacks in the 36-month follow-up period, compared with four in the placebo group. Six members of the vitamin B group had a stroke, compared with one person in the placebo group. They also found that diabetics who received vitamin B therapy experienced greater kidney damage than the placebo group.

Dr. Spence said he was astonished by the results and thought he was reading them wrong.

Now that they have been confirmed, Dr. Spence said he believes diabetics with kidney damage should not take high doses of vitamin B supplements.

"It doesn't mean that people with kidney impairment can't take ordinary one-a-day vitamins probably, but it means they shouldn't' be taking high doses," Dr. Spence said.

Although it's unclear why B vitamins seem linked to increased health risks, Dr. Spence said it may be because they are normally excreted in urine. Patients with kidney disease may not be able to flush the vitamins out, leading to a toxic buildup.

He added that patients should check their vitamin B levels with a health professional to determine whether they have a deficiency and if taking supplements are necessary.

Few, if any, physicians would routinely direct patients to take high doses of B vitamins to lower their homocysteine levels, said Judith Miller, a nephrologist and provincial clinical director of the Ontario Renal Network. However, based on some preliminary research, there was reason to believe the vitamins played a role in reducing the risk of heart attack, stroke and kidney damage.

Now the new study has proven that hypothesis false. Yet, Dr. Miller said it's possible the vitamin doses were too high and cancelled out any potential benefit.

While the study focuses on diabetics with existing kidney damage, one expert says it contains a valuable message for all people with diabetes.

Barbara Araneo, director of complications therapies with the Juvenile Diabetes Research Foundation in New York, said the population's overall shift toward greater consumption of vitamins and supplements can have serious unintended consequences for people with diabetes.

Good nutrition is a critical concern for the diabetic population, she said, but that could lead some to overload on certain vitamins - and potential problems. One of the major problems is that very little research has properly evaluated the effect vitamins can have on the health of diabetics.

"We don't know enough about over-the-counter supplements to be able to say 'Yes, you should take them,' " Dr. Araneo said. "The general theory is anything good for the general population is good for chronic diseases but you should talk to your primary care giver."

Dr. Spence's study was funded by a grant from the Canadian Institutes of Health Research and the Kidney Foundation of Canada. The B vitamins and placebo were provided by Pan American Laboratories, which produces homocysteine-related therapies. But no sponsors had a say in how the study was designed, according to the research team.

Dr. Spence disclosed in the study that he has received past consulting fees from Pan American Laboratories and Medice Arzneimittel Putter GmbH & Co., a German drug maker. He also has a patent pending with colleague Andrew House on a new therapy to lower homocysteine levels in dialysis patients.