Dr. David Spence of The University of Western Ontario and Dr. Meir Stampfer of the Harvard School of Public Health said vitamin B therapy was once widely used to lower homocysteine levels.
Too much of this amino acid in the bloodstream was linked to increased risk of stroke and heart attack, but several randomized trials found lowering homocysteine levels with B vitamins did not result in a cardiovascular benefit.
In fact, Spence, a scientist with the Robarts Research Institute at Western's Schulich School of Medicine & Dentistry, found vitamin B therapy actually increased cardiovascular risk in patients with diabetic nephropathy.
In an commentary published in the Journal of the American Medical Association, Spence and Stampfer said two key issues have been overlooked in the interpretation of the clinical trials -- the key role of vitamin B12, and the newly recognized role of renal failure.
"It is now clear that the large trials showing no benefit of vitamin therapy obscured the benefit of vitamin therapy because they lumped together patients with renal failure and those with good renal function," Spence said in the commentary. "The vitamins are harmful in renal failure, and beneficial in patients with good renal function, and they cancel each other out," said Spence, the author of "How to Prevent Your Stroke."
The commentary authors also contend most of the trials did not use a high enough dose of vitamin B12.