B vitamins harmful, not helpful, for diabetic nephropathy

Taking B vitamins reduces hyperhomocysteinemia in patients with diabetic nephropathy but increases their risk of vascular events, a new study found.

The randomized controlled trial included 238 Canadians with type 1 or 2 diabetes and nephropathy. The patients were given a daily tablet that included 2.5 mg of folic acid, 25 mg of B₆ and 1 mg of B₁₂ or placebo and were followed for 36 months. At the conclusion of the study, patients in the B-vitamin group had a mean decrease in their radionuclide glomerular filtration rate (GFR) of 16.5 mL/min/1.73 m² compared to only 10.7 mL/min/1.73 m² in the placebo group.

Those who took B vitamins also had double the risk of myocardial infarction, stroke, revascularization and all-cause mortality compared with the placebo group. The groups required dialysis at similar rates, but the B-vitamin group did see a decrease in plasma total homocysteine, while the placebo group had an increase. The study was published in the April 28 Journal of the American Medical Association.

While other trials have suggested no benefit and potential harm from pharmacologic doses of B vitamins, this is the first to show significant detrimental effects, the study authors said. It's possible that homocysteine-lowering would be protective against vascular disease, but that the benefits were outweighed by toxicity of B vitamins. If that is the case, alternate methods of lowering homocysteine may be needed, the authors said.

Given that patients with renal impairment would be less able to excrete the vitamins, it's not certain that these results would be generalizable to other populations, the authors noted. Still, they concluded that the use of high-dose B vitamins to lower homocysteine levels should be discouraged based on their results and other large-scale trials finding no benefit.