Added the following:
Do NOT Supplement with Higher than Normal Doses of Vitamins B6, B12 and Folic AcidA study of people with both Type 1 and Type 2 diabetes who had been diagnosed with diabetic kidney disease found that supplementing daily with a "Single tablet of B vitamins containing folic acid (2.5 mg/d), vitamin B6 (25 mg/d), and vitamin B12 (1 mg/d)" for three years was associated with faster deterioration in GFR and a doubling of stroke risk.
Effect of B-Vitamin Therapy on Progression of Diabetic Nephropathy:A Randomized Controlled Trial. Andrew A. House et al. JAMA 2010;303(16):1603-1609. doi: 10.1001/jama.2010.490
It's important to note that the amounts used in this study were megadoses many times greater than the RDA (Recommended Daily Allowance) for each.
If you are eating a diet that does not include grains or other foods that supply natural forms of these vitamins you won't be getting them in your diet and will need to supplement, but when you do, confine your supplementation to the RDA. You can find the RDA for the B vitamins HERE.