Effects of short-term treatment with metformin on serum concentrations of homocysteine, folate and vitamin B12 in type 2 diabetes mellitus: a randomized, placebo-controlled trial

Objective. Metformin is a key treatment option in type 2 diabetes. However, metformin may decrease vitamin B12 levels and increase levels of homo-cysteine, a cardiovascular risk factor. We investigated whether 16 weeks of treatment with metformin affects serum concentrations of homo-cysteine, folate and vitamin B12 in subjects with type 2 diabetes treated with insulin. Design. Placebo-controlled, randomized trial. Measurements: at baseline and 16 weeks later. Setting. This trial was conducted in the outpatient clinics of three general hospitals in The Netherlands. Subjects. A total of 745 patients with type 2 diabetes, treated with insulin and not known with a contraindication for the use of metformin, were approached; 390 gave informed consent and entered the study. Thirty-seven subjects dropped out (12 placebo and 25 metformin users). Intervention. Addition of metformin or placebo to insulin therapy. Primary outcome parameters. Serum homocysteine, folate, vitamin B12, indices of glycaemic control and body weight. Results. Amongst those who completed 16 weeks of treatment, metformin use, as compared with placebo, was associated with an increase in homocysteine of 4% (0.2 to 8; P = 0.039) and with decreases in folate [-7% (-1.4 to -13); P = 0.024] and vitamin B12 [-14% (-4.2 to -24); P < 0.0001]. In addition, the increase in homocysteine could be explained by the decreases in folate and vitamin B12. Conclusion. In patients with type 2 diabetes, 16 weeks of treatment with metformin reduces levels of folate and vitamin B12, which results in a modest increase in homocysteine. The clinical significance of these findings remains to be investigated.