Serum vitamin B12 status of patients with type 2 diabetes mellitus on metformin: A single-center cross-sectional study from Bangladesh

Background: Metformin use is a known cause of B12 deficiency in patients with type 2 DM (T2DM). Diabetic peripheral neuropathy (DPN) often has clinically indistinguishable clinical features of B12 deficiency-induced peripheral neuropathy (PN). Objective: The present study aims to assess serum vitamin B12 levels in patients with T2DM on metformin. Subjects and Methods: This cross-sectional study was conducted at a specialized endocrine outpatient clinic in Cumilla, Bangladesh, over six months from January 2020 to June 2020. Non-pregnant adults (>= 18 years age) receiving metformin for T2DM for at least six months were evaluated for PN and assessed for serum B12 levels. Results: Among 90 subjects evaluated, 28 (31.1%) had B12 deficiency and 6 (6.7%) had borderline B12 deficiency; 56 (62.2%) had normal B12 levels. Study subjects with subnormal B12 used metformin for a longer duration [8.5 (7.0-14.0) vs. 5.0 (2.25-10.0) years, median (IQR), P = 0.006], gram-years of metformin use was higher in them [12.0 (7.9-14.0) vs. 5.75 (2.0-13.6) years, median (IQR), P = 0.005] and they had a higher mean corpuscular volume [85.9 +/- 7.2 vs. 82.4 +/- 6.4 fL, mean +/- SD, P = 0.020] compared to those having normal B12 levels. Serum B12 levels had a strong negative correlation with duration of metformin use and gram-years of metformin use. B12 status did not influence the presence and severity of PN. Conclusions: A considerable number of patients with T2DM have subnormal B12 levels. Periodic screening for serum vitamin B12 level may be of clinical benefit in such patients.