Study to Identify Adequate Biomarker to Detect Vitamin B12 Deficiency in Type 2 Diabetic Patients Treated With Metformin

Inclusion Criteria: * Diagnosis of T2DM \> 6 months * Metformin treatment for at least 6 months * Age \> 18 years * Ability to give written informed consent * No metformin treatment in the last 6 months (control group) Exclusion Criteria: * Concurrent intake of preparations containing VB12 (within the last 3 months prior to study participation) * Lack of written and/or oral understanding in German, French, Italian or English languages * Diagnosis of Transcobolamin transporter defect * Diagnosis of Chronic or acute liver diseaseliver insufficiency with CHILD-PUGH scores B and C and acute hepatitis * Diagnosis of Renal disease (Creatinine-Clearance \<60 ml/min)renal insufficiency stadium III, IV and V (KDOQI) and acute renal diseases