Intermittent Fasting to Improve Insulin Secretion

Inclusion Criteria: * Body mass index (BMI) between 25 - 40 kg/m² * Understand and voluntarily sign an informed consent document prior to any study related assessments/procedures. * Subjects with prediabetes (IFG and/or IGT, HbA1c 5,4 % - 6,4 %, subphenotype cluster 3 or 5) or * Subjects with type 2 diabetes mellitus (diagnosed \<5 years prior to screening), HbA1c 6.0% - 9.5%, not receiving insulin or thiazolidinediones, and with an appropriate washout period for all other antidiabetic medications) Exclusion Criteria: * Subjects with diabetes mellitus type 1 (GAD-, IA2-AB positive) * Women during pregnancy and lactation * Treamtent with any medication effecting on glucose metabolism like anti-diabetic drugs or steroids * Subjects with a haemoglobin (Hb) ≤ 11.5 g/dl (for males) and Hb ≤ 10.5 g/dl (for females) at screening * Any pancreatic disease * Medical history of cancer and/or treatment for cancer within the last 5 years. * Known current presence or history of severe neurological or psychiatric diseases, schizophrenia, bipolar disorder * Known history of bariatric surgery * Severe liver or kidney diseases (Alanine Aminotransferase (ALT \[SGPT\]), Aspartate Aminotransferase (AST \[SGOT\]) above 3 x upper limit of normal (ULN) or Glomerular Filtration Rate (eGFR) ≤ 60 ml/min (MDRD formula) * Systemic infection (CRP \> 1 mg/dl) * Severe diabetic complications like chronic kidney disease (CKD), proliferating retinopathy or symptomatic cardiovascular disease * Presence of any contraindication for the conduct of an MRI investigation, such as cardiac pacemakers, ferromagnetic haemostatic clips in the central nervous system, metallic splinters in the eye, ferromagnetic or electronically operated active devices like automatic cardioverter defibrillators, cochlear implants, insulin pumps and nerve stimulators, prosthetic heart valves etc. * Persons with limited temperature sensation and / or elevated sensitivity to warming of the body * Persons with a hearing disorder or a increased sensitivity for loud noises * Claustrophobia * Participation in other clinical trials or observation period of competing trials up to 30 days prior to this study * Refusal to get informed of unexpected detected pathological findings