Effects of Metformin Treatment on Myocardial Efficiency in Patients With Heart Failure

Inclusion Criteria: * Patients with chronic heart failure uptitrated to recommended or maximally tolerated dose of ACE-I/ARB (unless contraindicated) and beta-blocker (unless contraindicated). If indicated, an aldosterone receptor antagonist should be given (unless contraindicated). An ICD and/or CRT should be implanted, if indicated. Patients with a CRT device should be treated for \> 3 months. * LVEF \< 45% * NYHA-class II, III or IV * Relatively preserved renal function (eGFR \> 30 ml/min) * Ability to understand the written patient information and to give informed consent * Negative urine-HCG for women of childbearing potential * Patients must have insulin resistance, defined as 1 or more of the following criteria: 1. HbA1c 5.5 - 6.4% (37 - 47 mmol/mol) within the last 12 months prior to enrolment 2. Impaired fasting glucose (IFG): Fasting P-glucose 5.6 - 6.9 mmol/l within 12 months prior to enrolment (patient in stable condition) 3. Impaired glucose tolerance (IGT) if OGTT has been performed at any time prior to enrolment: Fasting P-glucose \< 7.0 mmol/l and 2 hour P-glucose 7.8 -11.0 mmol/l Exclusion Criteria: * Metformin treatment within the last 3 months * Known allergy to metformin or major side effects to metformin treatment * Acute myocardial infarction, unstable angina or revascularization \< 3 months at the time of randomization * Planned coronary revascularization * Significant, uncorrected cardiac valve disease * Cardiac arrest or life threatening ventricular arrhythmias within the last 3 months (unless treated with an ICD) * Atrial fibrillation with poorly controlled ventricular rate at rest (\> 100 beats/min) * Hypertrophic or restrictive cardiomyopathy, infiltrative or storage myocardial disease, active myocarditis, or pericardial disease. * Planned major surgery * Female patients who are pregnant, nursing, or of childbearing potential while not practicing effective chemical contraceptive methods (i.e. oral, implanted, injectable, or transdermal contraceptive hormones; intrauterine device) * Age \< 18 years * Current abuse of alcohol or drugs * Cancer, with a life-expectancy of less than 2 years * Stroke within the last 6 months * Liver disease with P-ALAT \>3 times upper normal limit (it is possible to repeat this measurement once within a month) * Significant comorbidity or issue that makes the patient unsuitable for participation as judged by the investigator * Participation in another study involving long-term medical intervention (participation in device studies is allowed)