Oxidative Skeletal Muscle Metabolism in Chronic Heart Failure Patients With and Without Iron Deficiency

Inclusion Criteria: For HFrEF patients: * Diagnosis of chronic HF of either ischemic or non-ischemic etiology; * Stable, evidence-based medical therapy for HF; * LVEF \<40% measured \<5 year prior to inclusion; * NYHA class II - III (symptomatic HF) at moment of inclusion; For HFpEF patients: * Diagnosis of chronic HF of either ischemic or non-ischemic etiology; * LVEF \>40% and one of the following parameters, measured \<5 year prior to inclusion; * Left atrial volume index (LAVI) \>34 mL/m2 or * left ventricular mass index ≥115 g/m2 (for males) or ≥95 g/m2 (for females) or * E/e' ≥13 or * mean e' (septal and lateral) \<9 cm/s * NYHA class II - III (symptomatic HF) at moment of inclusion; * Serum NT-proBNP ≥125 pg/mL when in sinus rhythm; \>300 pg/mL when in atrial fibrillation. Additional inclusion criterion for subjects with ID: \- Iron deficiency, defined as TSAT \<20%. Exclusion Criteria: * Age \<18 years; * Unable or unwilling to undergo exercise MRI (e.g. pregnancy, physical disabilities, claustrophobia); * The presence of ferromagnetic material in/on the body which cannot be removed (e.g. non-MRI-compatible cardiac devices, tattoos containing ferrous ink); * History of erythropoietin stimulating agent, intravenous iron therapy and/or blood transfusion \<3 months prior to study enrolment; * Moderate anaemia, defined as Hb \<7 mmol/L for both men and women; * Oral iron therapy \>100 mg/day \<4 weeks prior to study enrolment; * Unable to understand study procedures; * Unable or unwilling to provide informed consent.