We will investigate the concentration of GDF-15 in blood samples in patients affected by mitochondrial disorders and compare it to the concentrations in patients affected by metabolic myopathies, muscular dystrophies and to a group of healthy controls. If there is a significant difference, GDF-15 may be a sensitive biomarker for mitochondrial disorders. We will further investigate the maximal oxidative capacity and the maximal workload capacity to study if and how this relates to the concentration of GDF-15.