The purpose of this study is to investigate the potential beneficial effects of a daily supplement of Resveratrol (1000mg/day) on physical ability and on muscle metabolism in patients with verified mitochondrial myopathy and patients with a verified fatty acid oxidation defect of VLCAD and CPTII deficiencies. Investigators hypothesize an improved muscle metabolism, mitochondrial function, fatty acid oxidation and thus improvement of physical ability.

Study design: double-blind, randomized, placebo-controlled, cross-over study.

To ensure enough participants, and due to the risk of a heterogeneous cohort, a cross-over design is chosen, where participants are their own controls. 10 patients with mitochondrial myopathy and 10 patients with fatty acid oxidation defects will be included. Eligible patients will be randomized using a 1:1 assignment ratio to receive placebo or RSV first. Each treatment will be administered orally twice daily for 8 weeks, followed by a 4 weeks wash-out, and afterwards a new 8-week treatment period.

During the 20-week trial period, subjects will visit the trial site on five occasions, for functional assessments (cycle ergometer testing), blood sampling and questionnaires.

Inclusion Criteria:

1. Patient is willing and able to provide written informed consent prior to participation.
2. Patient is ≥18 and ≤80 years of age at baseline.
3. Patients have genetically verified mitochondrial disorder or a fatty acid oxidation deficiency (VLCAD/CPTII).
4. Patient has a clinical presentation, signs or symptoms suggestive of myopathy (e.g., easy fatigability, exercise intolerance, muscle pain) in the opinion of the Investigator.
5. Patient is ambulatory.

Exclusion Criteria:

1. Patient has any prior or current medical conditions that, in the judgment of the Investigator, would prevent the patient from safely participating in and/or completing all study requirements.
2. Patient has symptoms of mitochondrial myopathy due to known secondary mitochondrial dysfunction (e.g., drug-induced myopathy).
3. Patient does not have the cognitive capacity to understand/comprehend and complete all study assessments.
4. Pregnancy or breastfeeding