The most frequent form of adult-onset mitochondrial disorders is mitochondrial myopathy, often manifesting with progressive external ophthalmoplegia (PEO), progressive muscle weakness and exercise intolerance. Mitochondrial myopathy is often caused by single heteroplasmic mitochondrial DNA (mtDNA) deletions or multiple mtDNA deletions, the former being sporadic and latter caused by mutations in nuclear-encoded proteins of mtDNA maintenance. Currently, no curative treatment exists for this disease. The investigators have previously observed that supplementation with an NAD+ precursor vitamin B3, nicotinamide riboside, prevented and delayed disease symptoms by increasing mitochondrial biogenesis in a mouse model for mitochondrial myopathy. Vitamin B3 exists in several forms: nicotinic acid (niacin), nicotinamide, and nicotinamide riboside, and it has been demonstrated to give power to diseased mitochondria in animal studies by increasing intracellular levels of NAD+, the important cofactor required for the cellular energy metabolism.

In this study, the form of vitamin B3, niacin, was used to activate dysfunctional mitochondria and to rescue signs of mitochondrial myopathy. Of the vitamin B3 forms, niacin, is employed, because it has been used in large doses to treat hypercholesterolemia patients, and has a proven safety record in humans. Phenotypically similar mitochondrial myopathy patients are studied, as the investigator's previous expertise indicates that similar presenting phenotypes predict uniform physiological and clinical responses to interventions, despite varying genetic backgrounds. Patients either with sporadic single mtDNA deletions or a mutation in a Twinkle gene causing multiple mtDNA deletions were recruited. In addition, for every patient, two gender- and age-matched healthy controls are recruited. Clinical examinations and collection of muscle biopsies are performed at the time points 0, 4 and 10 months (patients) or at 0 and 4 months (controls). Fasting blood samples are collected every second week until 4 months and thereafter every six weeks until the end of the study. The effects of niacin on disease markers, muscle mitochondrial biogenesis, muscle strength and the metabolism of the whole body are studied in patients and healthy controls.

The hypothesis is that an NAD+ precursor, niacin, will increase intracellular NAD+ levels, improve mitochondrial biogenesis and alleviate the symptoms of mitochondrial myopathy in humans.

Inclusion Criteria:

1. Manifestation of pure mitochondrial myopathy, with no major other symptoms or manifestations, caused by single or multiple deletions of mtDNA
2. Age and gender matched healthy controls for every patient
3. Agreed to avoid vitamin supplementation or nutritional products with vitamin B3 forms 14 days prior to the enrollment and during the study
4. Written, informed consent to participate in the study

Exclusion Criteria:

1. Inability to follow study protocol
2. Pregnancy or breast-feeding at any time of the trial
3. Malignancy that requires continuous treatment
4. Unstable heart disease
5. Severe kidney disease requiring treatment
6. Severe encephalopathy
7. Regular usage of intoxicants