The overarching aim of this observational study is to determine alterations in energy balance while exploring the underlying cellular mechanisms in human genetic models of mitochondrial stress.

In a case-control design, individuals with pathogenic mitochondrial DNA mutations will be compared to healthy controls matched for sex, age, and physical activity level. Participants will attend a screening visit and an experimental trial including assessments of energy expenditure, appetite sensation, energy intake, and muscle and subcutaneous adipose tissue biopsy samples.

Background: Pre-clinical models of mitochondrial stress are resistant to diet-induced obesity. Likewise, humans with primary mitochondrial diseases present a high prevalence of underweight (42%) as compared to a very low prevalence of obesity (2%). In this direction, recent data show a lower BMI across 17 cohorts of patients with mitochondrial diseases compared to national averages, suggesting mitochondrial stress-induced increments in resting energy expenditure as the primary driver of the lean phenotype. In recent years, the study of humans with genetic mutations has shown enormous potential to establish the mechanistic link between two physiological variables; indeed, if the mutation has a functional impact on one of those variables, then the direction of causality can be readily ascribed. Taken together, studies integrating assessments of energy balance with mitochondrial phenotyping in patients with rare mitochondrial disorders hold the potential to uncover putative mechanisms conferring protection from obesity in humans.

Objective: To determine alterations in energy expenditure/intake while exploring the underlying cellular mechanisms in individuals harboring mitochondrial DNA (mtDNA) mutations associated with mitochondrial stress.

Study design: Case-control study in individuals with mtDNA mutations (n=15) and healthy controls (n=15) matched for sex, age, and physical activity level.

Endpoint: Differences between individuals with mtDNA mutations and controls.

Eligibility criteria for individuals with mitochondrial DNA mutations

Inclusion criteria:

- Known mtDNA point mutations

Exclusion criteria:

• Use of antiarrhythmic medications or other medications which, in the opinion of the investigators, have the potential to affect outcome measures.
• Diagnosed severe heart disease, dysregulated thyroid gland conditions, or other dysregulated endocrinopathies, or other conditions which, in the opinion of the investigators, have the potential to affect outcome measures.
• Pregnancy

Eligibility criteria for controls

Exclusion criteria:

• Current and regular use of antidiabetic medications or other medications which, in the opinion of the investigators, have the potential to affect outcome measures.
• Diagnosed heart disease, symptomatic asthma, liver cirrhosis or -failure, chronic kidney disease, dysregulated thyroid gland conditions or other dysregulated endocrinopathies, or other conditions which, in the opinion of the investigators, have the potential to affect outcome measures
• Daily use of tobacco products
• Excessive alcohol consumption
• Pregnancy

Individuals with mitochondrial myopathy due to pathogenic mtDNA mutations are identified and recruited from the Copenhagen Neuromuscular Center or the Department of Clinical Genetics (Rigshospitalet).

Control volunteers are recruited via recruitment announcements in Denmark.