Mitochondria occupy a central role in cellular metabolism, generating ATP through oxidative phosphorylation while also participating in signaling pathways, apoptosis, calcium homeostasis, and reactive oxygen species regulation. Because of this central metabolic role, mitochondrial abnormalities have been proposed as contributing factors in a wide variety of diseases. In pediatric practice, the concept of “mitochondrial dysfunction” has increasingly entered discussions around neurodevelopmental disorders, chronic fatigue, developmental delays, and other nonspecific symptoms. At the same time, a growing number of direct-to-consumer tests claim to measure “mitochondrial health.” For clinicians, this landscape raises an important question: what does the evidence actually support?
Understanding the distinction between primary mitochondrial disease, secondary mitochondrial dysfunction, and broader claims of mitochondrial impairment is essential for pediatricians counseling families and evaluating children with complex symptoms.
Primary mitochondrial disease: a rare but well-defined group of disorders
Primary mitochondrial diseases (PMDs) are a heterogeneous group of genetic disorders caused by pathogenic variants affecting mitochondrial structure or function. These variants may occur in mitochondrial DNA (mtDNA) or nuclear genes encoding mitochondrial proteins. More than 350 genes have been implicated in mitochondrial biology, reflecting the complexity of mitochondrial physiology and the wide range of potential phenotypes.
PMDs are rare but clinically important because they often involve high-energy tissues such as the brain, skeletal muscle, and heart. As a result, children with mitochondrial disease frequently present with multisystem manifestations including developmental delay, hypotonia, seizures, cardiomyopathy, liver disease, renal dysfunction, or failure to thrive.
Classical mitochondrial syndromes illustrate this pattern. Examples include Leigh syndrome, MELAS (mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes), and Kearns–Sayre syndrome, each characterized by relatively recognizable clinical constellations.
Despite these recognizable syndromes, diagnosing mitochondrial disease remains challenging. Clinical phenotypes are highly variable, and many laboratory markers commonly used in evaluation are nonspecific. For example:
- Serum lactate has variable sensitivity (reported between 34–62%).
- Creatine kinase may be normal or mildly elevated depending on the disorder.
- Metabolic markers such as amino acid abnormalities lack specificity.
Historically, diagnosis relied heavily on muscle biopsy and biochemical testing of respiratory chain complexes. However, advances in genomic sequencing have shifted the diagnostic paradigm toward a “genetics-first” approach, using whole-exome or whole-genome sequencing alongside mtDNA sequencing to identify pathogenic variants.
Importantly, even with modern genetic techniques, the diagnosis of primary mitochondrial disease requires integration of clinical phenotype, molecular testing, and sometimes biochemical or histologic evidence rather than a single biomarker or screening test.
Secondary mitochondrial dysfunction and the expansion of the concept
Beyond rare genetic mitochondrial diseases, the concept of “mitochondrial dysfunction” has expanded in the literature. Many conditions—ranging from metabolic disorders to neurodegenerative diseases—have been associated with alterations in mitochondrial function.
For example, mitochondria are highly active in neurons and play critical roles in neural development, calcium signaling, and oxidative stress regulation.
Because of this, mitochondrial abnormalities have been investigated in neurodevelopmental disorders such as autism spectrum disorder (ASD) and attention-deficit/hyperactivity disorder (ADHD).
A recent systematic review and meta-analysis examining mitochondrial DNA copy number (mtDNA-cn) in ASD and ADHD included 14 studies involving over 1,200 participants. The pooled analysis suggested an association between altered mtDNA copy number and these conditions, although findings varied widely across studies.
Crucially, the results also highlight major limitations:
- The studies demonstrated extreme heterogeneity, with some showing increased mtDNA-cn, others decreased levels, and some no difference.
- When analyses were restricted to blood-based measurements, differences between patients and controls were not statistically significant.
- Mitochondrial DNA copy number varies substantially between tissues and is influenced by cell type composition, age, and physiologic stress.
These findings underscore an important point: biochemical markers of mitochondrial function measured in blood do not necessarily reflect mitochondrial function in other tissues such as the brain.
Thus, while mitochondrial pathways may be involved in complex diseases, these observations do not imply that children with neurodevelopmental disorders broadly have clinically meaningful mitochondrial disease.
Nonspecific symptoms and the risk of over-attribution
In recent years, some clinical narratives have proposed mitochondrial dysfunction as a potential explanation for nonspecific symptoms such as fatigue, developmental regression, behavioral symptoms, or chronic illness. While mitochondrial disease can produce broad systemic symptoms, the diagnosis requires objective clinical and molecular evidence.
Several factors make mitochondrial biology particularly prone to over-interpretation:
- Ubiquity of mitochondria.
Because mitochondria are present in nearly all cells and participate in numerous metabolic pathways, abnormalities in mitochondrial markers may arise in many unrelated conditions. - Dynamic mitochondrial responses.
Changes in mitochondrial DNA copy number may occur as adaptive responses to oxidative stress or energy demand rather than representing disease. - Measurement variability.
Differences in cell populations—such as leukocytes and platelets in blood—can significantly alter measured mitochondrial DNA levels. - High heteroplasmy variability.
Mitochondrial DNA mutations may exist at low levels even in healthy individuals and vary between tissues and over time.
These biological complexities make it difficult to interpret isolated laboratory findings without a compatible clinical syndrome. For pediatricians, this reinforces the importance of anchoring evaluation in clinical presentation rather than broad metabolic screening.
Direct-to-consumer testing and “mitochondrial health”
Parallel to the growing interest in mitochondrial dysfunction, several commercial tests claim to assess “mitochondrial health,” often using markers such as mitochondrial DNA copy number or related metabolites. These tests are increasingly marketed directly to consumers and parents.
From an evidence-based perspective, several limitations are important:
- Lack of validated clinical utility.
Biomarkers such as mtDNA copy number are still under investigation and have not been validated as diagnostic tools for mitochondrial disease in routine clinical practice. - Tissue specificity.
Measurements from blood or saliva may not correlate with mitochondrial function in clinically relevant tissues such as muscle or brain. - High biological variability.
Factors such as age, stress, inflammation, and cell composition can influence mitochondrial biomarkers independently of disease. - Absence of diagnostic algorithms.
Unlike established medical evaluation pathways—which integrate clinical phenotype, genetics, imaging, and biochemical testing—consumer tests typically interpret isolated markers without context.
For these reasons, current expert guidance emphasizes that mitochondrial disease diagnosis should be made within a structured clinical evaluation and confirmed through molecular testing rather than through screening biomarkers alone.
Practical considerations for pediatricians
When evaluating children with suspected mitochondrial disease, several clinical principles remain helpful:
- Look for multisystem involvement.
True mitochondrial disorders often affect multiple high-energy organs such as the brain, muscle, and heart. - Consider recognized syndromes.
Conditions such as Leigh syndrome or MELAS provide clearer diagnostic frameworks. - Use genetics strategically.
Comprehensive sequencing of nuclear and mitochondrial DNA increasingly serves as the most efficient diagnostic pathway. - Interpret metabolic markers cautiously.
Abnormal lactate or mitochondrial biomarkers should be interpreted in context and are rarely diagnostic in isolation. - Counsel families about uncertainty.
The expanding use of genomic sequencing often reveals variants of uncertain significance, which require careful interpretation and sometimes functional validation.
Conclusion
Mitochondrial biology plays a fundamental role in cellular physiology, and mitochondrial abnormalities have been implicated in a wide range of conditions. However, the concept of “mitochondrial dysfunction” has expanded far beyond the evidence base supporting primary mitochondrial disease.
Current research suggests that mitochondrial markers may vary in neurodevelopmental disorders and other conditions, but findings remain inconsistent and highly dependent on measurement methods and tissue type. In contrast, primary mitochondrial diseases remain rare genetic disorders that require careful clinical and molecular evaluation for diagnosis.
For pediatricians, the key challenge is distinguishing between these two domains: recognizing genuine mitochondrial disease while avoiding over-interpretation of nonspecific symptoms or unvalidated biomarkers. As consumer testing and online narratives continue to broaden the concept of mitochondrial dysfunction, maintaining a rigorous, evidence-based approach remains essential in pediatric practice.
References
- Bhai S, Hirano M. Diagnosis of primary mitochondrial diseases. Muscle Nerve. 2025;71:949-954.
Muscle and Nerve – 2025 – Bhai … - Al-Kafaji G, Jahrami HA, Alwehaidah MS, Alshammari Y, Husni M. Mitochondrial DNA copy number in autism spectrum disorder and attention deficit hyperactivity disorder: a systematic review and meta-analysis. Front Psychiatry. 2023;14:1196035. MitoDNAinASDADHD