What mitochondria do
Mitochondria are organelles inside almost every human cell. They evolved from ancient bacteria taken up into early eukaryotic cells, which is why they retain their own small circular genome distinct from the nuclear DNA. A typical human cell has hundreds to thousands of mitochondria. Cells with high energy demands — muscle, heart, brain, liver — have more.
The most familiar role of mitochondria is energy production. They generate the bulk of cellular ATP, the molecule cells use to power everything from muscle contraction to protein synthesis. But research has expanded the list of mitochondrial functions considerably:
- Calcium handling. Mitochondria buffer cellular calcium, which matters for signaling and contraction.
- Reactive oxygen species production. Some ROS are signaling molecules; excessive ROS is part of oxidative stress.
- Immune signaling. Mitochondrial damage-associated molecular patterns (mtDAMPs) can trigger innate immune responses.
- Apoptosis regulation. Mitochondria control programmed cell-death pathways.
- Biosynthesis. Intermediate metabolites of the citric-acid cycle feed many biosynthetic pathways.
This broader functional role means that mitochondrial changes can affect cell biology beyond simple energy production.
ATP and oxidative phosphorylation
ATP (adenosine triphosphate) is the cell's primary energy currency. The phosphate bonds in ATP store energy that is released when the molecule is hydrolyzed to ADP.
Cells produce ATP through two main routes:
- Glycolysis in the cytoplasm: a fast, less-efficient pathway that breaks down glucose without oxygen and produces a small amount of ATP plus pyruvate.
- Oxidative phosphorylation in mitochondria: the more efficient route, requiring oxygen, that produces the bulk of ATP through the electron transport chain and ATP synthase.
Oxidative phosphorylation involves five protein complexes (I-V) embedded in the inner mitochondrial membrane. Electrons from NADH and FADH2 (generated by the citric-acid cycle) flow through these complexes, pumping protons into the intermembrane space. The resulting proton gradient drives ATP synthase to produce ATP.
When researchers describe "impaired oxidative phosphorylation," they typically mean reduced flux through this chain, reduced activity of one or more complexes, or reduced ATP output. Methods to measure this require specialized equipment and are largely research-grade.
What "dysfunction" means in research
"Mitochondrial dysfunction" is a broad term used in the research literature for any deviation from normal mitochondrial function. It is not a clinical diagnosis. The term covers a wide range of states:
- Inherited mitochondrial diseases caused by mutations in mitochondrial or nuclear genes (such as MELAS, LHON, Leigh syndrome) — specific, often serious clinical conditions evaluated by mitochondrial-disease specialists.
- Acquired mitochondrial dysfunction from drug effects (some antiretrovirals, certain chemotherapies), environmental exposures, or inflammatory states.
- Aging-associated changes in mitochondrial number, dynamics, and respiratory capacity that are documented in research models.
- Functional impairment in research populations with conditions such as ME/CFS, long COVID, fibromyalgia, and others where research has reported altered mitochondrial parameters in subsets of patients.
The last category — functional impairment in post-viral and chronic-fatigue conditions — is the relevant one for long-COVID research and is the focus of this article.
Mitochondrial research in post-viral conditions
Mitochondrial biology has been studied in ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome) for decades, with mixed and sometimes contradictory results. Long-COVID research has reignited interest because of overlapping clinical features and improved methods.
A 2022 paper in Nature Communications by Hanson and colleagues reported altered metabolite profiles in ME/CFS patients consistent with impaired energy metabolism. A 2024 paper in Nature Communications by Appelman and colleagues at Amsterdam UMC examined skeletal muscle from long-COVID patients and reported reduced mitochondrial function and increased glycolysis after exertion, with parallel evidence of immune-cell mitochondrial changes.
Studies in immune cells have reported altered mitochondrial respiration in T cells from long-COVID patients. A 2022 paper in Science Translational Medicine by Klein and colleagues at Yale documented immune perturbations including features consistent with metabolic shifts in immune cell populations.
The picture is preliminary, heterogeneous, and at the level of group averages. Individual patient interpretation remains a clinical judgment that this article cannot substitute for.
How researchers measure mitochondrial function
Measuring mitochondrial function rigorously is challenging and largely outside routine clinical practice. Research methods include:
- High-resolution respirometry on muscle biopsy or isolated immune cells (Oroboros, Seahorse platforms): the gold standard for measuring oxygen consumption rates at defined respiratory states.
- Metabolomics in blood or urine: identifies metabolite patterns consistent with altered mitochondrial pathways (e.g., elevated lactate, altered acylcarnitines).
- 31P-magnetic-resonance spectroscopy of muscle: measures phosphocreatine recovery time after exercise as an in-vivo proxy for mitochondrial capacity.
- Lactate-to-pyruvate ratio: a clinical lab test that can suggest impaired oxidative metabolism in specific contexts.
- Mitochondrial DNA copy number in blood cells: a population-level marker, individually noisy.
- Genetic testing for inherited mitochondrial diseases when clinically indicated.
None of these is part of routine outpatient evaluation for fatigue. They are research methods or specialty tests ordered when there is specific clinical suspicion of a mitochondrial disease.
Nutrients researchers have examined
The research literature has examined several nutrients in the context of mitochondrial function. This section describes what researchers have examined — not what to take.
Coenzyme Q10 (CoQ10)
CoQ10 (ubiquinone/ubiquinol) shuttles electrons in the mitochondrial electron transport chain. Levels decline with age and with statin use. Small studies have examined CoQ10 supplementation in heart failure and statin-associated myopathy with mixed results. Larger trials in post-viral fatigue are limited.
NAD+ precursors
NAD+ (nicotinamide adenine dinucleotide) is a coenzyme central to oxidation-reduction reactions and a substrate for several enzyme families including sirtuins. Research has examined precursors (nicotinamide riboside, nicotinamide mononucleotide) for their ability to raise tissue NAD+. A 2022 study in Aging Cell by Brakedal and colleagues reported some metabolic effects in Parkinson's research populations. Specific evidence for benefit in long COVID remains preliminary.
B vitamins (B1, B2, B3)
Thiamine (B1), riboflavin (B2), and niacin (B3) are cofactors required for mitochondrial enzymes. Severe deficiency causes specific clinical syndromes. Mild deficiency is more common than recognized in some populations. The research framing is correction of deficiency where present, not high-dose supplementation as treatment.
L-carnitine, alpha-lipoic acid, creatine
Each has been examined in various mitochondrial research contexts with mixed and limited results. None is a treatment for post-viral conditions in current evidence.
What this does not mean
- This is not a diagnostic article. Fatigue has many causes.
- This is not a recommendation for any supplement or protocol. Discuss with your clinician.
- This is not evidence that mitochondrial dysfunction is uniform across patients or conditions.
- This is not a substitute for evaluation by a licensed clinician.
General practices the research community discusses
Several general factors are associated with mitochondrial biogenesis and function in research models. Framed as general wellness, not treatment:
- Regular physical activity within tolerance. Exercise is the most studied stimulus for mitochondrial biogenesis. In conditions with post-exertional malaise, this requires careful pacing — see exercise recovery research.
- Adequate sleep. Sleep affects mitochondrial repair and function in research models. See sleep architecture and cellular repair.
- Balanced nutrition with adequate protein, micronutrients, and antioxidant-rich foods.
- Avoiding tobacco, which directly impairs mitochondrial function in research.
- Moderating alcohol, which has mitochondrial toxicity at higher exposures.
Authoritative sources to read directly
- NIH RECOVER Initiative
- NIH NIDDK — metabolic research
- CDC ME/CFS
- United Mitochondrial Disease Foundation (UMDF)
- Mayo Clinic: Mitochondrial myopathies