Longevity Science · Cellular Energy Can you improve cellular energyafter 40? Yes, but not through a single supplement or biohack....
How Cells Maintain Mitochondria: Fusion, Fission, Mitophagy and Healthy Ageing
Longevity Science · Cellular Energy
How cells maintain mitochondria
How cells maintain mitochondria
through quality control.
Mitochondria constantly merge, divide, build new components and remove damaged material. This coordinated system, often called mitochondrial quality control, helps cells match energy supply to demand and maintain function across the lifespan.
4Core quality-control processes
16 wksTraining remodelled older muscle
145%Higher PARKIN in active older men
12 wksAerobic training shifted network balance
The Dynamic Network
Mitochondria are not
fixed cellular batteries
In skeletal muscle and many other tissues, mitochondria form interconnected networks. Their shape changes in response to energy demand, nutrient availability, cellular stress and physical activity. This plasticity matters because ATP production depends not only on how many mitochondria are present, but also on how effectively the network is organised and maintained.
Mitochondrial quality control is an umbrella term for several overlapping processes. Fusion and fission reshape the network. Biogenesis expands mitochondrial components. Mitophagy helps remove selected damaged or surplus material. These systems work together rather than acting as four isolated switches.
01
Fusion
Outer and inner membranes merge, allowing mitochondria to share contents and form longer connected structures.
02
Fission
A mitochondrial segment divides, supporting distribution, adaptation and the isolation of material that may need removal.
03
Mitophagy
Selected mitochondrial components are directed towards lysosomal recycling as part of cellular housekeeping.
04
Biogenesis
Cells increase mitochondrial proteins, enzymes, membranes and genetic material in response to repeated demand.
A healthy mitochondrial network is not one that is permanently fused or constantly recycled. Function depends on balance, timing, tissue type and the demand placed on the cell.
Evidence summary based on human skeletal-muscle biopsy researchFusion and Fission
Joining and dividing are both
part of maintenance
Fusion can help mix mitochondrial contents and support a more interconnected network. Fission can redistribute mitochondria within a cell and separate regions that may require repair or removal. Neither process is simply “good” or “bad”. Problems arise when the balance becomes poorly matched to the needs of the tissue.
Key Proteins Studied in Humans
- MFN1 and MFN2: proteins involved in fusion of the outer mitochondrial membrane.
- OPA1: a protein associated with inner-membrane fusion and cristae organisation.
- DRP1: a central regulator involved in mitochondrial division.
- FIS1: a fission-associated protein measured in human exercise studies.
- PARKIN: a protein involved in signalling pathways associated with removal of damaged mitochondria.
Human Clinical Trial · 12 Weeks
Aerobic Training Shifted the Network Towards Elongation
Ten sedentary adults with an average age of about 66 completed supervised aerobic training five days per week. Training improved aerobic capacity, insulin sensitivity and fat oxidation, while changing fusion-to-fission protein ratios towards a more elongated mitochondrial network. PMID 30408342
Mitophagy
Recycling is selective,
not a vague cellular detox
Mitophagy is a specialised form of autophagy in which mitochondrial material is targeted for lysosomal degradation. It helps prevent dysfunctional components from accumulating, but the process is difficult to measure directly in living humans. Most human studies rely on muscle biopsies and molecular markers, which provide useful evidence but not a real-time count of mitochondria being recycled.
- Markers are indirect: a higher or lower protein level does not by itself prove that total mitophagy flux has increased.
- Timing matters: measurements during exercise, one hour later and after months of training can show different patterns.
- Training history matters: recently trained adults may emphasise network growth, while lifelong training may show greater turnover and maintenance.
- Human and animal findings differ: mechanisms demonstrated in rodents should not automatically be presented as proven human outcomes.
Human Muscle-Biopsy Study · 22 Sedentary Adults
Four Months of Training Increased Mitochondrial Content
Older sedentary men and women completed 16 weeks of supervised exercise and were compared with lifelong trained adults. The intervention increased mitochondrial content and was dominated by fusion-related adaptation. Lifelong training showed a pattern more strongly associated with mitophagy, fusion and reduced fission, suggesting that short-term expansion and long-term maintenance may not look identical. PMID 30144291
145%Higher PARKIN protein in active older men compared with sedentary older men in one human study
Cross-Sectional Human Study · 33 Men
Regular Endurance Activity Was Associated With Mitophagy Markers
Active and sedentary younger and older men were compared using skeletal-muscle biopsies. PARKIN protein was 145% higher in active older men than sedentary older men, while active groups also showed higher fusion and oxidative-phosphorylation proteins. The design shows association, not proof that a single exercise programme caused every difference. PMID 31507451
Short-Term Versus Long-Term
One workout and years of training
do not produce the same snapshot
Researchers often measure autophagy or mitophagy markers at a single time point. That can be informative, but it may miss the full sequence of formation, transport and degradation. Acute exercise findings are especially sensitive to timing, intensity and whether the participant is fed or fasted.
01
Acute Exercise
A single session can rapidly change signalling and protein localisation, but a marker measured immediately afterwards may not reflect total turnover.
02
Training Adaptation
Repeated sessions can increase mitochondrial content, alter network proteins and improve the capacity for cellular recycling.
03
Lifelong Activity
Years of endurance activity may produce a maintenance pattern that differs from the early growth phase of a new training programme.
04
Measurement Limits
Human muscle biopsy studies are valuable, but small samples and indirect markers require careful interpretation.
Controlled Human Study
Training Increased the Capacity for Autophagosome Formation
One-legged exercise training altered autophagy regulation in human skeletal muscle. The researchers emphasised that human responses differed in important ways from findings often reported in cells and rodents. PMID 26614120
Acute Endurance Study
Mitophagy Was Not Uniformly Activated During Exercise
A human endurance-exercise study did not find clear activation of the measured fission and mitophagy pathways during the exercise bout, reinforcing that acute marker changes should not be oversimplified. PMID 28272266
What Supports Quality Control
Repeated demand is more credible
than a single mitophagy hack
The broadest human evidence supports regular exercise. Aerobic and interval training provide a direct mitochondrial stimulus. Resistance training protects muscle mass and function, even when its mitochondrial structural effects differ from aerobic training. Recovery and adequate nutrition help make repeated training sustainable.
- Exercise regularly: combine aerobic work, resistance training and daily movement rather than relying on one modality.
- Progress gradually: an effective programme is one that can be repeated without persistent exhaustion or injury.
- Maintain muscle: skeletal muscle is a major site of mitochondrial adaptation and whole-body glucose disposal.
- Avoid overclaiming fasting: nutrient deprivation can alter autophagy signals, but longer or more extreme fasting is not automatically better or appropriate for everyone.
- Address health conditions: metabolic, cardiovascular, endocrine and inflammatory conditions can affect mitochondrial function and exercise tolerance.
Read the Cellular Energy foundation article: How Your Cells Make Energy: ATP, Mitochondria and What Changes After 40.
Supplements and Mitophagy
No supplement has proven to
“renew” human mitochondria on demand
Some compounds are being studied for effects on mitochondrial biomarkers. One randomised trial of urolithin A in middle-aged adults reported improvements in selected strength and biomarker outcomes, but the primary peak-power endpoint did not significantly improve. This is research interest, not proof of universal mitochondrial rejuvenation.
Creatine, NR and micronutrients influence different parts of the wider energy system. Creatine supports rapid ATP buffering. NR provides a precursor within NAD+ metabolism. Vitamins and minerals support normal physiology when intake is inadequate. None should be described as a proven switch for fusion, fission or mitophagy.
Randomised Trial · 4 Months
Urolithin A Changed Selected Mitochondrial Biomarkers
Middle-aged adults receiving urolithin A showed improvements in selected muscle-strength and biomarker outcomes, while peak power, the primary endpoint, did not significantly improve. The findings are promising but do not establish a general anti-ageing effect. PMID 35584623
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References
[1]Konopka AR et al. Markers of human skeletal muscle mitochondrial biogenesis and quality control: effects of age and aerobic exercise training. J Gerontol A Biol Sci Med Sci. 2014;69(4):371-378. PMID 23873965
[2]Axelrod CL et al. Exercise training remodels human skeletal muscle mitochondrial fission and fusion machinery towards a pro-elongation phenotype. Acta Physiol. 2019;225(4):e13216. PMID 30408342
[3]Arribat Y et al. Distinct patterns of skeletal muscle mitochondria fusion, fission and mitophagy upon duration of exercise training. Acta Physiol. 2019;225(2):e13179. PMID 30144291
[4]Balan E et al. Regular endurance exercise promotes fission, mitophagy, and oxidative phosphorylation in human skeletal muscle independently of age. Front Physiol. 2019;10:1088. PMID 31507451
[5]Ruegsegger GN et al. High-intensity aerobic exercise training improves cardiometabolic health and skeletal muscle mitochondrial dynamics. J Appl Physiol. 2023;135(4):763-774. PMID 37616334
[6]Fritzen AM et al. Regulation of autophagy in human skeletal muscle: effects of exercise, exercise training and insulin stimulation. J Physiol. 2016;594(3):745-761. PMID 26614120
[7]Schwalm C et al. Activation of autophagy in human skeletal muscle is dependent on exercise intensity and AMPK activation. FASEB J. 2015;29(8):3515-3526. PMID 25957282
[8]Schwalm C et al. Lack of activation of mitophagy during endurance exercise in humans. Med Sci Sports Exerc. 2017;49(8):1552-1561. PMID 28272266
[9]Singh A et al. Urolithin A improves muscle strength, exercise performance, and biomarkers of mitochondrial health in a randomized trial in middle-aged adults. Cell Rep Med. 2022;3(5):100633. PMID 35584623
For informational purposes only. This article does not diagnose, treat or prevent disease and is not a substitute for medical advice. Human mitochondrial quality-control research is evolving, and molecular markers should not be interpreted as guaranteed clinical outcomes. Seek qualified advice before changing exercise, fasting or supplement routines.
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