Mitochondria are the structures inside your cells that turn food and oxygen into the energy the rest of the body uses. How many working ones a muscle holds is a large part of what fitness and endurance are. Aerobic training reliably makes muscle build more of them, one of the best established findings in exercise physiology.
The free radicals a workout produces are part of the signal that tells the cell to adapt, which is why high-dose antioxidant pills taken around training can blunt the gains. This page covers the machinery; the practices that act on it are linked at the end.
Findings & Outcomes
Mitochondria are the structures inside almost every cell that turn food and oxygen into usable energy. How many working ones a muscle holds is a large part of what fitness and endurance are, which is why the same machinery sits under so much of this section: Zone 2 work, VO2 max intervals, cold exposure, heat and a stretch of not eating all act on mitochondria at the cellular level. Understanding the structure once explains why those practices work, and each is linked at the end.
Building mitochondria through aerobic training is one of the best established findings in exercise physiology. The supplements sold as mitochondrial shortcuts are not established: they move laboratory markers, not outcomes a person can feel.
What Mitochondria Do
Mitochondria are small structures inside almost every cell, and they convert the energy in food into a form the cell can use. That form is a molecule called ATP, adenosine triphosphate: a muscle contracting, a nerve firing and an enzyme building a protein all run on it.
The conversion happens in two linked stages. First, fuel from food, mostly fat and glucose, is broken down into small carriers of electrons. Then those electrons are passed down a series of protein complexes embedded in the mitochondrion's inner membrane, the electron transport chain. As they move along it, they pump protons across the membrane, and the flow of those protons back through an enzyme called ATP synthase produces ATP. This whole process, using oxygen as the final electron acceptor, is oxidative phosphorylation, and it is why you breathe: the oxygen you take in is used up at the end of that chain. It is far more efficient than the alternatives. A cell that has to make energy without oxygen gets a small fraction of the ATP from the same glucose.
Two consequences follow, and both matter for everything below. Tissues that need a lot of steady energy, the heart and endurance muscle above all, are packed with mitochondria. And how many working mitochondria a muscle has, its mitochondrial content, sets how much energy it can produce aerobically, which is a large part of endurance capacity.
How Exercise Builds More Of Them
The number of mitochondria in a cell is not fixed. Cells build new mitochondrial material in a process called mitochondrial biogenesis, and the single most reliable trigger for it in muscle is aerobic exercise.
The protein that drives this is PGC-1 alpha. When a muscle contracts repeatedly it runs low on energy and fills with signals of that demand: rising calcium, a shift in the cell's energy charge sensed by an enzyme called AMPK, and a brief rise in reactive oxygen species. Those signals converge on PGC-1 alpha, which activates the set of genes needed to build more mitochondrial machinery. Repeat the stimulus across weeks and the muscle ends up with more mitochondria, and better ones.
The evidence for the outcome is strong. A systematic review of training studies in human muscle concludes that endurance training reliably raises mitochondrial content, with training volume the main driver of how much content rises and intensity driving the respiratory quality of what is built. A within-person trial makes the point cleanly: when ten men trained one leg with hard intervals and the other with steady work matched for total effort, the interval leg came out with higher mitochondrial enzyme activity, citrate synthase reaching 10.2 against 8.4 mmol per kg protein per minute. This is the cellular reason zone 2 training and VO2 max intervals work, and why aerobic fitness and mitochondrial content rise together.
Free Radicals Are Also Signals
For decades reactive oxygen species, the reactive by-products of burning fuel with oxygen, were treated as pure damage, the thing to be neutralized. In the right amount they are also a signal.
A working muscle produces a brief rise in these radicals, and that rise acts as one of the cues to adapt. It feeds into the same PGC-1 alpha response that builds mitochondria and into the cell's own defensive enzymes. This is the cellular version of hormesis, where a controlled dose of a stressor drives an adaptation that leaves the system stronger.
In a trial of 39 young men doing a four-week training program, exercise improved insulin sensitivity and switched on PGC-1 alpha and the muscle's own defensive enzymes only in the men who were not taking large daily doses of vitamin C at 1000 mg and vitamin E at 400 IU. The antioxidant supplements blunted the adaptation, because they removed the signal it was responding to.
High-dose vitamin C and E taken around training can cancel the insulin-sensitivity gain a workout produces. This is a caution about megadose supplements, not about antioxidants in food, which have not been shown to do this.
Density, Fitness And Metabolic Flexibility
Mitochondrial content does more than set endurance. Muscle rich in mitochondria is better at switching fuels, burning fat when it is plentiful and glucose after a meal, and this capacity to switch is called metabolic flexibility. It is blunted in obesity and type 2 diabetes, and it tracks with insulin resistance.
That connects this page to two others. Muscle is the largest place a meal's glucose goes, covered on muscle as an organ and insulin and glucose, and the quality of the mitochondria inside that muscle is part of how well it handles the fuel. Building mitochondrial capacity through training is, for this reason, one of the mechanistic threads under type 2 diabetes. The relationship is consistent and mechanistically grounded, though much of the human data is cross-sectional, so impaired flexibility is at least as much a marker of metabolic disease as a proven cause of it.
Mitochondria And Aging
Mitochondria sit at the center of one of the most influential ideas about why we age, and better evidence has revised it.
The original free radical theory of aging, later framed around mitochondria specifically, held that aging is driven mainly by the accumulation of oxidative damage from these organelles. It made a clean prediction: reduce the damage and you slow aging. That prediction has largely not held up. Antioxidant supplements have repeatedly failed to extend healthy lifespan or prevent age-related disease, and higher reactive oxygen species can accompany longer life. The revised view treats these radicals as both damage and signal, the same logic seen in the training studies scaled up to a lifetime. Oxidative damage does accumulate. What changed is the confident claim that removing it is straightforwardly good, and how much mitochondrial decline causes aging, as opposed to reflecting it, remains open.
The Supplement Question
If exercise builds mitochondria, the commercial question is whether a pill can do the same. The evidence here is thinner than the marketing, and each claim grades differently, so take them one at a time.
Urolithin A, a compound the gut makes from certain fruits, stimulates the clearing of damaged mitochondria. A first-in-human trial in sedentary older adults gave 500 to 1000 mg a day for four weeks, found it safe, and showed it shifted plasma acylcarnitines and muscle mitochondrial gene expression toward a molecular signature of better mitochondrial health. Those were biomarkers; the trial did not measure strength, endurance or any outcome a person can feel, so this sits at preliminary.
Coenzyme Q10 is a component of the electron transport chain, which makes it a plausible candidate. A meta-analysis of 28 trials in 830 people found that CoQ10 supplementation lowered blood markers of exercise-induced muscle damage, including creatine kinase by about 51 IU/L, but it did not establish a meaningful gain in performance, most trials were small and run in Asia, and lowering exercise-induced oxidative stress may work against the adaptation described above. See the CoQ10 page for the fuller picture.
NAD-boosting supplements, such as nicotinamide riboside, aim to raise a molecule mitochondria depend on. A six-week trial in about 24 older adults showed one such supplement did raise blood NAD safely, but it did not move most metabolic measures.
The pattern across all three is the same. Building mitochondria through training rests on strong, repeated human evidence. The specific supplement claims are emerging or not established, and none of them has yet matched what aerobic exercise does for free.
The Research & Studies
Everything here is based on the research we have collected and checked, sorted into groups and ordered with the strongest evidence first. Click any claim to open the studies behind it.
How it works
Endurance training reliably builds more muscle mitochondria; volume drives how many, intensity how well they work
Regular aerobic training builds more mitochondria in muscle. How much you do drives the number of them; how hard you go drives how well they work.
This systematic review of training-induced mitochondrial adaptations in human skeletal muscle concludes that endurance training increases both mitochondrial content, tracked by markers such as citrate synthase activity, and mitochondrial respiratory function. Total training volume is the main lever for content, while exercise intensity drives respiratory function, and the review notes a frequent dissociation between the two, so a rise in one does not guarantee a rise in the other. The exercise-to-mitochondria link itself is one of the best established findings in exercise physiology.
Who this may not transfer to:Pooled human training studies that skew heavily male, as most exercise physiology does; the direction holds broadly, but the volume-versus-intensity split rests on small, male-dominated trials.
The study · 1
Granata et al., training-induced changes in mitochondrial content and respiratory function in human skeletal muscle · Sports Med 2018
In 10 men, interval training raised citrate synthase to 10.2 versus 8.4 for work-matched steady cycling
When the same person trained one leg with hard intervals and the other with steady work matched for total effort, the interval leg built more mitochondrial capacity.
Ten young active men trained one leg with high-intensity interval cycling and the other with moderate-intensity continuous cycling, matched for total work at roughly 143 kilojoules per session, across six sessions in two weeks. Maximal citrate synthase activity reached 10.2 against 8.4 mmol per kg protein per minute, and mass-specific oxidative phosphorylation capacity was higher, on the interval leg, a within-person design that removes between-subject variation because each man is his own control.
Who this may not transfer to:Ten young active men. Whether the same intensity advantage holds in women, older adults or untrained people was not tested here, so it is an assumption rather than a finding for those groups.
The study · 1
MacInnis et al., superior mitochondrial adaptations after interval compared to continuous single-leg cycling matched for total work · J Physiol 2017
The brief rise in reactive oxygen species from exercise signals muscle to build mitochondria
The small burst of free radicals a workout produces is part of how the muscle learns to build more mitochondria. It is a signal, not just wear and tear.
This review sets out the mitohormesis account of exercise: mitochondria under the stress of contraction release reactive oxygen species that signal back to the cell nucleus, triggering a protective transcriptional response that includes mitochondrial biogenesis and upregulation of endogenous antioxidant enzymes. It is the mechanistic layer under the observation that blocking those radicals with high-dose antioxidant supplements can blunt the adaptation.
The study · 1
Merry and Ristow, mitohormesis in exercise training · Free Radic Biol Med 2016
Mitochondria-rich muscle switches between fat and glucose more readily; that flexibility falls in obesity and diabetes
Fitter, mitochondria-rich muscle switches easily between burning fat and sugar depending on what is available. Losing that flexibility goes with metabolic disease.
This review frames metabolic flexibility as the capacity of muscle and fat tissue to adjust fuel use to demand, oxidizing fat when it is plentiful and glucose after a meal. Mitochondrial capacity is central to that switching. Flexibility deteriorates in obesity and type 2 diabetes and is associated with insulin resistance, which is why building mitochondrial capacity through training is part of the mechanistic case for exercise in metabolic health.
The study · 1
Goodpaster and Sparks, metabolic flexibility in health and disease · Cell Metab 2017
Nicotinamide riboside raised blood NAD but did not improve metabolic measures in 24 older adults
An NAD-boosting supplement did raise NAD levels safely, but it did not produce a clear health benefit in the trial.
This randomized, double-blind, placebo-controlled crossover trial gave nicotinamide riboside, an NAD+ precursor, to healthy middle-aged and older adults for six weeks. It effectively raised NAD+ metabolism and was well tolerated. The main functional and metabolic outcomes did not clearly improve; a reduction in blood pressure and aortic stiffness appeared in a subgroup with elevated baseline values, which the authors flag as a hypothesis for future trials rather than an established effect.
Who this may not transfer to:About 24 healthy adults aged 55 to 79. Whether nicotinamide riboside helps younger people, or people with a metabolic condition, was not tested here.
Raising NAD is not the same as a proven benefit. The functional payoff these supplements are sold for has not been demonstrated in a healthy person, so the accurate position is emerging, not effective.
The study · 1
Martens et al., chronic nicotinamide riboside supplementation is well-tolerated and elevates NAD+ in healthy middle-aged and older adults · Nat Commun 2018
Urolithin A at 500 to 1000 mg shifted muscle mitochondrial markers but measured no strength or endurance outcome
Urolithin A safely nudged markers of mitochondrial health in older adults, but the study measured molecular signals, not strength, endurance or any health outcome.
This first-in-human study tested urolithin A, a gut-microbiome metabolite that stimulates mitophagy, the clearing of damaged mitochondria, in healthy sedentary elderly adults. Single and 4-week dosing at 500 to 1000 mg a day was safe and bioavailable, and modulated plasma acylcarnitines and skeletal-muscle mitochondrial gene expression, which the authors describe as a molecular signature of improved mitochondrial and cellular health. The endpoints were biomarkers; the trial did not measure strength, endurance, or a clinical outcome.
Who this may not transfer to:Healthy sedentary elderly adults. Whether urolithin A does anything for younger or active people, or improves a felt outcome in anyone, was not tested.
The study · 1
Andreux et al., the mitophagy activator urolithin A is safe and induces a molecular signature of improved mitochondrial and cellular health in humans · Nat Metab 2019
Blood Sugar
Vitamin C 1000 mg plus E 400 IU daily abolished the insulin-sensitivity gain from exercise in 39 young men
High-dose vitamin C and E taken around training wiped out the improvement in insulin sensitivity that exercise otherwise produced.
Thirty-nine healthy young men, 19 untrained and 20 pretrained, did a 4-week exercise program with or without vitamin C 1000 mg and vitamin E 400 IU daily. Exercise increased insulin sensitivity, measured by glucose infusion rate and plasma adiponectin, only in the men not taking antioxidants, in both the untrained and pretrained groups. The exercise-induced rise in ROS-responsive genes including PGC-1 alpha, and in the muscle's endogenous antioxidant enzymes, was likewise suppressed by the supplements. The interpretation is that the supplements removed the ROS signal the adaptation depends on.
Who this may not transfer to:Thirty-nine young men. The blunting effect has not been tested in women here, and antioxidant handling and training response both differ by sex, so the size of the effect in women is not established.
This is an argument against high-dose antioxidant supplements taken specifically around training, not against eating fruit and vegetables. Antioxidants from food have not been shown to interfere with the adaptation, and the effect studied here used large isolated doses.
The study · 1
Ristow et al., antioxidants prevent health-promoting effects of physical exercise in humans · Proc Natl Acad Sci USA 2009
Exercise Recovery
CoQ10 lowered muscle-damage markers (creatine kinase about 51 IU/L) across 28 trials but showed no performance gain
CoQ10 slightly lowers blood markers of muscle damage after exercise, but a real improvement in how you perform has not been shown.
This systematic review and dose-response meta-analysis pooled 28 randomized controlled trials totalling 830 participants. Coenzyme Q10 supplementation reduced creatine kinase by about 51 IU/L, lactate dehydrogenase by about 52 IU/L, myoglobin by about 22 ng/ml and malondialdehyde by about 0.73 micromol/L, with larger reductions at higher daily doses. The endpoints are markers of muscle damage and oxidative stress, not performance or a health outcome, and the authors caution that most trials were conducted in Asian populations and were small, limiting generalizability.
Who this may not transfer to:830 people across 28 trials, most run in Asia; the authors flag that generalizability to other populations is limited, and no performance or health outcome was established.
The study · 1
Talebi et al., effects of coenzyme Q10 supplementation on biomarkers of exercise-induced muscle damage, physical performance and oxidative stress: a systematic review and meta-analysis · Clin Nutr ESPEN 2024
Longevity And Mortality
Antioxidant supplements did not extend healthy lifespan, revising the free-radical theory of aging
The old idea that aging is mainly free-radical damage from mitochondria has been revised. Mopping up all free radicals with supplements does not extend healthy life, and some free radicals are useful signals.
This review revisits the free-radical theory of aging and reframes it as a cell-signaling account. The original theory predicted that reducing oxidative damage should slow ageing, but antioxidant supplementation has repeatedly failed to prevent age-related disease or extend healthy lifespan, and increased ROS can correlate with longevity rather than against it. The revised reading treats reactive oxygen species as dual agents, both damaging molecules and signaling messengers, so eliminating them wholesale can be counterproductive. This is the aging-scale version of the same mitohormesis logic seen in the training studies.
The study · 1
Vina et al., the free radical theory of aging revisited: the cell signaling disruption theory of aging · Antioxid Redox Signal 2013
Go Deeper
This page describes the machinery. The practices that act on it:
- Zone 2 training and VO2 max intervals, the clearest drivers of mitochondrial biogenesis.
- Cold exposure and heat exposure, hormetic stressors that share the same adaptive signaling.
- Time-restricted eating, which stresses the fuel-handling machinery in a different way.
- Hormesis, the general principle behind why a controlled stress makes a system stronger, and insulin and glucose and muscle as an organ, where mitochondria meet metabolism.
Common Questions
What do mitochondria actually do?
They turn the energy in food into a form your cells can spend. Fuel from fat and glucose is broken down, its electrons are passed along the electron transport chain inside the mitochondrion, and that process, oxidative phosphorylation, uses oxygen to make ATP, the molecule that powers muscle contraction, nerve signals and almost everything else. It is why you breathe: the oxygen is used up at the end of that chain.
Does exercise really build more mitochondria?
Yes, and it is one of the strongest findings in exercise physiology. Aerobic training triggers mitochondrial biogenesis through PGC-1 alpha, and human studies consistently show that endurance training raises mitochondrial content in muscle. Training volume mainly drives how many you build; intensity drives how well they work.
Why can antioxidant supplements be a problem around training?
Because the brief rise in reactive oxygen species a workout produces is part of the signal that drives the adaptation. In one trial of 39 young men, large daily doses of vitamin C and E blocked the improvement in insulin sensitivity that exercise otherwise produced. This is a caution about megadose supplements taken around training, not about antioxidants in food, which have not been shown to do this.
Do CoQ10, NAD boosters or urolithin A improve my mitochondria?
The claims are emerging, not established. Urolithin A safely shifted molecular markers in a first-in-human trial but no outcome a person can feel was measured. CoQ10 lowers some blood markers of muscle damage without a clear performance gain. NAD boosters raise blood NAD safely without a demonstrated health benefit. None has matched what aerobic training does, and training is free.
Are mitochondria the reason we age?
They are central to one theory of aging, but that theory has been revised. The old idea that aging is mainly oxidative damage from mitochondria predicted that antioxidants should slow aging, and they have not. Reactive oxygen species are now understood as both damage and signal, and how much mitochondrial decline causes aging, as opposed to reflecting it, is still an open question.
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All 9 sources on this page independently checked and cross-referenced.
Thomas Dehli, Founder & Editor, Sacred Lotus
Sacred Lotus has published Chinese medicine reference material since 2001. Integrative pages are held to the same standard as the herb and formula library: cite the source, grade the claim at its real strength, and say where the research has not looked. This page is educational and it is not medical advice. Last reviewed and updated August 10, 2026.
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