Aging, Sex, and Exercise Shape Brain Mitochondria Differently

Deep inside every neuron, mitochondria are working around the clock to keep the brain alive. These organelles generate the ATP that fuels synaptic transmission, maintains ion gradients, and powers axonal transport, while also buffering the calcium surges that accompany neuronal firing. When mitochondrial function falters, the consequences ripple outward: synaptic vesicle recycling slows, neurotransmitter release becomes erratic, and the early signs of cognitive decline begin to emerge. Now, a mini-review published in Frontiers in Aging by Jianhua Zhang and Victor Darley-Usmar of the University of Alabama at Birmingham and Yun Li of the University of Wyoming argues that two variables long treated as afterthoughts in mitochondrial research—biological sex and exercise—are far more important to brain mitochondrial health than most scientists have assumed.

The review synthesizes evidence from rats, mice, baboons, and postmortem human brains to show that mitochondrial decline with age is neither uniform across the brain nor identical between males and females. In one study of male Wistar rats aged 28 to 92 weeks, mitochondrial mass remained unchanged even as the activities of manganese superoxide dismutase, electron transport chain complex I, and complex IV dropped substantially, accompanied by increased mitochondrial swelling and oxidative stress markers. This finding suggests that functional decline can occur independently of changes in mitochondrial abundance, possibly reflecting the waning efficiency of mitochondrial quality control mechanisms such as mitophagy with advancing age.

When both sexes were included in a later study comparing animals aged 6 to 24 months, the picture became considerably more nuanced. Young males had higher mitochondrial DNA levels in the brain, consistent with the assumption that larger body and brain mass requires more energetic support. Yet when normalized for body weight, males actually had a lower brain-to-body weight ratio across the age span and lower mitochondrial protein levels at both mid-age and older ages compared to females. Citrate synthase activity, often used as a surrogate for mitochondrial content, was higher in females at 12 months, and electron transport chain complex IV activity was higher in females at every age tested. Age-by-sex interaction analyses were significant for body weight, brain weight ratio, total protein, total DNA, and citrate synthase activity, underscoring that bioenergetic function in the brain is fundamentally shaped by sex.

Perhaps most surprisingly, the same study found that mitochondrial DNA, citrate synthase activity, and complex V activity all increased with age in the brain, rather than declining as the conventional narrative of mitochondrial aging would predict. The authors caution that this apparent increase may reflect a compensatory response to declining mitochondrial efficiency, or could arise from decreased mitochondrial turnover, rather than indicating healthier mitochondria. Not all electron transport chain enzymes followed the same trajectory; complex IV activity remained unchanged across the age range. The authors emphasize that measuring a single parameter such as mitochondrial DNA or citrate synthase activity is not a proxy for overall mitochondrial functionality, and that integrated approaches using metabolomics and proteomics will be needed to capture the full picture.

Regional specificity adds another layer of complexity. In young C57BL6 mice, mitochondria isolated from female brains showed higher state 3 respiration through complexes I and II than those from males, and electron microscopy confirmed more mitochondria in female brains. Yet when cortical extracts were analyzed without mitochondrial isolation in a separate study, no sex differences in electron transport chain activities were apparent. These apparent contradictions were partially resolved by a systematic comparison of young and old mice of both sexes across four brain regions—cortex, hippocampus, cerebellum, and hypothalamus. Only the hippocampus exhibited sex differences in complex II and IV activities in young animals, with males showing higher values. In old mice, complex I, II, and IV activities declined with age in the cortex and hippocampus of males, but in females the decline was confined to the cortex, sparing the hippocampus entirely.

Beyond the electron transport chain, sex-dependent differences extend to mitochondrial membrane potential and proteostasis. A study of the hippocampus in C57BL/6J and SAMP8 mice found that oxidative stress was higher in aged males, while mitochondrial permeability transition pore function was elevated in aged females. Both sexes showed a significant decrease in the activity of Lonp1, a mitochondrial protease responsible for degrading damaged proteins, suggesting that mitochondrial protein turnover may be an underappreciated age-dependent factor in brain health.

Evidence from non-human primates and humans begins to bridge the translational gap. In baboon cortex spanning ages equivalent to roughly 26 to 91 human years, electron transport chain activities declined with age, but secondary analysis stratified by sex revealed that the overall age-related decline was largely driven by males. Intriguingly, walking speed measured two years before necropsy correlated positively with cortical mitochondrial activities, echoing epidemiological findings that muscle and cognitive frailty co-manifest in aging human populations. In postmortem human brains, the common 4977-nucleotide-pair mitochondrial DNA deletion was higher in subjects over 80 compared to those aged 67 to 77 in the cortex and putamen, and complex IV enzymatic activity decreased with age across multiple regions, though the decline lost significance when normalized to citrate synthase, hinting at an overall reduction in mitochondrial content.

The review then turns to exercise, one of the most accessible and widely recommended strategies for promoting brain health. Treadmill running in young and old male mice increased markers of mitochondrial biogenesis in the cortex and hippocampus, including PGC1α and citrate synthase mRNA levels and mitochondrial DNA content. Similar enhancements in brain mitochondrial respiration were observed in rats and mice after treadmill or free-wheel running, though the specific electron transport chain changes varied across studies, likely reflecting differences in exercise regimen, species, age, brain region, and methodology. In an Alzheimer’s disease model, male APP/PS1 mice subjected to treadmill exercise showed restoration of PGC1α protein levels and a reduction in damaged mitochondria in the CA1 region of the hippocampus, consistent with a neuroprotective mechanism operating through mitochondrial biogenesis.

Sex-dependent responses to exercise are beginning to emerge from the limited number of studies that include both sexes. In a nine-month voluntary wheel running protocol in female rats, most mitochondrial parameters remained unchanged, with only complex IV activity increasing in the cortex of the physical activity group. By contrast, a ten-week treadmill study in male and female Fisher 344 rats found that complex II SDHA protein levels increased in exercising females but decreased in exercising males, with no changes in complexes I, III, or IV. In a traumatic brain injury model using Swiss-Webster mice, low-intensity exercise improved ATP-linked oxygen consumption only in females, while males showed decreased oxygen consumption after high-intensity exercise, demonstrating that exercise intensity interacts with sex to shape cortical mitochondrial adaptation. In a neonatal hypoxia-ischemia model, maternal exercise during gestation elevated complex II enzymatic activities in the brains of pups of both sexes. An exploratory study using rats selectively bred for low or high aerobic running capacity found that high-capacity runner females exhibited higher state 3 respiration than low-capacity runner females, while the relationship in males was opposite at 18 months but reversed again at 24 months, providing indirect evidence that intrinsic aerobic capacity modulates brain mitochondrial function in a sex-dependent manner.

The authors identify several critical knowledge gaps that will shape future research. Few studies have sufficient statistical power to investigate sex differences in exercise-induced mitochondrial responses across the lifespan, and many rely on whole-brain or cortical measurements that obscure region-specific vulnerabilities relevant to neurodegenerative disease. The impact of cellular heterogeneity remains largely unexplored, as most studies use bulk tissue approaches that mask the distinct metabolic demands of individual cell types; advances in single-cell bioenergetic methodologies will be essential. Mechanistically, the relative contributions of mitochondrial biogenesis, mitophagy, and dynamics to exercise-induced improvements remain unclear, and systemic factors such as circulating proteins and metabolites induced by exercise likely differ between sexes. Translating findings from animal models to humans will require careful study of exercise mode, intensity, frequency, and duration. The authors also note that sex hormones, including estrogen, progesterone, and testosterone, decline with age in the brain and can regulate the expression of PGC1α, uncoupling proteins, citrate synthase, cytochrome c, and complex IV subunits through their nuclear receptors, providing a plausible mechanistic link between hormonal status and mitochondrial function. A clearer understanding of these sex-, region-, and cell-specific interactions, the authors conclude, will be essential for developing targeted strategies to preserve mitochondrial function and promote healthy brain aging.

Subject of Research: Sex differences in age-related brain mitochondrial decline and exercise-induced mitochondrial adaptation

Article Title: The impact of aging, sex, and exercise on brain mitochondria

Article References: Zhang, J., Darley-Usmar, V., & Li, Y. (2026). The impact of aging, sex, and exercise on brain mitochondria. Frontiers in Aging, 7, Article 1920914. https://doi.org/10.3389/fragi.2026.1920914

Image Credits: AI Generated

DOI: 10.3389/fragi.2026.1920914

Keywords: mitochondria, brain aging, sex differences, exercise, neurodegeneration, bioenergetics, impact, aging, brain, scientific research