Women's Biology
May 4, 2026
There’s a way of being tired that doesn’t make sense, and it has a specific cellular explanation that most women in midlife have never been given.
The hormonal and cellular shifts that change how women’s bodies produce energy in midlife, and the peptide research starting to map them.
There’s a way of being tired that doesn’t make sense.
You slept eight hours and woke up exhausted. You took a rest day after the workout that used to leave you feeling great, and you still feel heavy. You’ve eaten well, hydrated, supplemented, meditated, and your body still seems to be running on a different battery than it did a few years ago. Coffee that used to wake you up barely registers. Wine that used to be fine ruins three days. The math you used to do unconsciously, between effort and recovery, between what you put in and what came out, has stopped working.
This is one of the most common, least discussed experiences of women in their forties. It’s also, in most cases, a misunderstood one.
Women describing this kind of fatigue are often told it’s stress. Or sleep. Or aging. Or hormones, in the vague way that word gets used to mean everything and nothing. The truth is more specific, and more interesting, and supported by a body of research that has been quietly building for the last decade. The energy you have in your forties is not the energy you had in your thirties. It’s the same body, but the cellular machinery that produces energy has changed in ways that are now mostly understood, even if they’re rarely explained.
This piece is about that machinery. What it does, why it changes, and where the research on peptides is now intersecting with what women in midlife have been experiencing all along.
The cellular reality
Inside every cell in your body, in numbers ranging from a few hundred in some tissues to several thousand in metabolically active ones, are organelles called mitochondria. They are, in the most literal sense, what your body uses to turn food and oxygen into energy. Every breath you take, every step you climb, every thought you think, runs on the output of mitochondria.
The currency they produce is a molecule called adenosine triphosphate, or ATP. It is the universal energy substrate of biological life, and the human body produces and consumes its own body weight in it daily. When mitochondria function well, ATP is abundant and energy feels available. When mitochondria slow down, ATP production falls, and the felt experience is fatigue.
Mitochondrial function declines with age. This isn’t unique to women. But the trajectory of that decline is.[^1] In men, mitochondrial efficiency falls in a relatively linear pattern across decades. In women, it follows a different curve, one that includes a steep acceleration in the perimenopausal years, often beginning in the late thirties or early forties, sometimes earlier.
The reason is hormonal.
What estrogen was actually doing
For most of a woman’s adult life, estrogen has been quietly running a system most women never knew existed. Estrogen receptors are densely concentrated in mitochondria, and estrogen is one of the body’s most potent regulators of mitochondrial activity.[^2] It supports the structural integrity of mitochondrial membranes. It enhances the efficiency of the electron transport chain (the cellular machinery that actually produces ATP). It reduces the oxidative stress that damages mitochondria over time. And it promotes mitochondrial biogenesis, the process by which cells produce new mitochondria to replace aging ones.
When estrogen levels are high and stable, mitochondria function well. When estrogen levels become erratic and then decline, as they do in perimenopause, mitochondrial function follows suit.
A foundational body of work from researcher Roberta Diaz Brinton and colleagues at the University of Southern California has documented what they call the “perimenopausal bioenergetic transition,” a measurable shift in how the female brain and body produce and use energy during the years leading up to menopause.[^3] Their research, including translational neuroimaging studies combining FDG-PET and phosphorus-31 magnetic resonance spectroscopy, shows that perimenopausal women experience concurrent declines in cerebral glucose metabolism and oxidative phosphorylation efficiency.
Translated out of academic language: the brain, in particular, becomes less efficient at producing energy. And the brain consumes about 20% of the body’s total energy output despite representing only 2% of body weight. When the energy machinery slows down, the brain feels it first.
This is the cellular biology underneath what women in their forties describe as brain fog, fatigue that doesn’t respond to rest, and the diminished capacity to recover from things that used to be easy. It isn’t imagined. It isn’t laziness. It is a measurable shift in the energy economy of the body, and it has been mapped in the published literature.
What this changes about how the body responds to effort
The fatigue piece is the most discussed, but it isn’t the only consequence.
Recovery slows. Mitochondrial function is central to muscle repair after exercise. When mitochondria become less efficient, recovery takes longer, and the inflammation associated with hard physical effort lingers. Women who used to bounce back from a hard workout in 24 hours may suddenly find themselves needing 48 or 72 hours, or feeling residual heaviness even after that.[^4]
Body composition shifts. Estrogen-mediated mitochondrial function is closely linked to how the body handles fat. As mitochondrial efficiency falls and estrogen declines, fatty acid oxidation slows, while fat storage in visceral and abdominal regions becomes more pronounced. The classic perimenopausal shift toward central adiposity is, at the cellular level, a mitochondrial story before it’s a hormone story.[^5]
Insulin sensitivity decreases. Mitochondria play a central role in glucose metabolism, and their decline contributes to the insulin resistance that becomes more common in women’s forties and fifties. Blood sugar that used to be stable becomes more reactive. Carbohydrates that used to be fine produce more pronounced energy crashes.
Sleep quality changes. Mitochondrial function is implicated in the brain’s regulation of sleep architecture. As mitochondrial efficiency declines, the deep, restorative phases of sleep become harder to access, even when total sleep time looks normal on paper. This is part of why women in perimenopause often describe sleeping for hours and waking up still depleted.
Mood and cognition shift. The same brain regions involved in energy regulation are involved in mood and cognition. The Brinton lab’s research, along with multiple recent reviews, has linked perimenopausal mitochondrial dysfunction to the increased risk of depression, anxiety, and cognitive symptoms women experience during this transition.[^6]
None of these are separate problems. They are different views of the same underlying shift.
A new branch of research
For most of medical history, the mitochondrial decline of women’s midlife was treated as untreatable, beyond hormone replacement therapy itself. The thinking was: estrogen’s effects on mitochondria are real, but if a woman can’t or doesn’t want HRT, there isn’t much else to do.
That thinking has started to change, and it’s changed for an unusual reason: a new class of molecules called mitochondrial-derived peptides.
In 2015, researchers led by Changhan Lee and Pinchas Cohen at the University of Southern California published a paper in Cell Metabolism describing a peptide called MOTS-c.[^7] What made MOTS-c remarkable wasn’t just what it did, but where it came from. The peptide is encoded by the mitochondrial genome itself, the small set of DNA that lives inside mitochondria, separate from the nuclear DNA in the cell’s chromosomes. MOTS-c is, in other words, a peptide produced by the mitochondria themselves, apparently as a way of communicating with the rest of the cell about metabolic state.
The discovery was significant enough that it opened an entirely new field. Mitochondrial-derived peptides, including MOTS-c and a peptide called humanin discovered earlier, became the subject of rapidly expanding research because they appeared to act as retrograde signals from mitochondria to the nucleus, regulating metabolic adaptation, insulin sensitivity, and cellular stress response.
For women specifically, MOTS-c has produced one finding in the published literature that is worth pausing on. In a 2019 study by Lu and colleagues, published in the Journal of Molecular Medicine, MOTS-c treatment in ovariectomized mice (a standard model for postmenopausal hormonal change) prevented the obesity and insulin resistance that typically follow estrogen loss.[^8] The peptide also activated brown adipose tissue, reduced fat accumulation, and reduced inflammatory markers in white adipose tissue.
This is a rodent study, and the literal translation to women in midlife requires the careful caveats that any responsible reading of preclinical research demands. But it is the first peptide research that specifically engaged the metabolic question women in perimenopause are asking, in a model designed to mimic that hormonal transition. That alone is unusual.
A separate body of research has grown around a peptide called SS-31, also known as elamipretide, which selectively concentrates in mitochondria by binding to cardiolipin in the inner mitochondrial membrane.[^9] SS-31 has been documented to stabilize mitochondrial cristae structure, improve ATP production efficiency, and reduce reactive oxygen species at the source. It has been in active clinical trials for several mitochondrial-related conditions, including heart failure, age-related macular degeneration, and primary mitochondrial myopathy.
Neither of these peptides is FDA-approved for the kind of use most women might be considering. Both are sold as research chemicals for laboratory investigation. The published data is genuinely interesting, the clinical data is still emerging, and the women-specific evidence is in early stages. What has changed is that the research is now happening, and it is happening in a way that takes seriously the cellular biology underneath the lived experience of midlife women.
What this means for the woman experiencing it
The fatigue is not in your head. The slower recovery is not laziness. The brain fog is not aging in the abstract sense. It is a specific, measurable, increasingly understood shift in the energy machinery of the body, driven primarily by the loss of estrogen’s modulating effect on mitochondria.
This understanding doesn’t fix anything by itself. But it changes the questions worth asking. The interventions that address the underlying biology (hormone optimization where appropriate, exercise that supports mitochondrial biogenesis, sleep that allows mitochondrial repair, nutritional strategies that support cellular energy production) are categorically different from the interventions that don’t. Knowing which is which matters.
Peptide research is a part of this story. It is not the whole story, and it is not for everyone. But for women whose curiosity about their own biology has been outpacing the conventional wisdom available to them, the mitochondrial peptide literature is one of the more interesting frontiers in current research.
NextSelf Labs offers MOTS-c and SS-31 as research peptides for laboratory use, alongside other peptides whose roles in metabolic and energetic biology are increasingly well-characterized. The catalog is built around the proposition that women considering peptides deserve to know what the research actually shows. For a deeper look at what we mean by that, you can read our on the peptide research women were left out of.
The energy you have in your forties is not the energy you had in your thirties. The reason is cellular. Knowing that, and being curious about what the research is starting to say about it, is most of the work.
References
[^1]: López-Otín, C., Blasco, M. A., Partridge, L., Serrano, M., & Kroemer, G. (2023). “Hallmarks of aging: An expanding universe.” Cell, 186(2), 243 to 278. Mitochondrial dysfunction is established as one of the foundational hallmarks of biological aging.
[^2]: Klinge, C. M. (2008). “Estrogenic control of mitochondrial function and biogenesis.” Journal of Cellular Biochemistry, 105(6), 1342 to 1351. Comprehensive review of estrogen’s direct effects on mitochondrial function and biogenesis.
[^3]: Mosconi, L., Berti, V., Quinn, C., McHugh, P., Petrongolo, G., Varsavsky, I., et al. (2017). “Sex differences in Alzheimer risk: Brain imaging of endocrine vs chronologic aging.” Neurology, 89(13), 1382 to 1390. Brinton lab et al. Documents the perimenopausal bioenergetic transition through neuroimaging studies. See also: Yin, F., Yao, J., Sancheti, H., Feng, T., Melcangi, R. C., Morgan, T. E., et al. (2015). “The perimenopausal aging transition in the female rat brain: decline in bioenergetic systems and synaptic plasticity.” Neurobiology of Aging, 36(7), 2282 to 2295.
[^4]: Romero, S. A., Minson, C. T., & Halliwill, J. R. (2017). “The cardiovascular system after exercise.” Journal of Applied Physiology, 122(4), 925 to 932. Discusses mitochondrial recovery dynamics following exercise.
[^5]: Lizcano, F., & Guzmán, G. (2014). “Estrogen Deficiency and the Origin of Obesity during Menopause.” BioMed Research International, 2014, 757461. Documents the link between estrogen decline, mitochondrial dysfunction, and central adiposity.
[^6]: Wang, Y., Mishra, A., Brinton, R. D. (2020). “Transitions in metabolic and immune systems from pre-menopause to post-menopause: implications for age-associated neurodegenerative diseases.” F1000Research, 9, F1000 Faculty Rev-68. Recent review (2025): “Mitochondrial dysfunction in perimenopausal mood disorders: From hormonal shifts to neuroenergetic failure.” PMC12513434.
[^7]: Lee, C., Zeng, J., Drew, B. G., Sallam, T., Martin-Montalvo, A., Wan, J., Kim, S. J., Mehta, H., Hevener, A. L., de Cabo, R., et al. (2015). “The mitochondrial-derived peptide MOTS-c promotes metabolic homeostasis and reduces obesity and insulin resistance.” Cell Metabolism, 21(3), 443 to 454. The foundational paper introducing MOTS-c.
[^8]: Lu, H., Tang, S., Xue, C., Liu, Y., Wang, J., Zhang, W., Luo, W., & Chen, J. (2019). “Mitochondrial-Derived Peptide MOTS-c Increases Adipose Thermogenic Activation to Promote Cold Adaptation.” Journal of Molecular Medicine, 97(7), 1004 to 1015. See also: Lu, H., et al. (2019). “MOTS-c peptide regulates adipose homeostasis to prevent ovariectomy-induced metabolic dysfunction.” Journal of Molecular Medicine. PubMed: 30725119.
[^9]: Szeto, H. H. (2014). “First-in-class cardiolipin-protective compound as a therapeutic agent to restore mitochondrial bioenergetics.” British Journal of Pharmacology, 171(8), 2029 to 2050. Foundational paper on SS-31 (elamipretide) mechanism of action.
This piece reflects the views of NextSelf Labs and is intended for educational purposes. NextSelf Labs sells research peptides for laboratory use only. Products are not intended for human consumption, therapeutic use, or diagnostic application. Information presented here should not be construed as medical advice. Consult a qualified healthcare provider for any decisions regarding personal health, including hormonal therapies and interventions related to perimenopause and menopause.
