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Simon Parrott – Vitacuity Ltd
Persistent fatigue, especially the kind that disrupts thinking and doesn’t improve with sleep, may have a cellular origin. Your cells produce energy through a process involving several protein complexes working like an assembly line. The final step, Complex V, is where ATP (the molecule your body uses as fuel) is actually made. When this step slows down, neurons suffer most because they need constant, real-time energy to function, and they can’t store it locally. This creates the familiar pattern of brain fog, difficulty starting tasks, and needing disproportionate recovery time after mental effort.
Research on people with chronic fatigue and post-viral conditions shows measurable defects in this energy-making system. Studies using advanced imaging and cell analysis have found that mitochondria, the energy factories inside cells, aren’t producing ATP efficiently. Muscles show physical damage to mitochondria, and the body appears stuck in a harmful cycle: exertion causes cellular stress, which damages the mitochondria further, which reduces recovery capacity. This explains why pushing through fatigue can make things worse rather than better.
The research identifies several nutrients that support this energy system, particularly creatine, which helps cells quickly regenerate ATP. Magnesium is essential to the entire ATP production process, though the studies reviewed don’t directly test magnesium supplementation. Avoiding overexertion and pacing activities is not weakness but the physiologically correct response to a system already running at maximum capacity. This is an active area of research with specific molecular targets being identified for future treatments.
Verdict: Persistent cognitive fatigue can stem from real, measurable problems in how cells produce energy, and while we lack large-scale intervention studies, creatine supplementation and magnesium support have strong biological plausibility and safety profiles worth considering alongside pacing strategies.
What if the fatigue you feel, the kind that sleep doesn’t fix, the kind that makes your brain feel wrapped in fog by mid-morning, isn’t about willpower, lifestyle, or even stress? What if it’s happening at a level far more fundamental than any of those things: inside the tiny energy factories of your cells, where the biological machinery that powers every thought, every movement, and every heartbeat is quietly failing to do its job?
This isn’t a fringe idea. It’s where some of the most compelling fatigue research of the last five years is pointing. And the more scientists look at what’s actually happening inside the mitochondria of people who feel persistently exhausted, whether after a viral illness, a neurological event, or simply the cumulative grind of midlife, the more a single, uncomfortable truth keeps emerging: the problem isn’t that people are producing too little energy. It’s that the machinery built to convert fuel into usable cellular power is breaking down in specific, measurable ways.
Understanding that mechanism, and what it means for how you feel day to day, might be the most important thing you read this year.
*Vitacuity has analysed over 1.77 million research papers to bring you the most relevant, evidence-based insights on this topic. What follows is drawn from six key studies we identified as central to understanding the mitochondrial-fatigue connection.*
Let’s start with the basics, because the mechanism here is genuinely fascinating once you understand it.
Every cell in your body, and neurons especially, runs on a molecule called ATP (adenosine triphosphate). Think of ATP as the universal currency of biological energy. You can’t think, move, or regulate your mood without it. And the vast majority of your ATP is made inside mitochondria, through a process called oxidative phosphorylation, a kind of biological assembly line where electrons are passed along a series of protein complexes (imaginatively named Complex I through Complex V) to ultimately drive the synthesis of ATP [6].
Complex V, also known as ATP synthase, is the final step in this chain. It’s the machine that actually stamps out the ATP molecules. When Complex V works properly, the whole system hums along. When it doesn’t, you get a bottleneck: the earlier stages of the assembly line keep running, fuel keeps being consumed, but the final product, usable energy, comes out in reduced quantities [6].
For neurons, this is particularly catastrophic. Your brain accounts for roughly 20% of your body’s total energy consumption despite being only 2% of its mass. Neurons are among the most metabolically demanding cells in the body. They have almost no capacity to store energy locally, they rely on a continuous, real-time supply of ATP. When mitochondrial output drops, neurons are among the first to feel it, and the symptoms that follow, cognitive slowing, mental fatigue, difficulty initiating tasks, reduced concentration, and a recovery time after mental effort that feels wildly disproportionate to what you actually did, are precisely what researchers are now documenting [5].
This isn’t metaphorical brain fog. It has a measurable cellular basis.
Evidence grade: Promising, human cell study with a matched control group of 51 patients, but laboratory-based, not yet confirmed in large-scale clinical trials.
One of the most precise pieces of evidence for the mitochondrial-fatigue link comes from a 2020 study published in the *International Journal of Molecular Sciences* [6]. Researchers at a Canadian institution took blood cells from 51 people diagnosed with ME/CFS (myalgic encephalomyelitis/chronic fatigue syndrome) and an equal number of healthy, age- and gender-matched controls. They immortalised the cells so they could be studied continuously, then used a technique called Seahorse extracellular flux analysis, a method that measures oxygen consumption in living cells in real time, to assess mitochondrial function.
What they found was striking. The rate of ATP synthesis by Complex V, that final, critical step, was *significantly reduced* in ME/CFS cells compared to controls. At the same time, compensatory mechanisms were kicking in: the earlier complexes in the chain were working harder, the cells were consuming more oxygen, and there was increased “proton leak”, essentially, energy being lost as heat rather than captured as ATP.
The picture that emerges is of a system under pressure: the cell *knows* something is wrong and is trying to compensate, ramping up activity elsewhere to maintain steady-state ATP levels at rest. But the researchers made a crucial observation: this compensation may leave the cells with no reserve capacity. When energy demand suddenly spikes, during exercise, cognitive effort, or stress, there is nothing left in the tank to draw on. The system is already running at maximum just to maintain baseline [6].
This, the researchers argue, may explain one of the most debilitating features of ME/CFS and related conditions: post-exertional malaise. Not tiredness after effort, but a profound, delayed crash that can last days, triggered by levels of activity that healthy people wouldn’t even register [6].
Evidence grade: Promising, multiple recent studies converging on the same finding, including direct microscopy evidence, but mechanisms are still being untangled.
For years, researchers debated whether mitochondrial dysfunction in ME/CFS was real or a statistical artefact. A key problem was that most studies looked at white blood cells (leukocytes), easy to obtain but perhaps not the most relevant tissue. A 2025 review in the *Journal of Cachexia, Sarcopenia and Muscle* [2] changed that conversation by aggregating the growing evidence from skeletal muscle studies.
Here’s what the accumulated evidence now shows: electron microscopy, essentially, photographing the insides of muscle cells at extraordinary magnification, has *directly visualised* damaged mitochondria in the skeletal muscle of ME/CFS patients, with damage concentrated in a specific location (the subsarcolemmal region, which sits just beneath the cell membrane). This damage was not found in post-COVID patients who hadn’t developed ME/CFS, suggesting it represents a more advanced or distinct pathological process [2].
Further, MRI studies have shown that intracellular sodium levels in the muscles of ME/CFS patients are elevated, and that higher sodium correlates with weaker grip strength, which in turn correlates with symptom severity and prognosis. The proposed mechanism is a cascade: reduced blood flow leads to anaerobic metabolism, which floods cells with protons (hydrogen ions), which in turn drives sodium into the cells through a proton-sodium exchange mechanism, which then triggers calcium overload, which damages mitochondria [2].
What makes this particularly concerning is that the damage may be self-reinforcing. Exertion causes more ionic disruption, which causes more mitochondrial damage, which reduces the cell’s ability to recover, which makes the next bout of exertion even more damaging. Biopsy studies have found evidence of both necrosis (cell death) and regeneration in muscle tissue taken just one day after exercise in post-COVID ME/CFS patients, suggesting the body is in a constant, exhausting cycle of damage and attempted repair [2].
Evidence grade: Early stage, mechanisms proposed based on converging evidence, but direct causal human trials are still needed.
Not all fatigue is created equal. A 2025 paper published in the *Journal of Clinical Medicine* [5] makes a compelling case for what the authors call “Brain Fatigue Syndrome” (BFS), a distinct cluster of symptoms that arises specifically from neurological impact, whether from brain injury, autoimmune disease, neurological conditions, or systemic inflammatory illness.
The symptoms are recognisable to anyone who has experienced them: reduced endurance for cognitive tasks, disproportionately long recovery after mental effort, emotional instability, heightened sensitivity to stress and sensory input, difficulty *starting* tasks even when motivation exists, and sleep disturbances. Tension headaches triggered by mental exertion are also common [5].
At the cellular level, the researchers propose several interacting mechanisms. Astrocytes, the support cells of the brain, which regulate neurotransmitter levels and supply glucose to neurons, may become dysfunctional, impairing both glutamate signalling and glucose uptake. When neurons can’t get glucose efficiently, and when glutamate signalling is disrupted, normal cognitive processing is compromised at its most basic level. Mitochondrial dysfunction within neurons compounds this, reducing the ATP available for the energy-intensive work of firing signals and maintaining the ion gradients that make neural communication possible [5].
Blood-brain barrier dysfunction and activation of microglia (the brain’s immune cells) and mast cells add further layers of inflammatory disruption [5]. The result is a brain that is genuinely, measurably struggling, not one that simply needs its owner to push through or think more positively.
The researchers note that in many cases BFS resolves when the underlying injury or illness heals. But in a significant subset of people, it persists long after the apparent recovery, a finding that aligns with what we know about post-viral fatigue and long COVID [5].
Evidence grade: Promising, multiple converging lines of evidence from recent reviews, but large-scale RCTs for specific interventions are still limited.
The post-COVID era has brought an unwelcome but scientifically valuable opportunity: a large population of people experiencing measurable, documented fatigue following a well-characterised viral event. This has accelerated research into the underlying biology of post-viral fatigue in ways that might have taken decades otherwise.
A 2025 review in *Frontiers in Immunology* [4] synthesised current evidence using magnetic resonance spectroscopy (MRS), a non-invasive imaging technique that can measure the actual biochemical composition of tissue, to assess metabolic recovery in post-COVID patients. The findings confirm what cell studies have suggested: reduced ATP production, elevated oxidative stress, and disrupted mitochondrial biogenesis (the process by which cells make new mitochondria) are all measurable in post-COVID syndrome [4].
Specifically, MRS can track a marker called τPCr, the time it takes for phosphocreatine (a rapid-access energy store) to recover after exercise. In people with mitochondrial dysfunction, this recovery is slower, providing an objective window into cellular energy metabolism that doesn’t rely on self-reported symptoms alone [4].
The review notes that post-COVID mitochondrial abnormalities closely mirror those seen in chronic fatigue syndromes more broadly, a finding that supports the idea that these conditions share a common bioenergetic pathway rather than being entirely separate diseases [4]. Researchers debate whether systemic inflammation or direct viral damage is the primary driver, but the downstream consequence, impaired mitochondrial energy output, appears to be consistent across both proposed mechanisms [4].
A parallel 2025 review in the *Journal of Strength and Conditioning Research* [1] makes the case that creatine metabolism specifically may be disrupted in post-viral fatigue syndrome. The WHO now classifies post-viral fatigue syndrome as a neurological condition (ICD-11 code: 8E49), and the authors argue that creatine, which plays a central role in rapid ATP regeneration, particularly in the brain, deserves serious attention as both a diagnostic marker and a potential therapeutic target [1].
Similarly, a 2025 paper in *Physiology* [3] reviewing mitochondrial dysfunction in ME/CFS notes that a protein called WASF3 has emerged as a potential molecular culprit, it appears to disrupt the organisation of mitochondrial respiratory complexes, reducing their efficiency. ER (endoplasmic reticulum) stress is also implicated, creating a cascade that further impairs mitochondrial function [3]. These are early-stage findings, but they point toward specific molecular targets that future therapies could address.
This is where honesty matters, and at Vitacuity, we’d rather tell you what the research can’t yet confirm than oversell what it can.
The magnesium connection is implied, not directly tested in these studies. The research described here focuses on mitochondrial dysfunction, ATP production deficits, and cellular energy metabolism broadly. While magnesium is essential for ATP synthesis, ATP exists in the cell almost entirely as a complex bound to magnesium, and multiple mitochondrial enzymes require magnesium as a cofactor, the specific studies reviewed here do not directly measure magnesium levels in fatigued patients or test magnesium supplementation as an intervention. The mechanistic link is scientifically well-grounded, but this blog’s cited research focuses on the mitochondrial side of the equation rather than the magnesium intervention side specifically.
Causation remains difficult to establish. The studies reviewed here are largely observational, mechanistic, or review-based. We can see that people with ME/CFS have measurable mitochondrial dysfunction. We can see that this correlates with symptom severity. What we can’t yet say with certainty is whether correcting the mitochondrial dysfunction would resolve the fatigue, or whether both are downstream effects of some other upstream cause that we haven’t fully identified yet [2], [3].
Most interventional trials are short-term and small. The review of nutritional approaches for post-COVID mitochondrial dysfunction [4] notes that most clinical trials have focused on short-term outcomes and lack data on long-term efficacy. Protocols for supplements like CoQ10, NAC, and creatine vary widely across studies, making it difficult to draw firm conclusions about optimal dosing or duration [4].
We don’t yet have good ways to identify who needs what. The precision medicine approach being called for in the literature [4], matching specific nutritional interventions to specific patterns of mitochondrial dysfunction measured by MRS, is not yet available outside research settings. Testing for mitochondrial function severity isn’t something your GP can order on the NHS. Until that changes, personalised approaches remain aspirational rather than practical for most people.
The cell studies use immortalised blood cells, not neurons. The Complex V findings from the 2020 study [6] were observed in lymphoblasts, blood-derived cells, not brain cells. While the findings are compelling, we can’t assume identical mechanisms operate in neurons without direct evidence from neural tissue.
So what does all of this mean for a sensible, informed person in their 40s, 50s or 60s who wakes up tired, finds their brain running on half-power by 11am, and suspects something more fundamental is going on than simply needing an earlier bedtime?
First: take the biology seriously. The research reviewed here, drawn from six recent studies by Vitacuity’s analysis of over 1.77 million papers, makes a strong case that persistent cognitive fatigue and post-exertional exhaustion can have real, measurable cellular origins. This is not a character flaw. It is not simply stress. For a meaningful proportion of people, something is genuinely happening at the mitochondrial level that impairs the production or efficiency of ATP, and neurons, being the most energy-hungry cells in the body, feel it first and hardest [5], [6].
Second: the interventions with the most biological plausibility for supporting mitochondrial energy metabolism, creatine, CoQ10, and NAC, are all identified in the reviewed research [1], [4]. Of these, creatine monohydrate deserves particular mention: it’s cheap, well-studied for safety, and directly supports the rapid regeneration of ATP from ADP, exactly the system that appears compromised in post-viral and mitochondrial fatigue. The WHO’s classification of post-viral fatigue syndrome as a neurological condition adds legitimacy to exploring creatine’s role in brain energy metabolism specifically [1]. A typical research dose is 3–5g daily. The safety profile at this dose is excellent.
Third: the practical case for magnesium supplementation remains strong on mechanistic grounds, even though the specific studies here don’t test it directly. ATP exists in cells almost exclusively as Mg-ATP, magnesium is not optional for energy production, it’s structural to the process. Magnesium is also water-soluble in its most bioavailable supplemental forms (magnesium glycinate, malate, or citrate), meaning excess is excreted rather than accumulated. Deficiency is extremely common in Western populations, and the cost and risk of supplementing at 200–400mg daily is minimal. As a sensible, informed person: supplement daily. The risk of doing nothing when deficiency is common far outweighs the risk of supplementing at normal doses.
Fourth: don’t underestimate the importance of avoiding the crash. The research on post-exertional malaise is sobering [2], [6]. If you are genuinely experiencing the pattern described here, cognitive effort followed by a disproportionate, delayed recovery, the evidence suggests that pushing through is not just unhelpful but potentially harmful at the cellular level, driving further rounds of damage and ionic disruption [2]. Pacing isn’t weakness; in this context, it’s the physiologically correct response to a system that is already running at capacity.
Finally: this is a fast-moving area of research. The molecular targets being identified, Complex V, WASF3, ER stress pathways, represent future therapeutic opportunities [3]. What looks like a frustrating, poorly understood condition today may look very different in five years. Staying informed, supporting your mitochondrial health with evidence-backed nutrients where you can, and not catastrophising in either direction (this is not incurable, but it is also not something you can simply willpower your way through) is the most rational position the current evidence supports.
Your mitochondria are not failing you. But they may need some support.
[1] Creatine and post-viral fatigue syndrome: an update (2025). *Journal of the International Society of Sports Nutrition.* DOI: 10.1080/15502783.2025.2517278 PubMed: https://pubmed.ncbi.nlm.nih.gov/40481620/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12147496/
[2] Key Pathophysiological Role of Skeletal Muscle Disturbance in Post COVID and Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS): Accumulated Evidence (2025). *Journal of Cachexia, Sarcopenia and Muscle.* DOI: 10.1002/jcsm.13669 PubMed: https://pubmed.ncbi.nlm.nih.gov/39727052/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11671797/
[3] Mitochondrial Dysfunction in Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (2025). *Physiology.* DOI: 10.1152/physiol.00056.2024 PubMed: https://pubmed.ncbi.nlm.nih.gov/39960432/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12151296/
[4] Mitochondrial metabolic rescue in post-COVID-19 syndrome: MR spectroscopy insights and precision nutritional therapeutics (2025). *Frontiers in Immunology.* DOI: 10.3389/fimmu.2025.1597370 PubMed: https://pubmed.ncbi.nlm.nih.gov/40486513/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12141278/
[5] The Brain Fatigue Syndrome, Symptoms, Probable Definition, and Pathophysiological Mechanisms (2025). *Journal of Clinical Medicine.* DOI: 10.3390/jcm14103271 PubMed: https://pubmed.ncbi.nlm.nih.gov/40429267/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12111823/
[6] An Isolated Complex V Inefficiency and Dysregulated Mitochondrial Function in Immortalized Lymphocytes from ME/CFS Patients (2020). *International Journal of Molecular Sciences.* DOI: 10.3390/ijms21031074 PubMed: https://pubmed.ncbi.nlm.nih.gov/32041178/ PMC: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7036826/
This article is for informational purposes only and does not constitute medical advice. Food supplements should not be used as a substitute for a varied and balanced diet and healthy lifestyle. If you are pregnant, breastfeeding, taking medication or have a medical condition, consult your doctor before taking any supplement. These statements have not been evaluated by the Food and Drug Administration (FDA) or the Medicines and Healthcare products Regulatory Agency (MHRA). This product is not intended to diagnose, treat, cure, or prevent any disease.
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