Chronic Fatigue: Fatty-Acid Oxidation, Lactate Burden, and the Misclassification of Metabolic Divergence as Psychological Exhaustion

Disclaimer: I use scientific terminology in my research because I write academically. However, I do not always agree with the framing of the terminology used, even where it may be 'technically' accurate in the current environment. In such cases, I place the terminology in single inverted commas.

Chronic fatigue and ME/CFS are described in clinical settings through the language of fatigue, activity intolerance, deconditioning, and/or mood disturbance. However, this framing becomes inadequate where there is evidence, or reasonable suspicion, of underlying mitochondrial or metabolic vulnerability. In such cases, fatigue should be identified as a potential manifestation of impaired cellular energy production, reduced metabolic flexibility, and difficulty recovering after physiological demand.

A particularly important example is the role of fatty-acid oxidation. The HADHA gene encodes the alpha subunit of mitochondrial trifunctional protein, an enzyme complex involved in the final stages of long-chain fatty-acid β-oxidation. Variants affecting HADHA are associated with long-chain 3-hydroxyacyl-CoA dehydrogenase deficiency and mitochondrial trifunctional protein deficiency, conditions that alter the body’s ability to oxidise long-chain fatty acids efficiently (LCFAOD). These pathways are especially important during fasting, illness, prolonged exertion, physiological stress, and recovery, when the body typically increases reliance on fat-derived mitochondrial energy production.

Where long-chain fatty-acid oxidation is compromised, the body may have reduced capacity to access fat as a stable energy substrate. This can create a state of reduced fuel flexibility, in which the individual becomes more dependent on glucose metabolism and glycolytic compensation. Under conditions of exertion or stress, this may contribute to disproportionate lactate production, impaired lactate clearance, muscle pain, delayed recovery, post-exertional symptom exacerbation, and systemic energy instability. In this context, elevated lactate may act as a visible marker of a deeper metabolic bottleneck.

This has direct relevance to ME/CFS. Post-exertional malaise (PEM) is a pathological worsening of symptoms following physical, cognitive, sensory, emotional, or orthostatic demand. Studies have reported abnormal lactate findings in subsets of people with ME/CFS, including elevated resting blood lactate and abnormal lactate accumulation after repeated exercise. These findings support the possibility that at least some patients experience impaired metabolic recovery, altered mitochondrial function, or a shift toward anaerobic or glycolytic energy production under demand.

The same metabolic logic may also be relevant to neurodivergent populations. Autism and related neurodevelopmental profiles have been associated in multiple studies with mitochondrial ‘dysfunction’ in a subset of individuals, including altered lactate, pyruvate, lactate-to-pyruvate ratio, oxidative stress, and other markers of impaired energy metabolism. This does not imply that neurodivergence itself is pathological. Rather, it suggests that some neurodivergent individuals may carry underlying metabolic, mitochondrial, connective-tissue, immune, or redox vulnerabilities that make modern environmental demands disproportionately exhausting or destabilising.

A person with chronic fatigue symptoms in the context of a confirmed fatty-acid oxidation variant should not be reduced to a psychological formulation without proper metabolic investigation. Psychological support may help someone cope with the consequences of chronic illness, but it cannot substitute for evaluation of mitochondrial fuel handling, fatty-acid oxidation, lactate/pyruvate metabolism, carnitine status, acylcarnitine profile, glucose stability, rhabdomyolysis risk, cardiac involvement, liver involvement, neuropathy, or retinal involvement where these are biologically relevant.

Unfortunately, in relation to fatigue, mainstream clinical pathways often perform generic blood work and then cease further investigation when those results appear within normative-set ranges. Many patients experiencing chronic fatigue, exertional intolerance, post-exertional malaise, pain, and metabolic collapse therefore do not have documented evidence of an underlying physiological, genomic, mitochondrial, immune, or biochemical vulnerability. This does not mean such vulnerability is absent. It often means that the correct investigation has not been offered, the relevant tests are not routinely available, or the patient has been placed into a psychological pathway before biological mechanisms have been properly explored.

In my own case, I knew for years that something was physically happening. The fatigue I experienced felt like instability of energy production: a cellular struggle to meet demand, followed by disproportionate pain, weakness, and recovery collapse. At the time, however, I did not have the genomic evidence I now possess. I did not know that my raw data contained a homozygous ‘pathogenic’ variant in HADHA, a gene directly involved in mitochondrial long-chain fatty-acid oxidation. I only knew, from embodied lived experience, that the psychological explanation being offered did not fit the biological reality of what I was experiencing.

The misclassification of metabolic exhaustion as psychosomatic illness represents a category error. It relocates the problem from cellular energy production to personality, belief, mood, or behaviour. In doing so, it risks obscuring the actual mechanism of illness and may expose metabolically vulnerable patients to inappropriate advice, including exertional strategies that exceed their biochemical capacity. For patients with fatty-acid oxidation divergence, the central clinical question is what happens metabolically when they are required to produce energy.

A more coherent framework would treat chronic fatigue, exertional intolerance, lactate burden, and post-exertional malaise as possible signs of impaired energy delivery. In individuals with HADHA-related vulnerability, the pathway may be conceptualised as follows: long-chain fatty-acid oxidation impairment reduces mitochondrial fuel flexibility; reduced fuel flexibility increases reliance on glycolysis under demand; glycolytic compensation increases lactate burden and energy inefficiency; impaired recovery then manifests clinically as pain, weakness, cognitive dysfunction, post-exertional malaise, and chronic fatigue.

This model does not claim that all ME/CFS is caused by HADHA variants, nor that all neurodivergent people have fatty-acid oxidation ‘disorders’. Rather, it argues that biologically identifiable metabolic vulnerabilities must be recognised when present. Where such variants exist, they should alter the clinical interpretation of fatigue. The patient may be operating within a constrained energy system in which ordinary physiological demands produce disproportionate metabolic cost.

This reframing has wider implications for chronic illness medicine. It challenges the assumption that unexplained symptoms are psychological by default, and instead demands a more precise investigation of mitochondrial function, metabolic switching, immune signalling, vascular regulation, oxidative stress, and genetic susceptibility. For patients whose lived experience has been dismissed as psychosomatic, this synthesis offers a biologically coherent explanation: their fatigue may represent a functional divergence of cellular energy delivery that is incompatible with modern diets, environments, and clinical assumptions.

The Energetic Cost of Cognitive Divergence: Neurodivergent Processing, Brain Energy Demand, and Chronic Fatigue

It has been demonstrated how divergent fatty-acid oxidation, lactate, and reduced mitochondrial fuel flexibility may contribute to chronic fatigue and post-exertional collapse. However, this metabolic framework becomes even more important when considered alongside neurodivergent cognition. The issue is not only that some neurodivergent individuals may have reduced metabolic flexibility; it is also that divergent modes of cognition may require greater energetic investment.

The brain is an energetically expensive organ. Complex cognition, sensory integration, prediction, attention switching, emotional regulation, language processing, pattern recognition, social interpretation, and executive control all require metabolic support. These processes depend on adenosine triphosphate (ATP) production, mitochondrial function, glucose handling, oxygen delivery, vascular regulation, neurotransmitter cycling, ion gradients, and glial support. Cognitive work is therefore expensive biochemical labour.

This has particular relevance for neurodivergent populations. Many autistic, ADHD, dyslexic, bipolar, gifted, hyperlexic, traumatised, and/or otherwise divergent individuals process the world through increased pattern recognition, heightened sensory registration, rapid associative thinking, deep analytical synthesis, or intensified internal modelling. These capacities are often misread socially as personality traits, behavioural differences, anxiety, avoidance, overthinking, or emotional dysregulation. Yet from a neurobiological perspective, they may also represent high-demand processing states requiring substantial bioenergetic support.

This also creates a double vulnerability. If a person has a brain that processes more sensory, cognitive, social, emotional, or environmental information than the surrounding system recognises, they may already be operating with higher baseline energetic demand. If that same person also has mitochondrial vulnerability, divergent fatty-acid oxidation, lactate burden, connective-tissue instability, immune activation, redox stress, dysautonomia, or poor metabolic switching, then the energetic cost of cognition may exceed the body’s capacity to replenish itself.

Fatigue therefore may be the downstream consequence of a nervous system performing high-cost computation within a constrained energy economy. The individual may appear avoidant, withdrawn, inconsistent, overwhelmed, emotionally reactive, or unable to sustain ordinary demands, when the underlying issue is that cognitive and sensory processing are consuming more energy than the body can reliably supply.

This is especially important in relation to ME/CFS and post-exertional malaise, because exertion is not only physical. Cognitive exertion, sensory exposure, social interaction, emotional stress, light, sound, temperature change, decision-making, and sustained attention can all produce delayed deterioration. This suggests that the energetic conversion is not confined to muscles. It involves the brain, autonomic nervous system, immune system, vascular system, and mitochondrial network as a whole.

For some individuals, the combination of high cognitive load and constrained metabolic capacity may create a persistent mismatch between demand and energy availability. This mismatch may be especially significant in neurodivergent individuals who are expected to mask, interpret ambiguous social cues, suppress sensory distress, maintain executive function, regulate emotional responses, tolerate unsuitable environments, and perform sustained productivity under systems designed around different neurological and metabolic assumptions. Masking itself may be understood as an energy-intensive form of cognitive labour: continuous self-monitoring, behavioural translation, sensory suppression, and social prediction.

The result is metabolic overextension. A divergent brain is doing much more work than is visible from the outside, while a vulnerable energy system may be less able to buffer that demand. This creates a biologically coherent explanation for why neurodivergent individuals may experience disproportionate fatigue, burnout, pain, cognitive crash, and delayed recovery after tasks that appear externally minor.

This also helps explain why psychological interpretations are often inadequate. A person may be told they are anxious, avoidant, unmotivated, resistant, or catastrophising, when their nervous system may instead be exceeding its energetic threshold. Psychological support may help with distress, self-understanding, grief, pacing, or trauma from dismissal, but it cannot erase the energetic cost of complex cognition, sensory processing, mitochondrial vulnerability, or metabolic constraint.

A progressive framework would recognise neurodivergent fatigue as an interaction between increased neurocognitive demand and reduced metabolic resilience. Fatigue emerges not because the person is psychologically fragile, but because their brain and body are managing a higher processing load within a limited energy system. The failure is in clinical models that separate cognition from metabolism, and then misread metabolic exhaustion as psychological dysfunction.

Artist: H.J.Ruprecht, 1872

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