What Causes Brain Fog in ME/CFS, Fibro, and Long COVID

What is Brain Fog?

Have you ever read the same sentence five times and still couldn’t understand it? Or lost your train of thought mid-sentence, searching for a word that should come easily?

That’s not just forgetfulness. That’s brain fog.

In conditions like ME/CFS, fibromyalgia, and long COVID / Post-Acute Sequelae of COVID (PASC), brain fog is one of the most disabling symptoms patients experience. It affects memory, focus, processing speed, and decision-making—often interfering with daily independence.

Patients describe it in different ways: difficulty concentrating, trouble learning new information, losing words mid-sentence, or feeling mentally “slowed.” Regardless of how it presents, the underlying question remains the same.

Brain fog is not a single process. It reflects overlapping disruptions in immune signaling, brain energy metabolism, neurotransmitters, and gut-brain interactions. Below are six of the most well-supported mechanisms—particularly relevant in postviral illness.

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What Causes Brain Fog? (Overview)

The most common contributors to brain fog in ME/CFS, fibromyalgia, and long COVID / PASC include:

• Neuroinflammation and oxidative stress

• Gut-derived gases (ammonia, hydrogen sulfide, methane)

• Lactate and impaired brain energy metabolism

• Environmental exposures and pollutants

• Senescent (“zombie”) cells

• Neurotransmitter imbalance (glutamate excitotoxicity)

These processes often overlap rather than occur in isolation.

1. Neuroinflammation and Oxidative Stress

One of the most consistent findings in both ME/CFS and long COVID / PASC is chronic low-grade neuroinflammation—ongoing inflammation within the brain and central nervous system.

This process is driven in part by microglia, the brain’s resident immune cells. When activated, microglia release inflammatory cytokines and reactive oxygen species (free radicals). These interfere with neuronal communication, particularly in regions responsible for working memory, attention, and processing speed.

We see a similar phenomenon in “chemo brain,” where oxidative stress disrupts neuronal repair and activates microglia, leading to measurable cognitive dysfunction. Imaging studies in these patients show changes in brain regions tied to learning and memory.

In ME/CFS and fibromyalgia, the same pattern appears to hold. Oxidative stress acts as a trigger for neuroinflammation, which in turn disrupts signaling efficiency in the brain. The result is slowed thinking, reduced attention, and cognitive fatigue.

2. Gut-Derived Gases and the Gut-Brain Axis

For many patients, brain fog does not originate solely in the brain—it begins in the gut. Gut bacteria produce metabolites and gases that can influence brain function. These include ammonia, hydrogen sulfide, and methane. Under certain conditions, these compounds can enter systemic circulation and affect the central nervous system.

Ammonia

Ammonia is continuously produced in the body and is normally converted to urea for safe excretion. While severe elevations are associated with liver disease, even small increases—referred to as minimal hepatic encephalopathy—can impair cognitive function (Nardone et al., 2016; Walker, 2012).

Ammonia is neurotoxic. It disrupts neuronal metabolism, increases brain lactate, and alters astrocyte function, leading to subtle swelling and impaired signaling. Even mild elevations can interfere with attention, memory, and processing speed.

Importantly, ammonia is also produced by gut bacteria. Species such as Proteus mirabilis, Klebsiella pneumoniae, Clostridium perfringens, and Helicobacter pylori contain the enzyme urease, which converts urea into ammonia (Mora & Arioli, 2014).

Metabolomics research in ME/CFS has identified abnormalities in the urea cycle, suggesting impaired ammonia handling may be part of the disease process (Yamano et al., 2016). Even in the absence of liver disease, low-grade elevations may contribute to persistent brain fog.

Hydrogen Sulfide

Hydrogen sulfide is another microbial gas with neurological implications. Produced by bacteria such as Desulfovibrio and Bilophila wadsworthia, it can impair mitochondrial function, reducing energy production in both gut and brain cells.

Diets high in fat and protein may increase hydrogen sulfide production, particularly in susceptible individuals.

Methane

Methane, produced by organisms such as Methanobrevibacter smithii, is associated with slowed gut transit and constipation. While less directly neurotoxic, altered gut motility can influence microbial balance and metabolite production, indirectly contributing to cognitive symptoms.

3. Lactate and Impaired Brain Energy Metabolism

One of the more consistent biochemical findings in ME/CFS is elevated brain lactate.

Lactate accumulates when energy production becomes inefficient. In the brain, this suggests impaired mitochondrial function—meaning neurons are not generating energy effectively.

This matters because the brain is highly energy-dependent. Even small disruptions in ATP production can impair cognition.

Lactate may originate from two sources:

• Central production due to impaired brain metabolism

• Peripheral production from gut bacteria (D-lactate)

Elevated brain lactate has been shown to correlate with symptom severity in ME/CFS. In long COVID / PASC, the picture is more complex, but similar metabolic disruptions are likely involved.

Ammonia may further worsen this process by increasing lactate production and impairing energy pathways, linking gut metabolism directly to brain function.

4. Environmental Triggers and Pollutant Exposure

Environmental exposures can directly and indirectly affect brain function.

Ultrafine particles from pollutants, pesticides, and mold toxins can enter the brain through the bloodstream or via the olfactory nerve. Once there, they can activate microglia and promote neuroinflammation.

There is also a lung-brain signaling pathway. Inhaled pollutants stimulate inflammatory responses in lung tissue, leading to cytokine release that can travel to the brain and amplify immune activation.

Chronic exposure may “reprogram” microglia, making them more reactive over time and sustaining inflammation even after the exposure ends.

Air pollution has been associated with reduced cognitive performance and productivity, likely through these inflammatory pathways (Schmidt, 2019).

5. Senescent “Zombie” Cells in Long COVID / PASC

A newer and less widely recognized contributor to brain fog is cellular senescence.

Senescent cells—sometimes called “zombie cells”—are damaged or aged cells that fail to undergo normal programmed cell death. Instead, they persist and release inflammatory signaling molecules.

Autopsy studies in COVID-19 patients have shown increased accumulation of these senescent cells. These cells contribute to a pro-inflammatory environment, which may affect both systemic and brain function.

This area of research is still evolving, but it offers a compelling explanation for persistent symptoms in long COVID / PASC, including fatigue and cognitive dysfunction.

Therapies targeting senescent cells (senolytics) are currently under investigation, though this remains an emerging field.

6. Glutamate Imbalance and Excitotoxicity

Another important mechanism is neurotransmitter imbalance—specifically, excess glutamate.

Glutamate is the brain’s primary excitatory neurotransmitter. When levels become too high, it can lead to overstimulation of neurons, a process known as excitotoxicity.

This can impair cognition, reduce mental clarity, and contribute to symptoms such as anxiety and the “wired but tired” feeling many patients describe.

Ammonia may play a role here as well. It interferes with enzymes responsible for breaking down glutamate, making it harder for the brain to regulate excitatory signaling.

The result is a system that is overstimulated but inefficient—leading to mental fatigue and impaired function.

Why Brain Fog Feels Worse After Eating

Many patients report that brain fog worsens after meals.

This may be related to:

• Changes in blood sugar regulation

• Increased microbial fermentation in the gut

• Production of metabolites such as ammonia or D-lactate

Post-meal shifts in circulation and metabolism may also temporarily reduce energy availability to the brain, compounding cognitive symptoms.

Read more → Post-Meal Fatigue in ME/CFS and Long COVID

Why Brain Fog Is So Common in ME/CFS and Long COVID / PASC

Brain fog is especially prominent in postviral conditions because multiple systems are affected simultaneously:

• Immune activation (neuroinflammation)

• Metabolic dysfunction (lactate, mitochondrial impairment)

• Gut dysbiosis and metabolite production

• Autonomic and vascular changes

Rather than a single cause, brain fog reflects the combined effect of these disruptions.

Final Thoughts on Brain Fog

Brain fog is not a normal part of aging, nor is it simply a matter of stress or poor sleep.

It reflects measurable biological dysfunction involving the brain, immune system, metabolism, and gut. In ME/CFS, fibromyalgia, and long COVID / PASC, these processes often overlap, making brain fog one of the most persistent and disabling symptoms. Understanding the underlying mechanisms is the first step toward managing it.

FAQ: Brain Fog Causes

• What are the most common causes of brain fog?

  The most common causes include neuroinflammation, impaired brain energy metabolism, gut-derived metabolites, neurotransmitter imbalance, and environmental exposures.

• Why do I have brain fog all the time?

  Persistent brain fog often reflects ongoing immune activation or metabolic dysfunction rather than temporary fatigue alone.

• Can gut health cause brain fog?

  Yes. Gut bacteria can produce compounds such as ammonia and hydrogen sulfide that affect brain function and cognition.

• Why is brain fog worse after eating?

  It may be related to blood sugar fluctuations, microbial fermentation, and increased production of metabolites like lactate or ammonia.

• Is brain fog caused by inflammation?

  Inflammation—particularly neuroinflammation—is one of the most well-supported mechanisms behind brain fog.

References

Walker, V (2012) Severe hyperammonaemia in adults not explained by liver disease. Annals of Clinical Biochemistry: International Journal of Laboratory Medicine. 49(3): 214-228.

Nardone, R., Taylor, A. C., Höller, Y., Brigo, F., Lochner, P., & Trinka, E. (2016). Minimal hepatic encephalopathy: A review. Neuroscience Research, 111, 1–12.

Jin YY, et al (2018) Blood Ammonia as a Possible Etiological Agent for Alzheimer's Disease. Nutrients. 10(5). pii: E564.

Yamano, E et al. (2016) Index markers of chronic fatigue syndrome with dysfunction of TCA and urea cycles. Nature Scientific Reports volume 6, Article number: 34990.

Mora D. & Arioli S. (2014) Microbial Urease in Health and Disease. PLoS Pathog. 10(12): e1004472.

Schmidt S. (2019) Brain Fog: Does Air Pollution Make Us Less Productive? Environmental Health Perspective. 127:5.