essay · neurometabolism

Brain fog is an energy problem.

A blurred screen on long sessions, mistyped letters, the lag before a thought arrives. Three small symptoms with a single cause underneath them: neurons running short of ATP.

Bioenergetics Notes / Apr 26, 2026 / 8 min read

Most explanations of brain fog reach for a neurotransmitter. Low dopamine. Rising serotonin. Faltering acetylcholine. Those names show up because they are what we know how to measure, not because they are the cause. Underneath all three is a simpler problem: the cells that do the thinking are running short of fuel.

The fuel is ATP, the small molecule that cells use to power almost everything they do. A neuron that cannot make enough ATP cannot fire on time, cannot recover between firings, and cannot maintain the charge differences across its membrane that make signaling possible. The neurotransmitter shifts that everyone talks about are downstream of that.

The brain is the most expensive tissue in the body. It is roughly 2% of your body weight and 20% of the energy you burn at rest, and unlike a muscle, it has nothing in reserve. Almost all of that energy goes to one job: holding a steady voltage difference across the membrane of every neuron. The inside of a resting neuron sits at about −70 millivolts relative to the outside, and the cell has to constantly pump sodium out and potassium back in to keep it there. Each time the cell fires, it spends that gradient down and then has to rebuild it before the next firing arrives. Disturb the gradient and the neuron stops firing on time.

That ATP is made inside small structures called mitochondria, sometimes called the cell's powerhouses. They take glucose (or fat) and oxygen in, and they put ATP, water, and carbon dioxide (CO₂) out. Most of an ordinary cell holds dozens to thousands of mitochondria depending on how energy-hungry the tissue is. Neurons sit at the top of that range. When that machinery slows down, whether from low thyroid, damaged membranes, gut inflammation, low blood sugar, or shallow breathing, the symptoms of fog show up in a predictable order.

fig.01 About half of a neuron's ATP goes to a single job: pumping sodium out and potassium in to maintain the gradients that make firing possible. Cells that fire faster spend more, and the most expensive cells (inhibitory interneurons, Purkinje cells, photoreceptors) reach their ceiling first.

01 · symptomsWhere ATP runs out first

The three classic symptoms of fog (e.g., blurred vision under sustained focus, fumbled fine motor control, and slowed thinking) are not random. They appear in this order because three particular parts of the brain run closest to their energy ceiling at all times. When fuel gets tight, those parts go first.

Ex. 1Vision blurring under sustained focus

Your retina is one of the most energy-hungry pieces of tissue you own. The light-sensing cells burn ATP fast just to do their normal job, and the small muscle that adjusts your eye's focus stays in continuous low-grade contraction the whole time you are looking at something. When ATP runs short, both fatigue.

There is a second hit on top of that. The blood supply to your brain depends on having enough carbon dioxide (CO₂) in your blood. CO₂ is what tells brain blood vessels to stay open. Stress breathing (fast, shallow, mouth open) blows off CO₂, which narrows those vessels and starves the visual cortex first. This is also why visual fog tracks so reliably with high-serotonin states: serotonin is itself a vasoconstrictor, acting on the same smooth muscle in blood vessel walls, and on top of that it interferes with mitochondrial respiration directly.^[1] Reduced cerebral blood flow plus lower oxidative output in each cell is enough to register as the wading-through-fog feeling on its own.

Ex. 2Fumbled fine motor control

Coordination is timing, and timing is run by a class of neurons in the cerebellum (Purkinje cells) that fire fast and burn through ATP faster than almost any other cell in the body. Their job is to inhibit: to say "not yet," "not that one," "stop." When fuel runs short, inhibitory neurons fail before excitatory ones, so coordination breaks down before strength does. The mistyped letter is the brake giving up: the inhibitor cell ran out of ATP first, while the muscle and the firing motor neuron behind it are still fine.

Ex. 3Slowed reaction and processing

Every time a neuron fires, it has to spend energy putting itself back the way it was: pumping ions back across the membrane, scooping neurotransmitter molecules out of the synapse, repackaging them for next time. All of this happens in the millisecond gaps between firings. Under marginal ATP, those gaps stretch out. The signal still goes through, but it arrives a little late, a little noisier, on top of a baseline that is itself a little fuzzier. That is the half-second lag you can sometimes feel between stimulus and response.

02 · mechanismThe cascade and its weak points

Here is what actually happens to a sugar molecule once it reaches a neuron.

The first step, glycolysis, takes place in the open part of the cell. It is fast, ancient, and yields almost nothing: just two ATP per glucose, plus a leftover called pyruvate. Anything more has to happen inside a mitochondrion.

Once pyruvate is inside, it gets fed into a circular reaction (the citric acid cycle) that strips the carbons off it and harvests the energy as electrons. Those electrons are then passed down a chain of proteins (the electron transport chain) embedded in the inner mitochondrial membrane. As they move along, the chain uses the released energy to pump protons (H⁺) out of the matrix and onto the other side of the membrane. The gradient that builds up is sharper than it sounds: roughly a tenfold concentration difference and a 150 millivolt charge difference across a membrane only a few nanometers thick. That stored difference is the cell's main energy reservoir, and ATP synthase is the only legitimate way back across it.

Then those protons get to fall back through a single doorway, a tiny molecular turbine called ATP synthase, and that fall is what makes ATP. About thirty more ATP per glucose come out of this second half. It is also where almost everything that can go wrong, does.

fig.02 The first stretch of the chain (glycolysis, in the cytosol) yields just two ATP per glucose. The much larger payoff comes from the citric acid cycle and the electron transport chain, both running inside the mitochondrion. That is also where almost every common breakdown point lives.

Five things break this machinery most often, and they tend to compound on each other.

Gut endotoxin is bacterial debris that leaks across the gut wall when digestion is unhappy. Once in the bloodstream it triggers inflammation, suppresses thyroid output, and raises serotonin, histamine, and nitric oxide. It is often the acute trigger of fog after a heavy meal or a poor night's sleep.

Lactate is what cells make when they cannot oxidize glucose properly, which means something in the chain above is already failing. Once lactate accumulates it inhibits pyruvate dehydrogenase, the enzyme that hands pyruvate off to the mitochondrion, so a small failure becomes a self-reinforcing one.

Polyunsaturated fats (PUFAs), the kind found in seed oils and most processed foods, get embedded in the inner mitochondrial membrane over the course of years. They oxidize easily, and when they do they damage the membrane that holds the proton gradient, which damages the gradient itself. PUFAs are sometimes called the dietary "forever chemicals" for this reason: once incorporated into membranes they can persist for years before turnover.

Low thyroid sets the density of the respiratory enzymes themselves: how many electron transport chains a cell can build in the first place. This is the most common chronic driver of fog, and the easiest to miss on standard blood panels because the numbers can sit just inside "normal" while the cells run lean. Running lean here means a cell with fewer mitochondrial enzymes than it should have for its workload, so each step of oxidation is a little slower and the steady state ATP supply is a little thinner. Nothing shows up as deficient until demand spikes.

Nitric oxide, released under any inflammatory exposure, binds the last enzyme in the electron transport chain (cytochrome c oxidase) and shuts it down directly. This is the fastest known route from an inflammatory exposure (a stressful conversation, a bad meal, a flu) to feeling unable to read a paragraph.

fig.03 The electron transport chain pumps protons up into the intermembrane space, building a gradient. ATP synthase lets them fall back through and uses the spinning to phosphorylate ADP. PUFA peroxidation damages the membrane that holds the gradient; nitric oxide blocks the pumps; low thyroid drops the density of the enzymes that run the whole machine.

03 · neurotransmittersThe neurotransmitter readout

Neurotransmitter levels are what the brain looks like under a metabolic state. They are not the cause of that state, they are the readout of it. Three patterns are worth holding in mind.

Serotonin

In the bioenergetic frame, serotonin is principally a shutdown signal. It rises sharply when the cell senses metabolic distress: low oxygen, low blood sugar, gut inflammation, infection, or systemic inflammation from any cause.^[2] Once high, it narrows blood vessels, suppresses dopamine release, and reduces blood flow to the brain. That last one accounts for most of the wading-through-fog feeling. About 90% of the body's serotonin is made in the gut, not the brain, by enterochromaffin cells responding to the contents of the lumen. Excess serotonin from a leaky or inflamed gut spills into circulation, raises systemic 5-HT tone, and reaches the brain via vagal signaling and (in inflamed conditions) directly across a more permeable blood-brain barrier.

Dopamine

Dopamine does roughly the opposite. It is the molecule of focus, drive, and the ability to start. The brain builds it in two steps from a dietary amino acid (tyrosine), and each step needs cofactors (small helper molecules like iron, oxygen, and B6) that are themselves only made or kept stable when the cell's oxidative metabolism is healthy. So when energy state collapses, dopamine collapses with it. The lived signature is poor focus, low motivation, and the can't-initiate quality of fog.

fig.04 The first step from tyrosine to L-DOPA is rate-limiting, oxygen-dependent, and exquisitely sensitive to thyroid status. Each cofactor (BH4, iron, oxygen, B6) is itself made or maintained by the same oxidative metabolism, so an upstream energy problem surfaces as a dopamine problem at the synapse.

Acetylcholine

Acetylcholine is the brain's attention and memory signal. Cholinergic projections from the basal forebrain set the gain on the cortex during focused tasks, and acetylcholine in the hippocampus is required for laying down new memories. Building it requires a starter molecule called acetyl-CoA, which is made by pyruvate dehydrogenase, the same enzyme lactate inhibits. So when the metabolic system jams up, acetylcholine and dopamine drop together and lactate rises in the same step.

Dopamine and acetylcholine also oppose each other in the striatum: dopamine release inhibits cholinergic interneurons, and those interneurons in turn modulate dopamine release. The healthy state is a balance between the two. Under metabolic stress both drop in absolute terms, but cholinergic tone often falls faster relative to dopamine, which is part of why the cluster of symptoms looks the way it does (poor recall, weak attention, sluggish initiation) rather than an even fade across the board.

Glutamate, GABA, and histamine

Glutamate is the brain's main excitatory signal. Cleaning it out of the synapse between firings is one of the most ATP-expensive things any neuron does, because it depends on glutamate transporters that have to be powered against a steep concentration gradient. When energy gets tight, that cleanup fails first. Background glutamate rises, neurons become subtly over-stimulated, and the cell wastes ATP trying to recover from signals that should not have arrived.

GABA, the brain's main inhibitory signal, is built directly from glutamate by a single enzyme (glutamic acid decarboxylase) that requires vitamin B6 as a cofactor. Marginal B6 or chronic stress reduces GABA synthesis at the same time that excess glutamate is overshooting. The combination is the paradoxical wired-but-can't-think state: too much background firing, not enough inhibition to quiet it.

Histamine adds the inflammatory component. Mast cell activation, often triggered by gut endotoxin or food intolerance, releases histamine into circulation. Histamine then induces nitric oxide synthase, which raises nitric oxide, which (as in the gut endotoxin paragraph above) directly inhibits cytochrome c oxidase. This is the fastest route from a single inflammatory meal to staring at a paragraph without absorbing any of it.

04 · synthesisThe energy budget under all of it

Life is driven by nothing else but electrons, by the energy given off by these electrons while cascading down from the high level to which they have been boosted up by photons. Albert Szent-Györgyi, Introduction to a Submolecular Biology (1960)

Szent-Györgyi's framing is the bioenergetic frame in one sentence. The brain is not a machine that runs on neurotransmitters. It is a machine that runs on a continuous descent of electrons through the respiratory chain, paid out one ATP at a time. Brain fog is what happens when that descent slows down.

The vision blur, the typing slip, the half-second lag are early warning signals from the cells with the highest energy demand in the body. The neurotransmitter shifts that follow are a downstream readout of an upstream problem with how those cells are being fueled.

Working backward from symptoms to physiology, the variables most worth evaluating tend to fall into a small set:

Thyroid status. Thyroid hormone sets how many mitochondrial enzymes a cell builds, so a slow thyroid means cells have fewer respiratory enzymes than they should and cannot make ATP fast enough even when glucose and oxygen are plentiful. The most common chronic driver of fog, and the easiest to miss on standard panels because TSH and total T4 can sit just inside normal while free T3 (the active hormone at the cell) is genuinely low.
PUFA load. Polyunsaturated fats from seed oils get embedded in the inner mitochondrial membrane and oxidize over the course of years. The damage is slow and cumulative, but it directly erodes the membrane that holds the proton gradient.
Oxidative stress. Reactive oxygen species (ROS) are byproducts of normal respiration, kept in check by vitamin E, vitamin C, and glutathione. When the balance tips, oxidants damage the same membranes and enzymes the cell depends on, and a failing electron transport chain then leaks more electrons in the wrong direction, producing still more ROS in a self-reinforcing loop. Smoking is the most concentrated exogenous source: each puff delivers on the order of 10¹⁵ free radicals plus carbon monoxide that competes with O₂ on hemoglobin, reducing oxygen delivery to the brain on top of the chemical insult.^[4] Alcohol, ultra-processed foods, and chronic infection do the same thing more slowly.
Gut endotoxin. When the gut barrier is inflamed or permeable, lipopolysaccharide (LPS), a fragment of the outer wall of gram-negative bacteria, leaks into the portal blood and reaches the liver and systemic circulation. LPS is detected as foreign by immune cells, which trigger the release of cytokines, serotonin, histamine, and nitric oxide. Thyroid output drops at the same time. This is often the acute trigger of an episode of fog after a heavy meal, alcohol, poor sleep, or any state that compromises gut barrier integrity.
Breathing pattern and CO₂ tolerance. CO₂ is not a waste gas; it is the primary regulator of cerebral blood flow. Chronic over-breathing (mouth breathing, sighing, anxious breathing) blows off CO₂ faster than the body produces it, which constricts cerebral arterioles and reduces oxygen delivery to neurons.^[3] The result looks like a disease but is purely a habit, and is reversible by training tolerance back up.
Mineral and B-vitamin sufficiency. A handful of small molecules sit at the heart of the energy machinery: thiamine (B1) especially, then magnesium, B6, iron. Any one of them being marginal can throttle the whole system.
Cortisol-to-progesterone and estrogen-to-progesterone ratios. Cortisol drives gluconeogenesis (breaking down muscle and other tissue for glucose) and suppresses thyroid conversion. Progesterone protects mitochondrial respiration and opposes both. When cortisol or estrogen sits high relative to progesterone for long stretches, the body is biased toward catabolism (tearing tissue down) and away from oxidative metabolism (running the cell efficiently). Brain cells feel that shift the same way every other tissue does.

None of these are exotic. All of them sit upstream of every neurotransmitter the typical workup tries to push around.

What this changes

The practical consequence of taking the bioenergetic frame seriously is that the standard fog workup (test the neurotransmitters, push them around with drugs that target receptors) is downstream of the real action. The work to actually do is metabolic: get enough thyroid hormone to where the cell can use it, lower the chronic inflammatory load that suppresses oxidation, stop the obvious oxidative insults (smoking is the loudest one, repeated alcohol exposure not far behind), eat in a way that does not pump PUFA into mitochondrial membranes, breathe in a way that does not strangle blood flow to the brain, and keep the small handful of cofactors at the heart of the machinery (thiamine, magnesium, B6, iron) at sufficiency. None of this is fast. All of it works on the same root variable, and the symptoms come along with it.

05 · sourcesSources

This article is a synthesis of widely-held positions in the bioenergetic and physiological literature. The framing is most strongly indebted to the published work of Ray Peat, Albert Szent-Györgyi, Otto Warburg, and the standard cellular respiration chapters in Guyton & Hall's Textbook of Medical Physiology. Specific anchor references for the inline citations:

[1] Peat, Ray. Generative Energy (1994), and essays at raypeat.com on serotonin, vasoconstriction, and mitochondrial inhibition. See also Watts AG, Cell Reports (2011) on serotonergic regulation of cerebrovascular tone.
[2] The relationship between serotonin and metabolic stress is reviewed in Berger M et al., Annual Review of Medicine (2009), "The expanded biology of serotonin." Peat treats serotonin as a stress mediator across many essays; this is one of the more contested positions, and the user is encouraged to verify the specific mechanistic claims.
[3] Buteyko KP. The role of CO₂ in cerebral autoregulation is standard physiology; see Brian JE, Anesthesiology (1998), "Carbon Dioxide and the Cerebral Circulation." For the breathing-pattern angle specifically, see McKeown's The Oxygen Advantage (2015) and the underlying Buteyko literature.
[4] Pryor WA, Stone K. Annals of the New York Academy of Sciences (1993), "Oxidants in cigarette smoke: radicals, hydrogen peroxide, peroxynitrate, and peroxynitrite." For the broader oxidative-stress and mitochondrial-decline framework, see Halliwell B and Gutteridge JMC, Free Radicals in Biology and Medicine (5th ed., 2015).

This is a reading framework, not medical advice. The bioenergetic perspective draws on the work of Ray Peat and the broader tradition that treats cellular respiration as the central organizing variable of physiology.