Why Dysbiosis, SIBO, Hydrogen Sulfide Intolerance, MCAS, and Long COVID May All Be Downstream of a Single Event

For thirty years, the story of gut disease has been a story about microbes — too few good ones, too many bad ones, fix the bugs and you fix the patient. The Host Capacity Model says we have been reading the story backwards. The microbiome is not the author of chronic gut dysfunction. In a great many complex cases, it is the signature — the visible residue of a host that has lost the metabolic capacity to hold its own ecosystem in place.

The single most important sentence in this entire framework is this: a healthy microbiome does not survive because the right organisms arrive. It survives because the host builds and defends the conditions those organisms require. When the host can no longer build those conditions — when one specialized intestinal cell stops being able to burn fuel and consume oxygen — the ecosystem doesn't just drift. It flips. And once it flips, it locks.

This article is the long version of that argument, written so that you can follow every step of the mechanism even if you've never opened a biochemistry textbook. By the end you will understand why "low butyrate" and "can't use butyrate" are completely different diseases, why some people react badly to the very supplements meant to help them, and why chasing organisms with antimicrobials, probiotics, and binders so often fails to produce lasting change.

The standard story — and why it stops short

Open almost any conversation about gut health and it begins in the same place: dysbiosis, an imbalance in the community of microbes living in the intestine.

We talk about low microbial diversity. We talk about depleted butyrate producers — the beneficial bacteria that ferment fiber into short-chain fatty acids. We talk about elevated Proteobacteria and expanded Enterobacteriaceae (families of bacteria that bloom in inflamed, leaky, oxygen-exposed guts). We talk about hydrogen-sulfide organisms, methane organisms, histamine-producing bacteria, parasites, yeast. We talk about low Faecalibacterium prausnitzii, low Roseburia, low Akkermansia, low Bifidobacterium, low Lactobacillus.

All of that is real. All of that matters. I am not here to dismiss a single line of a stool report.

But notice what the standard story never asks. It catalogs which organisms are present or missing, and then it tries to add back the missing ones and kill the excess ones. It almost never asks the prior question:

Why did the host lose the ability to maintain this ecosystem in the first place?

Because here is the uncomfortable truth that anyone who has treated complex cases eventually runs into: you can give the "right" probiotics, the "right" fibers, the "right" antimicrobials, and the ecosystem still refuses to stabilize. The good organisms won't take. The bad ones come back. You are pouring seeds onto soil that can no longer support them — and then blaming the seeds.

The Host Capacity Model (HCM) is my attempt to describe that soil. It asks a different question, and that single change of question reorganizes everything downstream of it.

Meet the colonocyte: the cell that decides who is allowed to live in your gut

To understand the model, you have to meet its central character.

A colonocyte is one of the epithelial cells that line the wall of your large intestine — the single layer of cells standing between the trillions of microbes in your gut lumen and the rest of your body. The instinct is to imagine these cells as a passive wall, a row of bricks. That instinct is wrong, and the error is the source of a huge amount of confusion in this field.

The mature colonocyte is not a brick. It is a metabolically specialized engine, and one of its jobs — arguably its most important ecological job — is to control the oxygen environment of the colon.

Here is the setup. Your gut wall is richly supplied with blood, so the tissue side of the colonocyte sits in oxygen. But the lumen — the open channel where your microbes live — is supposed to be almost completely oxygen-free, a condition called physiological hypoxia (literally "normal low-oxygen"). This matters enormously, because most of the beneficial gut bacteria are obligate anaerobes: organisms that are poisoned by oxygen. Faecalibacterium, Roseburia, Eubacterium, Anaerostipes, the Clostridial clusters IV and XIVa — these are the keystone species of a healthy colon, and they can only dominate when the neighborhood stays anaerobic.

So who keeps the lumen anaerobic? The colonocyte does. And the way it does it is the heart of this entire model.

How butyrate becomes oxygen control: the mechanism, step by step

Healthy colonocytes don't run on sugar the way most of your cells do. They run, preferentially, on butyrate — a short-chain fatty acid (a small fat molecule) that anaerobic bacteria produce when they ferment dietary fiber and resistant starch. Up to roughly 70% of a colonocyte's energy comes from butyrate. This is one of the most beautiful arrangements in human biology: the bacteria feed the very cell that builds the home they need.

Follow the butyrate as it enters the cell:

Transport in. Butyrate crosses into the colonocyte through dedicated transporter proteins — chiefly MCT1 (monocarboxylate transporter 1, gene SLC16A1) and SMCT1 (sodium-coupled monocarboxylate transporter 1, gene SLC5A8). Hold onto that second one. SLC5A8 will come back later as a hidden lock in the whole system.

Activation. Inside the cell, butyrate is tagged with coenzyme A to become butyryl-CoA, the "activated" form that the mitochondria can process.

Beta-oxidation. Butyryl-CoA enters beta-oxidation inside the mitochondria — the metabolic assembly line that chops fats apart for energy — producing acetyl-CoA, the universal fuel molecule that feeds the cell's central furnace.

The TCA cycle. Acetyl-CoA enters the tricarboxylic acid (TCA) cycle, also called the Krebs cycle — the spinning wheel at the center of cellular metabolism. Each turn of the wheel strips high-energy electrons off the fuel and loads them onto two carrier molecules, NADH and FADH₂.

The electron transport chain (ETC). NADH and FADH₂ deliver those electrons to the electron transport chain, a series of four protein complexes (Complex I through IV) embedded in the inner mitochondrial membrane. The electrons cascade down the chain, releasing energy that the cell uses to make ATP, its energy currency.

The final step — and the ecological payoff. At the very end of the chain, at Complex IV (cytochrome c oxidase), those spent electrons are handed off to oxygen, which is consumed and turned into water.

That last step is the one almost everyone overlooks, and it is the linchpin of the Host Capacity Model.

Butyrate oxidation consumes oxygen. Vast amounts of it, relative to the alternative. The colonocyte, by burning butyrate at its surface, acts like a biological vacuum cleaner that pulls oxygen out of the mucosal environment before it can leak into the lumen. This is exactly what Kelly and colleagues demonstrated in 2015: butyrate drives epithelial oxygen consumption, which stabilizes a master regulatory protein called HIF (hypoxia-inducible factor) that further reinforces the barrier — and antibiotic depletion of butyrate-producers collapses both at once.

So the colonocyte's metabolism is the colon's oxygen control system. When that engine runs, the lumen stays anaerobic, and the anaerobes — the good guys — keep their home-field advantage.

This is also why butyrate is so much more than "fuel." Through this same machinery and its signaling side-effects, butyrate maintains physiological hypoxia by driving oxygen consumption (the ecological function we just walked through); activates PPAR-γ (peroxisome proliferator-activated receptor gamma), a nuclear receptor that locks the colonocyte into its oxygen-burning, fat-oxidizing metabolic program (Byndloss and colleagues showed in Science in 2017 that when this PPAR-γ signal is lost, the cell switches metabolic mode, oxygen and nitrate leak into the lumen, and Enterobacteriaceae like E. coli and Salmonella bloom); acts as an HDAC inhibitor, blocking histone deacetylases — enzymes that control which genes are switched on — which is how a simple fat molecule ends up reprogramming gene expression, calming inflammation, and inducing regulatory T cells (the immune cells that teach the gut to tolerate its own microbes, demonstrated by Furusawa and by Arpaia, both in Nature in 2013); and signals through receptors GPR41, GPR43, and GPR109A (HCAR2), coordinating epithelial and immune communication.

One molecule. Energy source, oxygen sink, gene regulator, immune educator, barrier builder. Butyrate is the keystone — and the colonocyte is the cell that has to be healthy enough to use it.

The oxia-dysbiosis loop: the engine that turns a small failure into a chronic disease

Now we can describe the central mechanism of chronic gut dysfunction — the self-reinforcing cycle I call the oxia-dysbiosis loop.

Picture what happens when the colonocyte's mitochondria start to fail — from inflammation, infection, toxin exposure, viral reactivation, nutrient depletion, or any of the redox insults we'll come to in a moment.

A struggling cell can't run oxidative phosphorylation (the oxygen-hungry, butyrate-burning pathway) efficiently anymore. So it does what stressed cells do: it falls back on glycolysis — generating ATP from glucose without needing much oxygen. This is sometimes called a Warburg-like shift, after the same metabolic switch seen in cancer cells.

Here is the catch. A glycolytic colonocyte consumes far less oxygen. And the moment epithelial oxygen consumption falls, the vacuum cleaner shuts off. Oxygen — and, via inflammation-driven iNOS, nitrate — begins to leak into the lumen.

Watch the dominoes fall: oxygen leaks into the niche, and the obligate anaerobes (the good guys) lose their competitive advantage because oxygen is poison to them. Facultative anaerobes and oxygen-tolerant organisms — Enterobacteriaceae, the inflammatory pathobionts — surge in to fill the space, now respiring on the leaked oxygen and nitrate. (Rivera-Chávez and colleagues showed in 2016 that simply depleting butyrate-producing Clostridia is enough to oxygenate the gut and let Salmonella expand aerobically.) These organisms shed more LPS (lipopolysaccharide, the inflammatory coat of gram-negative bacteria), flagellin, and other microbial-associated molecular patterns — molecular "danger signals" your immune system is built to detect. Those signals trip pattern-recognition receptors (TLR4, TLR5, NOD-like receptors), firing up NF-κB, the master switch of inflammation. Inflammation further damages colonocyte mitochondria. Oxygen consumption falls further. More oxygen leaks. More dysbiosis.

The gut becomes more oxygenated than it was ever meant to be, that oxygen favors the wrong organisms, those organisms drive inflammation, inflammation cripples the mitochondria that were supposed to consume the oxygen — and the loop closes on itself.

This is the part of the model that explains chronicity. It explains why so many people get stuck. It is not that they keep catching the wrong bacteria. It is that they are trapped in a loop that perpetuates the very conditions that select for the wrong bacteria. The microbiome looks abnormal not only because bad organisms arrived, but because the host stopped maintaining the low-oxygen world the good organisms require.

Peroxynitrite and the iron-sulfur cluster catastrophe: how the engine actually breaks

So far I've said the colonocyte's mitochondria "start to fail." That's a placeholder. Let's open it up, because the specific way they fail is what ties the gut to inflammation, infection, mast cells, and long COVID.

In chronic inflammation, a gene called NOS2 — encoding inducible nitric oxide synthase (iNOS) — gets switched on hard, flooding the tissue with nitric oxide (NO). At the same time, damaged mitochondria and inflammatory enzymes called NADPH oxidases (NOX) spill out superoxide, a reactive oxygen molecule.

Nitric oxide and superoxide find each other and react almost instantly to form peroxynitrite (ONOO⁻) — one of the most destructive reactive nitrogen species in all of biology. Peroxynitrite is not a vague "oxidative stress." It is a precision-guided wrecking ball, and it has a favorite target. That target is the iron-sulfur cluster.

Iron-sulfur clusters are tiny scaffolds — built from iron atoms and inorganic sulfur, in arrangements like 2Fe-2S and 4Fe-4S — that sit at the catalytic heart of many of the most important enzymes in your mitochondria. Think of them as the spark plugs of cellular respiration: small, easily fouled, and absolutely essential. Peroxynitrite oxidizes and tears these clusters apart.

Look at what depends on them: Aconitase, a TCA-cycle enzyme, carries a 4Fe-4S cluster — damage it and the central metabolic wheel seizes. Complex I of the electron transport chain holds multiple iron-sulfur centers that ferry electrons from NADH — damage them and the front of the chain stalls. Complex II carries iron-sulfur centers feeding electrons from succinate. Complex III holds the Rieske iron-sulfur protein, a critical relay in the middle of the chain. Lipoic acid synthase is itself an iron-sulfur enzyme — cripple it and you lose endogenous lipoic acid, which means pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase (the gateways that let carbohydrate-derived carbon enter mitochondrial oxidation) both falter. Ferrochelatase, the final enzyme of heme synthesis, is tied into iron-sulfur biology as well.

Now watch how this becomes a trap rather than a single injury: peroxynitrite damages iron-sulfur clusters → damaged clusters impair mitochondrial respiration → impaired respiration leaks more superoxide → that superoxide reacts with the nitric oxide that's still flooding in → producing more peroxynitrite → which damages more iron-sulfur clusters.

This is not "inflammation." This is a redox-metabolic collapse that feeds itself, and it is happening inside the very cell that is supposed to be burning butyrate and consuming oxygen. The peroxynitrite trap and the oxia-dysbiosis loop are not two separate problems. They are the same engine seen from the inside and the outside.

The Fenton trap: free iron, lipid peroxidation, and a phospholipid named cardiolipin

When iron-sulfur clusters disintegrate, they don't vanish — they dump their iron into the cell's labile iron pool (the small reservoir of loose, reactive, unbound iron).

Loose iron in an inflamed cell is dangerous, because of a piece of chemistry called the Fenton reaction: in the presence of hydrogen peroxide, free ferrous iron generates the hydroxyl radical, the single most reactive and indiscriminate oxidant the body can produce. Hydroxyl radicals don't signal; they simply shred whatever they touch.

What they touch, in a mitochondrion, is fat. They initiate lipid peroxidation — a chain reaction that rips through the fatty membranes of the cell. And one membrane fat matters more than any other here: cardiolipin, a phospholipid found almost exclusively in the inner mitochondrial membrane, where it physically organizes the respiratory complexes into efficient supercomplexes. When cardiolipin is oxidized, the electron transport chain loses its scaffolding and its electrons leak even more freely — which makes more superoxide, which makes more peroxynitrite, which damages more iron-sulfur clusters, which releases more iron.

If that pattern of iron-driven lipid peroxidation sounds like it has a name, it does — it's the same chemistry underlying ferroptosis, a form of regulated cell death. The colonocyte isn't just running poorly. It is being pushed toward the edge.

The takeaway: by the time a complex case reaches you, you are often not looking at a cell that won't burn butyrate. You are looking at a cell that physically cannot — its spark plugs are fouled, its furnace is seized, its membranes are oxidized, and its iron is leaking.

The distinction nobody makes: butyrate production vs. butyrate utilization

Now we can state one of the most important and most overlooked ideas in this entire field. Low butyrate on a stool test and an inability to use butyrate are two completely different problems, and they require opposite interpretations.

A stool report can show low butyrate for the obvious reason: production is low because the butyrate-producing bacteria are depleted. That's a supply problem, and the instinct to feed fiber and reseed Clostridia makes sense.

But in many complex cases, butyrate is present — sometimes even abundant — and the colonocyte still can't use it. The supply is fine. The utilization is broken. The cell lacks the mitochondrial capacity, the redox balance, the intact iron-sulfur clusters, the functioning NAD/FAD machinery, or the electron transport competence to actually oxidize the butyrate sitting right in front of it.

This is why some people feel worse on butyrate supplements, on fiber, on the exact interventions that are supposed to help. Pushing more substrate into a cell that can't process it is like flooding the carburetor of a car whose engine won't turn over. (And there is real biology here — Donohoe and colleagues showed butyrate is the colonocyte's energy substrate, and when it can't be oxidized, it accumulates and changes the cell's behavior entirely.)

If you only read stool butyrate, you cannot tell these two patients apart. One needs more butyrate. The other needs you to fix the engine before you add fuel. Confusing them is one of the most common and consequential mistakes in functional gut work — and it is invisible unless you are thinking in terms of host capacity, not just microbial output.

Hydrogen sulfide: a host clearance problem wearing a microbial costume

Hydrogen sulfide (H₂S) deserves its own section, because it is so badly misunderstood.

H₂S is not a villain. At physiological levels it is a gasotransmitter — a signaling gas your body makes on purpose, involved in blood-vessel tone, mitochondrial signaling, neuromodulation, and mucosal defense. The problem is never the existence of hydrogen sulfide. The problem is always concentration, location, rate of production, and rate of clearance.

On the production side, certain microbes generate H₂S from sulfur substrates — sulfate-reducing bacteria like Desulfovibrio, and bile-tolerant sulfur metabolizers like Bilophila wadsworthia. When these expand, sulfide production rises. This is the part most testing focuses on.

But here is what almost everyone misses: clearing hydrogen sulfide is a mitochondrial job. Your body disposes of H₂S through a dedicated enzymatic unit. SQOR (sulfide quinone oxidoreductase) initiates clearance by oxidizing sulfide and passing its electrons into the CoQ pool — the same coenzyme-Q hub the electron transport chain uses. ETHE1, TST/rhodanese, and SUOX carry the downstream steps that finish converting sulfide into harmless, excretable forms.

Read that again, because it's the whole point: sulfide clearance dumps electrons into the same electron transport chain that's already failing in our model. If the CoQ pool and the ETC are congested — because iron-sulfur clusters are damaged and the chain is stalled — then sulfide oxidation slows down right alongside everything else. Sulfide accumulates. And accumulated H₂S does something vicious: it inhibits Complex IV (cytochrome c oxidase) — the exact enzyme that consumes oxygen at the end of the chain.

So now the colonocyte is being suffocated from two directions at once: its iron-sulfur clusters are wrecked at the front of the chain, and hydrogen sulfide is poisoning the oxygen-consuming step at the back of the chain. This is the H₂S–Complex IV poisoning axis, and it explains why sulfur problems and energy problems travel together.

This reframes sulfur intolerance completely. When someone reacts badly to high-sulfur foods, to NAC, to glutathione, to garlic and eggs and cruciferous vegetables, the reflexive interpretation is "too much sulfur." The Host Capacity Model offers a deeper one: the host has lost the mitochondrial capacity to process sulfur, because the electron flow that clearance depends on is already compromised. It is not that there is too much sulfur. It is that there is too little capacity. Same input, collapsed processing.

Glutathione is not a supply problem. It's a capacity problem.

The same logic — and it is the signature logic of this entire model — applies to glutathione, your master cellular antioxidant.

Low glutathione is almost universally read as a building-block problem: the body lacks the raw materials (cysteine, glycine, glutamate) to make it, so we supply precursors and call it solved. Sometimes that's right. In chronic inflammatory gut dysfunction, it is frequently wrong, and the wrongness matters.

Glutathione can run low for reasons that have nothing to do with how much you're making: it is being consumed too fast — persistent oxidative stress and lipid peroxides relentlessly burn through reduced glutathione; recycling is broken — used (oxidized) glutathione has to be regenerated by an enzyme called glutathione reductase, which requires both NADPH (a cellular reducing-power molecule) and FAD (derived from vitamin B2, riboflavin), and if NADPH or riboflavin run short, the recycling stalls and glutathione "looks low" even when synthesis is fine; NADPH itself is depleted — drained by immune activation, by NADPH-oxidase activity producing superoxide, and by the constant demand to recharge antioxidants; selenium is limiting, so the selenium-dependent glutathione peroxidases that neutralize lipid hydroperoxides can't keep up.

So when glutathione is low, the right question is never just "are we deficient in sulfur amino acids?" It is a capacity question: Can the host recycle glutathione? Can it regenerate NADPH? Can it limit peroxynitrite? Can it stop the lipid peroxidation that's burning glutathione faster than any supplement can replace it?

This is why pouring glutathione or NAC into some patients does nothing — or makes them worse. You are topping up a bucket with a hole punched in the bottom by the very redox collapse we've been describing.

The NAD⁺/CD38 drain: how inflammation steals the cell's energy currency

There is one more leak in the system that ties the whole picture together, and it centers on NAD⁺ — the molecule that carries electrons into the electron transport chain and powers the cell's repair and longevity enzymes (the sirtuins).

In a healthy cell, NAD⁺ is abundant. But chronic inflammation drives up an enzyme called CD38, an NAD⁺-consuming enzyme expressed heavily on activated immune cells. CD38 chews through NAD⁺ at a ferocious rate. As inflammation persists, the NAD⁺ pool drains.

The consequences ripple straight back into everything we've discussed: less NAD⁺ means less fuel for the electron transport chain — exactly when the chain is already crippled by iron-sulfur damage; less NAD⁺ means the sirtuins (NAD⁺-dependent enzymes that maintain mitochondrial health and tune metabolism) go quiet, removing one of the cell's main repair programs; less NAD⁺ deepens the metabolic shift away from oxidative phosphorylation — the very shift that started the oxia-dysbiosis loop.

So inflammation doesn't just damage the machinery; it simultaneously drains the currency the machinery runs on. The CD38/NAD⁺ axis is the financial side of the collapse, while the peroxynitrite/iron-sulfur axis is the structural side. They fail together, and they fail in the same direction.

SLC5A8 silencing: the self-reinforcing lock

Remember SLC5A8, the gene for the SMCT1 transporter that carries butyrate into the colonocyte? It closes one of the cruelest loops in the entire model.

In inflamed and metabolically stressed colonic tissue, SLC5A8 is frequently epigenetically silenced — switched off by chemical marks (DNA methylation) on its promoter, the same way it is silenced in colon cancer. And here's the trap: one of the things that normally keeps SLC5A8 switched on is butyrate itself, acting through its HDAC-inhibitor activity.

Trace the circle: a stressed cell can't use butyrate well, so it gets less of butyrate's gene-regulating, HDAC-inhibiting signal, so SLC5A8 gets silenced, so even less butyrate can be transported into the cell, so the cell uses even less butyrate, and the silencing deepens.

The cell locks the door to its own fuel supply, and butyrate — the molecule that could have kept the door open — can no longer get in to do it. This is a textbook example of why these conditions become self-perpetuating at the molecular level, and why simply raising luminal butyrate doesn't always reach the cell that needs it most.

The hidden regulators: local thyroid signaling and iron trafficking

Two more systems round out the picture, and both reinforce the same theme — what's in the blood is not what's in the tissue.

Local thyroid signaling. We usually judge thyroid status from blood markers — TSH, free T4, free T3. But thyroid hormone is activated and inactivated locally, in tissues, by enzymes called deiodinases. Inflammation upregulates deiodinase 3 (D3), which inactivates thyroid hormone right at the tissue level. The result: a colonocyte can be functionally hypothyroid — its mitochondrial, beta-oxidation, and transporter genes under-stimulated — while the patient's blood thyroid panel reads perfectly normal. Not every gut case is a thyroid case. But systemic blood markers genuinely cannot see local epithelial thyroid suppression, and that blind spot matters.

Iron trafficking. Iron is never simply "high" or "low" — it has to be routed correctly. Chronic inflammation reshapes the whole logistics network: it raises hepcidin (the hormone that locks iron away inside cells by blocking the exporter ferroportin), alters ferritin storage, and drops transferrin saturation. The classic inflammatory signature is high ferritin with low serum iron and low transferrin saturation — iron sequestered, not absent. And here's the paradox that ties back to our core mechanism: while the body looks iron-restricted in the blood, the colonocyte may be drowning in loose, reactive iron internally — exactly the labile iron pool that damaged iron-sulfur clusters keep dumping. Systemically iron-poor, locally iron-toxic. The two can be true at the same time, in the same person, and only a systems view can hold both.

Putting it together: the convergence map

No single marker proves the Host Capacity Model. That's not a weakness — it's the nature of systems biology. The model is built by pattern recognition, by watching independent data streams converge on the same story.

Here is the convergence I'm looking for when I work a complex case: dysbiosis (loss of butyrate-producers, expansion of Proteobacteria and facultative anaerobes, H₂S organisms) points back to epithelial oxygenation, which points back to butyrate oxidation, which points back to mitochondrial electron flow, which points back to iron-sulfur cluster integrity, which points back to peroxynitrite burden, which points back to iNOS, NADPH oxidase, mast cells, LPS, viral reactivation, and inflammatory signaling.

And running in parallel: hydrogen sulfide sits at the intersection of microbial production and mitochondrial clearance; glutathione sits at the intersection of NADPH, FAD/riboflavin, selenium, and lipid peroxidation; NAD⁺ is drained by CD38 even as the chain that needs it is failing; SLC5A8 silencing locks the colonocyte out of its own fuel.

The data I read to test this convergence includes stool / microbiome testing (loss of butyrate-producing ecology, expansion of facultative anaerobes and Proteobacteria-like patterns, sulfide-associated organisms, plus host-inflammation markers — calprotectin, secretory IgA, occult blood, pancreatic elastase, bile-acid clues); organic acids testing (elevations that may flag mitochondrial congestion and TCA bottlenecks — succinate, citrate, cis-aconitate, alpha-ketoglutarate, pyruvate, lactate, hydroxymethylglutarate — indirect and never to be over-read in isolation, but powerful when they line up with stool ecology and blood chemistry); blood chemistry (ferritin, serum iron, transferrin saturation, TIBC; CBC indices, RDW, MCV, MCH, platelets, neutrophil-to-lymphocyte ratio; CRP, ESR; liver enzymes, bilirubin, albumin, uric acid; fasting glucose and insulin; thyroid markers; vitamin D, B12, folate, magnesium, copper, ceruloplasmin, zinc, selenium); and symptom pattern (sulfur intolerance and rotten-egg gas; reactions to NAC, glutathione, or butyrate; worsening with probiotics; histamine and mast-cell reactions; post-infectious or post-viral onset; antibiotic and food-poisoning history; motility pattern; neuroinflammation, fatigue, exercise intolerance, autonomic dysfunction).

When those four streams point at the same underlying picture — impaired colonocyte bioenergetics, altered epithelial oxygen handling, oxidative-nitrosative stress, iron-sulfur vulnerability, sulfide-clearance failure, redox depletion, immune activation, and secondary microbial collapse — that's not a coincidence. That's the host terrain showing itself through every window at once.

What this changes in practice

If the model is right, the entire sequence of care shifts. The standard approach asks, which organisms do I add or remove? The Host Capacity approach asks first, can the host actually sustain the ecosystem I'm trying to rebuild? — and if the answer is no, it works on capacity before, or alongside, working on the bugs.

It means recognizing that pushing butyrate, fiber, NAC, glutathione, or probiotics into a host whose machinery is broken can be useless or actively counterproductive — and knowing which patient is which before you reach for the bottle.

It means treating sulfur intolerance and glutathione depletion as capacity signals to be decoded, not deficiencies to be brute-forced.

It means reading dysbiosis as a message about the terrain, not just a list of targets.

None of this dismisses the microbiome. The microbes are real, they drive inflammation and metabolite imbalance and immune activation, and they absolutely belong in the analysis. But the host decides whether the ecosystem is allowed to recover. If the host cannot hold its oxygen gradient, run its mitochondria, recycle its glutathione, clear its sulfide, and keep its barrier intact, then no amount of seeding will make the garden grow.

The core claim

So here is the whole model in three sentences. Dysbiosis is often the consequence, not the cause. The colonocyte's loss of bioenergetic control — its inability to burn butyrate, consume oxygen, protect its iron-sulfur clusters, clear sulfide, and recycle its antioxidants — is what lets the ecosystem flip and then locks it in place through self-reinforcing loops. The microbiome matters, but the host terrain decides what the microbiome is allowed to become.

The future of chronic gut care cannot only be about killing pathogens, adding probiotics, and raising butyrate. The deeper, harder, more honest question is the one this model is built to ask: Does the host still have the biological capacity to sustain the ecosystem we are trying to rebuild?

That question is the core of the Host Capacity Model. And in my experience with complex cases, it is the question that finally explains the people who never got better when everyone was only ever looking at the bugs.

Key references

Byndloss MX, Olsan EE, Rivera-Chávez F, et al. Microbiota-activated PPAR-γ signaling inhibits dysbiotic Enterobacteriaceae expansion. Science. 2017;357(6351):570–575. doi:10.1126/science.aam9949

Litvak Y, Byndloss MX, Bäumler AJ. Colonocyte metabolism shapes the gut microbiota. Science. 2018;362(6418):eaat9076. doi:10.1126/science.aat9076

Rivera-Chávez F, Zhang LF, Faber F, et al. Depletion of butyrate-producing Clostridia from the gut microbiota drives an aerobic luminal expansion of Salmonella. Cell Host & Microbe. 2016;19(4):443–454.

Kelly CJ, Zheng L, Campbell EL, et al. Crosstalk between microbiota-derived short-chain fatty acids and intestinal epithelial HIF augments tissue barrier function. Cell Host & Microbe. 2015;17(5):662–671. doi:10.1016/j.chom.2015.03.005

Wang RX, Henen MA, Lee JS, Vögeli B, Colgan SP. Microbiota-derived butyrate is an endogenous HIF prolyl hydroxylase inhibitor. Gut Microbes. 2021;13:1938380.

Donohoe DR, Garge N, Zhang X, et al. The microbiome and butyrate regulate energy metabolism and autophagy in the mammalian colon. Cell Metabolism. 2011;13(5):517–526.

Furusawa Y, Obata Y, Fukuda S, et al. Commensal microbe-derived butyrate induces the differentiation of colonic regulatory T cells. Nature. 2013;504(7480):446–450.

Arpaia N, Campbell C, Fan X, et al. Metabolites produced by commensal bacteria promote peripheral regulatory T-cell generation. Nature. 2013;504(7480):451–455.

These foundational papers establish the colonocyte-oxygen-butyrate axis. The Host Capacity Model is my own synthesis that extends this axis into iron-sulfur biology, hydrogen-sulfide clearance, NAD⁺/CD38 dynamics, glutathione capacity, and local tissue regulation as an integrated framework for complex chronic cases.

Mohammed Attallah is the founder of Biomelogic, an independent systems-biology consulting practice. He works as a non-clinician mechanistic case analyst, using the Host Capacity Model to map the upstream terrain driving complex chronic presentations — gut dysfunction, MCAS, SIBO, histamine intolerance, post-viral syndromes, and long COVID. This article is educational and mechanistic; it is not medical advice, diagnosis, or treatment.

If your case has never made sense through the lens of "just fix the bacteria" — that may be exactly the point. Biomelogic consultations include a full case review, a working session, and a written mechanistic summary. Contact: research@biomelogic.net — www.biomelogic.net