Rethinking Chronic Dysbiosis, Mast Cell Activation, and the Failure of Intestinal Ecological Control

I have been away from social media and have not written on Substack or Facebook for almost three weeks.

That absence was not because I had stopped working on the microbiome, mast cell activation, chronic gut dysfunction, or the other subjects I normally discuss. It was because I have been working more deeply than usual.

I have been spending a significant amount of time reviewing complex client cases, comparing recurring patterns across different clinical presentations, and continuing to develop what I call the Host Capacity Model.

I have also been using Google Gemini and several other advanced artificial intelligence models to challenge the framework from different angles.

I want to be precise about what that means.

I am not using artificial intelligence as a scientific authority. AI can rapidly connect pathways, identify possible interactions, and generate mechanistic hypotheses. It can also produce explanations that sound highly sophisticated while quietly exceeding the evidence, confusing one enzyme with another, misapplying findings from one tissue to another, or converting an interesting animal experiment into an unjustified clinical conclusion.

I have therefore been using AI as a pressure-testing tool.

I ask it to challenge the model, search for missing metabolic bottlenecks, identify contradictory pathways, and generate alternative explanations. I then separate the resulting ideas into four categories: established physiology, mechanistic inferences supported by experimental evidence, plausible but unproven hypotheses, and speculation that should not yet be treated as clinically meaningful.

That distinction is essential.

I am not returning to say that I have discovered a universal cause of SIBO, dysbiosis, IBS, mast cell activation, food intolerance, inflammatory bowel disease, or chronic postinfectious illness.

I have not developed a universal treatment.

I do not believe that one organism, one metabolite, one receptor, one nutrient, or one mitochondrial enzyme can explain every case.

What I am developing is a framework for asking a different question.

For years, much of microbiome medicine has revolved around the question: Which organisms are elevated, and how can we suppress them?

The question behind the Host Capacity Model is different:

What changed in the host environment that allowed those organisms to expand, persist, and repeatedly return?

That shift sounds subtle, but it changes the entire way we interpret chronic dysbiosis.

1. The Microbiome Does Not Exist Independently of the Host

A stool test can show increased Proteobacteria, reduced obligate anaerobes, altered short-chain fatty acids, elevated opportunistic organisms, reduced diversity, or an increase in organisms associated with histamine, hydrogen sulfide, lactate, methane, or other metabolites.

Those findings may be important.

But they are often interpreted as though the microbes independently decided to reorganize the ecosystem.

They did not.

Every microbial ecosystem is constrained by the environment in which it lives.

The organisms that expand are determined partly by which nutrients are available, which gases are present, which electron acceptors can be used for respiration, how quickly intestinal contents move, how acidic or alkaline the environment is, how much mucus is produced, which bile acids reach the area, how much secretory IgA is present, and whether the immune system is tolerating or attacking the community.

The microbiome is therefore not merely a list of organisms.

It is an ecological output of the relationship among microbial metabolism, epithelial metabolism, immune signaling, intestinal motility, nervous-system activity, mucus production, bile-acid chemistry, diet, vascular oxygen delivery, and tissue repair.

The Host Capacity Model begins with the idea that chronic dysbiosis may sometimes be downstream of a partial loss of host ecological control.

The host may no longer be maintaining the biochemical conditions that once favored a stable, predominantly anaerobic, barrier-supporting microbial community.

This does not make the microbes irrelevant.

It means that the microbial pattern may be both a cause of disease and a consequence of the environment created by the host.

2. What I Mean by Host Capacity

Host capacity is not one laboratory marker.

It is the combined ability of intestinal tissue to receive microbial products, process them appropriately, maintain energy production, control local oxygen availability, regulate inflammatory chemistry, preserve the mucus barrier, renew the epithelium, and restore homeostasis after injury.

A host with adequate intestinal capacity must be able to perform several tasks simultaneously.

The epithelium must absorb short-chain fatty acids and other microbial metabolites. Colonocytes must transport those molecules into the cell and direct them toward the appropriate metabolic pathways. Mitochondria must accept the resulting carbon and electrons without developing excessive reductive pressure or oxidative injury. The respiratory chain must continue transferring electrons toward oxygen. The epithelial surface must consume enough oxygen to preserve the low-oxygen environment required by obligate anaerobes.

The tissue must regulate nitric oxide production so that inflammation does not continuously generate nitrate and related respiratory substrates for facultative organisms. Goblet cells must produce and organize mucus. Secretory lineages must generate antimicrobial peptides and signaling molecules. Stem cells must renew the appropriate balance of absorptive and secretory cells. Tight junctions must control paracellular exposure. Mast cells, macrophages, lymphocytes, sensory neurons, and epithelial cells must communicate without entering a chronic amplification loop.

Host capacity therefore describes the ability of the intestine to function as an ecological regulator.

The central problem in some chronic cases may not be that the host has completely lost one pathway. The problem may be that several pathways have become marginal at the same time.

A modest loss of butyrate oxidation, a modest increase in epithelial oxygenation, a modest increase in nitrate, a modest reduction in mucus, and a modest increase in mast-cell reactivity may interact to create a major change in the ecosystem.

Systems biology is often defined by these nonlinear interactions.

3. The Colonocyte Is Not a Passive Wall

The intestinal epithelium is often described primarily as a barrier.

That description is incomplete.

A mature colonocyte is also a metabolic governor of the microbial ecosystem.

Oxygen reaches the intestinal tissue through the circulation beneath the epithelium. If that oxygen were allowed to diffuse freely into the lumen, the colon would no longer support the same anaerobic microbial community.

Mature colonocytes prevent much of this from happening by actively consuming oxygen.

In the healthy colon, butyrate produced by microbial fermentation is taken up and metabolized by differentiated colonocytes. Butyrate supports a PPARγ-dependent oxidative program that increases fatty-acid oxidation and epithelial oxygen consumption.

This produces what is often called physiological epithelial hypoxia.

The term can be misleading. It does not mean that the entire intestine is pathologically deprived of oxygen. It means that the epithelial surface maintains a controlled low-oxygen state because oxygen is being actively consumed before it can diffuse into the lumen.

This metabolic oxygen sink is an important component of colonization resistance.

Experimental models have shown that depletion of butyrate-producing organisms can reduce epithelial PPARγ signaling, increase epithelial iNOS expression, increase luminal nitrate, increase oxygen availability, and permit the expansion of facultative Enterobacteriaceae.

These organisms can use oxygen and nitrate for respiration, giving them an energetic advantage over organisms restricted primarily to fermentation.

This means that oxygen and nitrate are not merely inflammatory byproducts.

They are ecological resources.

A shift in host metabolism can change which microbial strategies are energetically profitable.

4. Butyrate Is Not Simply a Supplement or a Stool Measurement

Butyrate is commonly discussed in an overly simplified way.

Low butyrate is considered bad. High butyrate is considered good. More butyrate is assumed to be beneficial.

Butyrate biology is more complex because its effects depend on which cell receives it, whether that cell can metabolize it, where the cell is located, and whether epithelial architecture remains intact.

For butyrate to support colonocyte oxygen consumption, several events must occur successfully.

Microbes must first produce it from fermentable substrates. The butyrate must reach the epithelial surface. It must cross the apical membrane through monocarboxylate transport systems or passive diffusion according to the local pH gradient. It must then enter the metabolic machinery of the cell.

After activation to butyryl-CoA, short-chain acyl-CoA dehydrogenase begins its oxidation. Electrons then move through electron-transfer flavoprotein and ETF dehydrogenase into the coenzyme Q pool. The carbon skeleton is progressively converted into acetyl-CoA, which can enter the tricarboxylic-acid cycle.

Electrons transferred through NADH, FAD-linked reactions, the coenzyme Q pool, Complex III, cytochrome c, and Complex IV ultimately allow oxygen to be reduced to water.

The entire pathway matters.

The presence of butyrate does not prove that butyrate is being oxidized.

The presence of butyrate-producing organisms does not prove that the host is metabolically using their product.

A high fecal butyrate measurement could represent robust microbial production. It could also represent reduced absorption, impaired utilization, rapid transit, altered water content, or a mismatch between production and consumption.

A low fecal measurement could represent inadequate production. It could also represent efficient uptake, substrate scarcity, abnormal sampling, or altered transit.

Concentration is not the same as flux.

This distinction is fundamental to the Host Capacity Model.

5. The Butyrate Effect Depends on Cellular Identity

Butyrate is a fuel for mature colonocytes, but it is not simply a fuel for every epithelial cell.

Mature colonocytes located near the luminal surface possess the oxidative machinery required to consume significant quantities of butyrate.

Stem and progenitor cells located deeper within the crypt have different metabolic priorities.

Research has shown that differentiated colonocytes normally form a metabolic barrier that consumes butyrate before excessive quantities reach the stem-cell niche. When crypt architecture is disrupted or differentiated colonocytes cannot metabolize butyrate adequately, butyrate can reach the proliferative compartment and suppress stem and progenitor-cell proliferation through a FOXO3-dependent mechanism.

This produces an important principle:

A molecule cannot be classified as universally beneficial or harmful without defining the receiving cell and the metabolic state of that cell.

Butyrate reaching a metabolically competent mature colonocyte can support oxidative metabolism and ecological stability.

The same molecule reaching an exposed stem-cell compartment after mucosal injury can restrain proliferation and delay repair.

The problem is not that butyrate suddenly becomes inherently toxic.

The problem is that tissue architecture and metabolic compartmentalization have changed.

This may help explain why the same microbiome-focused strategy can help one person and worsen another.

6. The Butyrate, Oxygen, and PPARγ Loop

One of the strongest sections of the Host Capacity Model is the relationship among microbial butyrate, colonocyte metabolism, epithelial oxygen consumption, PPARγ signaling, iNOS regulation, nitrate availability, and microbial respiration.

In a healthy state, fermentable substrates are metabolized by anaerobic organisms, producing butyrate. The butyrate is absorbed by colonocytes and oxidized in mitochondria. This increases epithelial oxygen consumption, helps preserve physiological hypoxia, restricts facultative respiration, supports obligate anaerobic communities, and promotes continued fermentation and butyrate production.

This is not simply a metabolic pathway.

It is a host-microbial feedback system.

The dysfunctional loop may move in the opposite direction.

Loss of fermenters or impaired colonocyte oxidation can reduce butyrate signaling and utilization. Epithelial oxygen consumption falls. More oxygen reaches the mucosal surface. Inflammatory signaling increases iNOS and nitrate production. Facultative organisms gain a respiratory advantage. Microbial metabolites and inflammatory exposure change, further suppressing epithelial metabolism.

Once established, the ecosystem may continue reproducing the conditions that maintain it.

This is one reason why direct suppression of an organism may produce temporary improvement without lasting ecological stability.

If the organism is removed but the environmental advantage remains, another organism with a similar metabolic strategy may occupy the same niche.

7. Dysbiosis May Sometimes Be a Problem of Respiratory Opportunity

Microbiome discussions usually focus on taxonomy.

They ask which species increased and which species decreased.

The Host Capacity Model adds a thermodynamic question:

Which organisms have access to the most energetically favorable form of metabolism in the current environment?

An obligate anaerobe relying primarily on fermentation extracts relatively limited energy from a substrate.

A facultative organism capable of using oxygen or nitrate as a terminal electron acceptor can generate considerably more energy from available carbon.

When inflammation increases nitrate and epithelial dysfunction increases oxygen availability, the host unintentionally creates a more profitable metabolic environment for organisms capable of respiration.

The expansion of Enterobacteriaceae can therefore be interpreted not only as an increase in potentially inflammatory organisms, but also as a biological signal that respiratory electron acceptors may have become more available.

This does not mean every increase in Enterobacteriaceae is caused by colonocyte failure.

Transit, infection, antibiotics, diet, immune deficiency, medications, bile acids, and other variables can contribute.

The point is that microbial expansion cannot be interpreted independently of the energetic environment.

8. Hydrogen Sulfide Is Physiological Until Capacity Is Exceeded

Hydrogen sulfide is another molecule that is often placed into a simplistic good-or-bad category.

Hydrogen sulfide is produced by microbial metabolism and by host enzymes. At controlled concentrations, it participates in normal signaling and can be oxidized by mitochondria.

Colonocytes possess a specialized sulfide-oxidation pathway.

Sulfide oxidoreductase, commonly abbreviated SQOR or SQR, initiates mitochondrial sulfide oxidation by transferring electrons from sulfide toward the coenzyme Q pool. Downstream enzymes further process the sulfur intermediates toward thiosulfate and sulfate.

This system allows the colonocyte to detoxify a molecule that would otherwise interfere with respiration.

The problem arises when sulfide exposure exceeds the oxidation capacity of the tissue.

High sulfide exposure can inhibit cytochrome c oxidase at Complex IV, reducing the ability of the respiratory chain to consume oxygen.

Experimental studies have also found that sulfide can impair colonocyte butyrate oxidation at the short-chain acyl-CoA dehydrogenase step, producing accumulation of butyryl-CoA and reducing downstream metabolites.

These findings were produced under experimental conditions and cannot be converted directly into a universal human clinical model, but they establish biological plausibility for sulfide-induced impairment of butyrate metabolism.

The most defensible version of the hypothesis is therefore this:

In a susceptible host with limited epithelial sulfide oxidation and respiratory capacity, excessive sulfide flux may contribute to impaired butyrate oxidation, reduced oxygen consumption, and loss of epithelial ecological control.

This is a phenotype-specific hypothesis.

It does not mean that every sulfur-containing food is harmful.

It does not mean that every sulfide-producing organism should be eliminated.

It does not mean that hydrogen sulfide is inherently pathological.

The relevant variables are dose, location, host clearance capacity, mitochondrial state, and the balance between sulfide production and oxidation.

9. A Correction to the Earlier Iron-Sulfur Cluster Hypothesis

One reason I have taken time away from publishing is that the model needed scientific correction in several areas.

One example involves iron-sulfur clusters.

Earlier AI-generated versions of the model described both SQOR and succinate dehydrogenase as iron-sulfur proteins.

That is not accurate.

SQOR is a flavin-dependent quinone oxidoreductase with covalently associated FAD and catalytic cysteine chemistry. It is not primarily an iron-sulfur cluster enzyme.

Succinate dehydrogenase, or mitochondrial Complex II, is different.

Its SDHA subunit contains FAD, while SDHB contains three iron-sulfur clusters that transfer electrons from the flavin toward the quinone-binding site.

Iron-sulfur cluster integrity may therefore matter greatly for Complex II function and for other mitochondrial enzymes, but it should not be inaccurately applied to every component of sulfide oxidation.

This distinction matters because sophisticated-sounding inaccuracies can corrupt the entire logic of a model.

The goal is not to create the most elaborate explanation.

The goal is to create the most defensible explanation.

10. Succinate May Be a Metabolic Junction, Not a Universal Master Cause

Succinate has become an important component of my thinking, but it must be handled cautiously.

Succinate can originate from host mitochondrial metabolism or microbial metabolism.

It can remain inside mitochondria, accumulate in the cytosol, enter the extracellular space, appear in the intestinal lumen, or participate in microbial cross-feeding.

Its meaning depends on its source and location.

Inside the mitochondrion, succinate is normally oxidized to fumarate by succinate dehydrogenase, which is also Complex II of the respiratory chain.

If Complex II flux is impaired, if the tricarboxylic-acid cycle is altered, or if inflammatory metabolism changes the direction and use of the pathway, succinate can accumulate.

Inside the cytosol or nucleus, elevated succinate can interfere with enzymes that require α-ketoglutarate.

Outside the cell, succinate can function as a signaling ligand through the succinate receptor SUCNR1.

In the intestinal lumen, succinate may serve as a microbial metabolite, a cross-feeding intermediate, or a signal detected by specialized epithelial cells.

These are not equivalent compartments.

A fecal succinate measurement does not tell us the concentration inside an intestinal stem cell.

A urinary succinate measurement does not directly measure colonocyte Complex II activity.

A plasma measurement does not reveal what is occurring at the mucosal surface.

The model must preserve those compartmental boundaries.

11. Succinate and the Tuft-Cell Type 2 Immune Circuit

One experimentally supported connection involves tuft cells.

Tuft cells are rare chemosensory epithelial cells that can detect luminal signals and communicate with the immune system.

In the mouse small intestine, luminal succinate can activate SUCNR1 on tuft cells. This initiates canonical taste-signaling machinery involving phospholipase C, intracellular calcium signaling, TRPM5 activity, and the release of interleukin 25.

Interleukin 25 activates group 2 innate lymphoid cells, which produce interleukin 13.

Interleukin 13 then acts on epithelial progenitors, expanding tuft-cell and goblet-cell programs and amplifying type 2 immunity.

This is an elegant sensory circuit.

It may help the host respond to helminths, protists, and particular microbial conditions.

But it must not be generalized beyond the evidence.

The strongest data involve small-intestinal tuft cells and type 2 immunity in animal models.

They do not prove that luminal succinate is the universal driver of human mast cell activation syndrome.

They do not prove that every elevation in succinate will activate this pathway.

They do not prove that reducing succinate would resolve a mast-cell phenotype.

Succinate signaling can also be protective depending on the infectious and anatomical context.

The current Host Capacity Model therefore treats the succinate–tuft-cell pathway as one possible bridge between microbial metabolism and type 2 immune remodeling, not as a universal explanation for MCAS.

12. Butyrate and Succinate May Regulate the Same Immune Circuit in Opposite Directions

Recent research adds another layer of complexity.

Butyrate-producing microbes and butyrate itself have been shown to restrain tuft-cell differentiation and type 2 immune activation through an epithelial HDAC3-dependent mechanism.

In experimental models and human organoids, butyrate limited expansion of the tuft-cell program, while loss of this microbial influence increased succinate-driven tuft-cell and ILC2 responses.

This suggests that the immune consequences of microbial metabolism may depend partly on the balance between metabolites.

Succinate may signal that a particular luminal ecological condition is present.

Butyrate may limit excessive expansion of the corresponding sensory and type 2 immune program.

This is more sophisticated than describing one metabolite as inflammatory and another as anti-inflammatory.

The system behaves more like a rheostat.

Relative concentration, anatomical location, cellular receptor expression, epithelial state, and the surrounding cytokine environment determine the response.

13. The α-Ketoglutarate and Succinate Balance May Connect Metabolism to Chromatin

α-Ketoglutarate is not only a tricarboxylic-acid-cycle intermediate.

It is also an essential cofactor for a large family of dioxygenase enzymes.

These include TET DNA-modifying enzymes and Jumonji-domain histone demethylases.

These enzymes use α-ketoglutarate, oxygen, and catalytic iron to modify DNA or histone marks.

Succinate is produced as part of their reaction and can also compete with α-ketoglutarate when it accumulates.

This creates a potential connection between mitochondrial metabolic state and chromatin regulation.

The important concept is not that a single succinate-to-α-ketoglutarate ratio controls every gene.

The important concept is that metabolic flux can influence the availability of cofactors and competitive metabolites used by chromatin-modifying enzymes.

Metabolism can therefore influence which transcriptional programs remain accessible.

This may help explain how an inflammatory event can continue affecting epithelial behavior after the initiating trigger has disappeared.

14. Inflammation Can Leave a Metabolic and Epigenetic Memory

A 2024 study examined intestinal stem cells after inflammatory injury in models of gastrointestinal graft-versus-host disease.

The researchers found that inflammatory exposure changed intestinal stem-cell metabolism, caused succinate accumulation, altered the epigenome, and reduced the later capacity of those stem cells to differentiate and regenerate.

Importantly, some of these changes persisted after the original inflammatory exposure had resolved.

This supports a concept that may be highly relevant to chronic illness:

Removing the original trigger does not necessarily return the tissue immediately to its original biological state.

An infection may be gone.

An antibiotic course may have ended.

A dietary exposure may have stopped.

The acute inflammatory signal may have declined.

But the stem-cell compartment may retain altered metabolic and chromatin states that change how the tissue responds to the next challenge.

This does not mean that the tissue is permanently locked.

Epigenetic regulation is dynamic.

It means that tissue history matters.

15. Human Intestinal Stem Cells Can Retain Inflammatory Memory

The concept of epithelial memory is now supported by emerging human data.

A 2026 study developed organoids from previously inflamed and noninflamed intestinal regions from the same patients with ulcerative colitis.

Even after long-term culture outside the original immune and inflammatory environment, organoids derived from previously inflamed tissue retained distinct chromatin-accessibility patterns.

When challenged again, those organoids demonstrated heightened transcriptional responses and faster wound closure, but also reduced clonogenic capacity and impaired barrier characteristics.

The epithelial stem-cell compartment had retained a memory of prior inflammation.

The interpretation is nuanced.

This memory may not be entirely pathological.

A tissue that has survived a previous inflammatory injury may become primed to respond more rapidly to a future insult.

That adaptation could improve immediate wound closure.

But the same primed state may reduce regenerative flexibility, impair barrier quality, or increase the probability of exaggerated responses and relapse.

An adaptation that is useful during acute danger can become costly when it remains active chronically.

16. Metabolism May Help Determine Which Epithelial Cells Are Produced

The intestinal epithelium is continuously renewed.

Stem cells generate several specialized lineages.

Absorptive enterocytes and colonocytes process nutrients and microbial metabolites. Goblet cells produce mucus. Paneth cells support the stem-cell niche and contribute antimicrobial factors, particularly in the small intestine. Enteroendocrine cells release metabolic and neuroactive signals. Tuft cells sense luminal chemistry and coordinate immune responses.

A healthy barrier requires the correct balance of these cell types.

A major development in 2025 showed that mitochondrial metabolism helps influence this lineage decision.

The study found that OGDH, the enzyme that metabolizes α-ketoglutarate toward succinyl-CoA, is expressed differently across absorptive and secretory lineages.

Absorptive cells expressed more OGDH under the regulation of HNF4 transcription factors. This supported the energetic and biosynthetic requirements of absorptive cells while consuming α-ketoglutarate.

Secretory progenitors expressed less OGDH, allowing α-ketoglutarate to accumulate.

The increased α-ketoglutarate supported TET-dependent increases in 5-hydroxymethylcytosine and promoted differentiation toward secretory lineages.

In mouse models and intestinal organoids, OGDH suppression or cell-permeable α-ketoglutarate shifted differentiation toward goblet-cell and Paneth-cell programs.

However, complete OGDH suppression was harmful to absorptive cells, demonstrating that the objective is not simply to minimize OGDH activity.

Different lineages have different metabolic requirements.

This finding has significantly expanded my model.

I was originally focused primarily on whether mature colonocytes could oxidize butyrate and maintain epithelial hypoxia.

I am now equally interested in whether the stem-cell niche is regenerating the appropriate cellular architecture required to maintain that metabolism.

17. Mitochondria Are Not Identical, Even Within the Same Stem-Cell Population

Another 2025 study showed that intestinal stem cells can asymmetrically inherit mitochondria of different chronological ages.

A subset of stem cells enriched in older mitochondria produced more α-ketoglutarate and demonstrated a greater ability to regenerate the Paneth-cell niche.

The α-ketoglutarate signal promoted TET-dependent epigenetic changes that supported Paneth-cell formation.

This is important because it challenges another oversimplification.

Older mitochondria are not automatically dysfunctional.

New mitochondria are not automatically superior.

Mitochondrial age, redox state, metabolic output, cellular context, and quality control can create functionally distinct organelle populations.

The relevant question is not simply whether mitochondria are old or young.

The question is what metabolic information those mitochondria are delivering to the cell.

18. Lineage Imbalance May Reproduce Barrier Weakness After Every Turnover

The epithelial surface is replaced continuously.

If the stem and progenitor compartment repeatedly produces an abnormal balance of cell types, the barrier may remain vulnerable even after superficial inflammation improves.

A relative deficiency in mature goblet cells could reduce mucus production and organization.

A reduction in appropriate Paneth-cell or secretory functions could weaken antimicrobial compartmentalization.

An abnormal tuft-cell program could alter type 2 immune sensing.

An inadequate population of metabolically competent mature colonocytes could reduce butyrate oxidation and oxygen consumption.

In that situation, each epithelial turnover may recreate part of the dysfunctional state.

This is still a developing hypothesis.

The OGDH and α-ketoglutarate studies were largely performed in mouse models and organoids, with supporting observations in human inflammatory tissue.

They do not prove that every person with SIBO, IBS, MCAS, food intolerance, or postinfectious symptoms has a secretory-lineage defect.

They do, however, establish that intermediary metabolism can participate directly in cell-fate decisions.

That is a major conceptual change.

19. Mast-Cell Activation Cannot Be Reduced to Histamine

My work with complex clients has also reinforced that mast-cell activation should not be reduced to histamine alone.

Histamine is important, but mast cells can release or generate many other mediators, including tryptase, chymase, prostaglandins, leukotrienes, cytokines, chemokines, growth factors, and other signaling molecules.

Each mediator has different receptors, tissue targets, clearance pathways, and time courses.

Mast cells sit at a strategic anatomical interface.

They are positioned near epithelial surfaces, sensory nerves, blood vessels, smooth muscle, lymphatic structures, and immune cells.

This allows them to integrate information from several systems simultaneously.

They can respond to immunoglobulin signaling, complement, cytokines, neuropeptides, microbial products, epithelial danger signals, mechanical stimuli, hormones, and metabolic conditions.

Once activated, they can influence vascular permeability, sensory signaling, smooth-muscle contraction, intestinal secretion, epithelial junctions, immune recruitment, and motility.

Mast-cell activation is therefore better understood as a network event than as a single-mediator excess.

20. Mast Cells Can Directly Weaken Epithelial Containment

Experimental work has shown that mast-cell tryptase can activate protease-activated receptor 2 on colonocytes.

This signaling increases intracellular calcium, activates ERK-related pathways, reorganizes perijunctional actin, redistributes ZO-1 and occludin, lowers transepithelial resistance, and increases paracellular permeability.

This creates a biologically plausible amplification loop.

Epithelial injury increases microbial and dietary antigen exposure. This can increase mast-cell activation. Mast-cell proteases and inflammatory mediators can further disrupt epithelial junctions, increasing exposure again.

The mast cell may therefore be both a sensor of barrier failure and a contributor to its persistence.

That does not establish mast cells as the primary cause in every patient.

It means that once the loop begins, identifying the original starting point may become difficult.

21. The Nervous System Is Part of the Same Circuit

The intestine is densely innervated.

Sensory neurons, enteric neurons, autonomic signaling, epithelial cells, immune cells, and mast cells participate in continuous bidirectional communication.

Neural stress signaling can influence epithelial permeability and mast-cell activity.

Mast-cell mediators can activate or sensitize enteric and sensory neurons.

Those neurons can then alter motility, secretion, vascular tone, pain perception, and further immune signaling.

This can generate a neuroimmune feedback loop in which the original mucosal event is no longer the only factor maintaining symptoms.

A person may begin with infection or dysbiosis.

Over time, altered motility, visceral hypersensitivity, autonomic dysfunction, conditioned immune responses, and mast-cell activation may become independent contributors.

This is why the Host Capacity Model cannot be exclusively mitochondrial.

Mitochondria may be one important regulatory layer, but the full model must include microbial ecology, epithelial biology, immune signaling, vascular function, and the nervous system.

22. The Self-Reinforcing Loop I Am Currently Investigating

The current working model can be summarized as a network rather than a single linear pathway.

An initiating stress may include infection, inflammation, antibiotics, altered bile acids, dietary disruption, ischemia, chronic stress signaling, toxic exposure, immune activation, reduced motility, or another injury.

That stress changes epithelial metabolism.

Colonocytes may reduce oxidative substrate use and rely more heavily on glycolytic or inflammatory metabolic programs.

Epithelial oxygen consumption falls.

More oxygen becomes available near the mucosal surface.

Inflammatory signaling increases iNOS and the production of nitrate-related electron acceptors.

Facultative organisms gain a respiratory advantage.

The microbial community reorganizes.

Butyrate production, butyrate utilization, lactate cross-feeding, sulfide flux, succinate dynamics, bile-acid metabolism, and other metabolite pathways change.

The altered metabolites and microbial products increase epithelial, immune, mast-cell, and neural signaling.

Barrier integrity becomes less stable.

Stem-cell metabolism and lineage allocation may change.

The regenerated epithelium may contain fewer metabolically competent or secretory cells.

Reduced host capacity then recreates the ecological conditions that sustain dysbiosis.

The cycle may therefore move from initiating injury to epithelial metabolic suppression, reduced oxygen consumption, greater oxygen and nitrate availability, facultative microbial expansion, altered metabolite flux, immune and mast-cell amplification, barrier and regenerative dysfunction, and then further loss of epithelial metabolism.

Different people may enter this loop at different points.

For one person, the initial driver may be infectious.

For another, it may be immune-mediated.

For another, motility may be primary.

For another, bile-acid disruption may be the central factor.

For another, mitochondrial vulnerability may precede the dysbiosis.

For another, dysbiosis may produce the mitochondrial stress.

Similar symptoms do not prove identical mechanisms.

23. Why the Same Strategy Can Help One Person and Worsen Another

A recurring problem in microbiome medicine is that a strategy that helps one person can cause substantial worsening in another.

This is often attributed vaguely to individual sensitivity.

The Host Capacity Model tries to define what that sensitivity may represent biologically.

The response to a fermentable substrate depends on which microbes receive it and whether downstream cross-feeding organisms are present.

The response to butyrate depends on whether mature colonocytes can use it and whether crypt architecture is intact.

The response to sulfur depends on microbial sulfur metabolism and the host’s capacity to oxidize sulfide.

The response to histamine depends on production, degradation, receptor sensitivity, barrier exposure, mast-cell activity, and hepatic and intestinal clearance.

The response to a microbial organism depends on the oxygen environment, immune state, bile acids, mucus, transit time, and the existing ecological network.

The intervention is not entering an empty system.

It is entering a dynamic biological state.

A substance can therefore be appropriate in one metabolic state and destabilizing in another.

This is one reason I am cautious when symptom worsening is automatically described as die-off.

Worsening could represent increased fermentation, altered motility, osmotic stress, metabolite accumulation, mast-cell activation, epithelial exposure, excessive substrate delivery, drug intolerance, or another adverse response.

Without evidence, die-off is often a story rather than a mechanism.

24. The Model Requires Distinct Phenotypes

The Host Capacity Model will not be useful if it becomes a universal explanation applied identically to every person.

I am currently developing the framework around distinct mechanistic phenotypes.

One phenotype may be dominated by loss of butyrate oxidation and increased epithelial oxygenation.

Another may involve high sulfide flux and limited sulfide-oxidation capacity.

Another may involve inflammatory succinate accumulation and persistent stem-cell memory.

Another may be dominated by type 2 immune signaling, tuft-cell activity, and mast-cell amplification.

Another may involve mucus and secretory-lineage failure.

Another may be driven predominantly by transit impairment and repeated small-intestinal fermentation.

Another may involve bile-acid signaling, antimicrobial effects, and altered fat digestion.

Another may be primarily postinfectious, with persistent neural and immune sensitization after the original pathogen has disappeared.

These phenotypes can overlap.

The purpose of the model is not to force every patient into one category.

The purpose is to identify which loops are most likely to be maintaining the individual system.

25. What I Am Actively Working on Now

The first area I am developing is a more complete map of colonocyte bioenergetics.

This includes butyrate transport, activation to butyryl-CoA, ACADS activity, electron transfer through ETF and ETF dehydrogenase, coenzyme Q redox balance, Complex II behavior, Complex IV inhibition, mitochondrial membrane integrity, NAD⁺ and NADH balance, and the regulation of mitochondrial quality.

The second area is the sulfide and redox interface.

I am trying to distinguish physiological sulfide signaling from excessive sulfide exposure, microbial production from host production, luminal burden from intracellular exposure, and normal SQOR-driven oxidation from respiratory inhibition.

The third area is the succinate and α-ketoglutarate axis.

This includes Complex II flux, microbial succinate production, SUCNR1 signaling, TET activity, chromatin regulation, stem-cell memory, OGDH expression, and epithelial-lineage allocation.

The fourth area is epithelial oxygen ecology.

I am examining how colonocyte oxygen consumption, PPARγ, iNOS, nitrate, HIF signaling, and facultative microbial respiration interact.

The fifth area is the mast-cell and epithelial interface.

This includes tryptase, PAR2, leukotrienes, prostaglandins, cytokines, epithelial alarmins, type 2 immune circuits, sensory nerves, vascular signaling, and motility.

The sixth area is measurement.

The major challenge is determining how these mechanisms can be assessed in humans without pretending that indirect markers provide more certainty than they actually do.

26. The Limitations of Current Clinical Testing

Most clinical testing measures static outputs.

A stool test may detect microbial DNA, inflammatory proteins, digestive markers, antimicrobial-resistance genes, or selected metabolites.

It usually cannot directly measure colonocyte oxygen consumption.

It cannot directly measure mitochondrial membrane potential in the intestinal epithelium.

It cannot determine whether butyrate is being oxidized.

It cannot measure oxygen tension at the mucosal surface.

It cannot reveal the NAD⁺-to-NADH ratio inside colonocyte mitochondria.

It cannot determine whether Complex II is operating normally in a specific epithelial compartment.

It cannot measure the α-ketoglutarate concentration inside an intestinal stem cell.

It cannot directly determine TET activity or chromatin accessibility.

It cannot show real-time cross-feeding among organisms.

A urinary organic-acids result showing succinate is not a direct measurement of colonocyte succinate.

A fecal butyrate value is not a direct measurement of butyrate oxidation.

A stool abundance of a mucus-associated organism is not a direct measurement of mucus thickness.

A normal plasma histamine level does not exclude tissue mast-cell activation.

A low-diversity score does not identify the mechanism that reduced diversity.

We must stop asking tests to answer questions they were not designed to answer.

27. What the Host Capacity Model Does Not Claim

The model does not claim that microbes are unimportant.

It does not claim that infections or overgrowths are imaginary.

It does not claim that every case begins with mitochondrial dysfunction.

It does not claim that succinate is the universal cause of mast-cell activation.

It does not claim that hydrogen sulfide is always harmful.

It does not claim that butyrate is always beneficial.

It does not claim that α-ketoglutarate supplementation can reproduce the cell-specific metabolic conditions observed in experimental models.

It does not claim that one protocol can restore every dysfunctional pathway.

It does not replace diagnosis.

It does not establish MCAS in every person with food reactivity, flushing, fatigue, brain fog, abdominal symptoms, or autonomic complaints.

It does not convert findings from mouse colitis into guaranteed human treatment responses.

The model is a map of interacting mechanisms.

It is not yet a complete clinical decision system.

28. What Would Make the Model Scientifically Useful

A model is only useful if it can be tested and potentially proven wrong.

The Host Capacity Model predicts that at least some forms of persistent dysbiosis should correlate with altered epithelial metabolism and increased availability of respiratory electron acceptors.

It predicts that microbial communities may change after host metabolic recovery, even without direct targeting of every organism.

It predicts that fecal metabolite concentrations will sometimes diverge from mucosal utilization.

It predicts that previously inflamed epithelial tissue may respond differently even after the initial inflammatory trigger has resolved.

It predicts that different mechanistic phenotypes will respond differently to the same microbial substrate or signal.

It predicts that restoration of a stable microbial ecosystem will require restoration of epithelial containment, oxygen control, and regenerative competence in at least a subset of patients.

The model would be weakened if human studies showed that epithelial oxidative metabolism remains normal across persistent dysbiosis.

It would be weakened if mucosal oxygen and nitrate availability had no meaningful relationship with facultative microbial expansion.

It would be weakened if stem-cell inflammatory memory were restricted to very narrow experimental conditions.

It would be weakened if improving host epithelial metabolism did not alter microbial ecology or barrier behavior.

Those possibilities must remain open.

29. Why I Have Been Quiet

I did not want to continue writing simply to remain visible.

The more complex cases I see, the less comfortable I become with simple explanations.

“Kill the bacteria” is not a complete model.

“Feed the good bacteria” is not a complete model.

“Lower histamine” is not a complete model.

“Repair the gut” is not a complete model.

“Support mitochondria” is not a complete model.

Each may describe one legitimate part of the system.

The difficult work is identifying which layer is upstream, which is compensatory, which is maintaining the cycle, and which apparent abnormality is actually an attempt by the body to protect itself.

That is what I have been working on.

I do not have every answer.

I do not believe anyone currently does.

But I believe we can ask more precise questions.

Instead of asking only what is living in the intestine, we should also ask what kind of environment the intestine is creating.

Instead of asking only how much butyrate is present, we should ask whether it is being transported, metabolized, and spatially contained.

Instead of asking only whether succinate is elevated, we should ask where it came from, where it accumulated, which cells can sense it, and whether it reflects host metabolism, microbial metabolism, or both.

Instead of asking only whether mast cells are activated, we should ask which signals are repeatedly reaching them and whether epithelial containment has failed.

Instead of asking only which organisms should be removed, we should ask why the current ecosystem rewards their metabolic strategy.

The microbiome is not merely a collection of passengers living inside the body.

The host constructs the habitat.

It controls the oxygen gradient.

It supplies or restricts electron acceptors.

It metabolizes microbial products.

It produces mucus.

It regulates exposure.

It renews the epithelial surface.

It trains and restrains the immune response.

When those functions are intact, the ecosystem has a greater probability of stabilizing itself.

When those functions deteriorate, dysbiosis may repeatedly return even after the organisms temporarily change.

This is the central direction of the Host Capacity Model:

Persistent dysbiosis may sometimes reflect not only the presence of undesirable organisms, but a loss of the host’s capacity to create and maintain a healthy intestinal habitat.

The organisms are real.

The inflammation is real.

The mast-cell activation is real.

The mitochondrial dysfunction may be real.

But none of these systems operates alone.

The next stage of my work is to determine how these layers interact, how the major phenotypes differ, which assumptions survive scientific pressure testing, and which parts of the model must be abandoned or revised.

I am not presenting a cure.

I am building a more complete map.

After stepping away to work on it, I am ready to begin sharing that map