Revisiting the Four Humors: Why Hippocrates Was Right About the Gut-Brain Axis

Overview

For over two millennia, the foundational pillar of Western medicine was the Theory of the Four Humours. Established by Hippocrates and later expanded by Galen, this system posited that human health, temperament, and disease were dictated by the balance of four distinct bodily fluids: blood (sanguine), phlegm (phlegmatic), yellow bile (choleric), and black bile (melancholic). By the mid-19th century, with the advent of germ theory and the rise of cellular pathology, humoralism was discarded into the dustbin of "primitive" history. It was mocked as a superstitious relic of an era before microscopes and synthetic pharmaceuticals.

However, we at INNERSTANDING believe that the dismissal of Hippocratic wisdom was one of the greatest editorial errors in medical history. We are currently witnessing a seismic shift in the biological sciences—a "neo-humoral" revolution. Modern research into the microbiome-gut-brain axis is revealing that Hippocrates was not merely guessing; he was observing the macro-effects of a micro-biological reality. When Hippocrates famously stated, "All disease begins in the gut," he was providing a blueprint for what we now recognise as the primary driver of systemic health: the internal biochemical environment.

The "Humours" were never just fluids; they were metabolic signatures. Today, we know that the "black bile" associated with melancholia (depression) closely correlates with the toxic metabolites produced by a dysbiotic gut and impaired bile acid metabolism. We know that the "phlegmatic" state of lethargy and respiratory congestion mirrors the systemic inflammation and mucosal hyper-responsiveness triggered by an imbalanced microbiome. The ancient Greeks understood that our "disposition"—our very soul and personality—was a downstream product of our digestion.

This article serves as a comprehensive re-evaluation of this ancient paradigm. We will explore how the vagus nerve, the enteric nervous system (ENS), and trillions of commensal bacteria form a complex regulatory network that dictates everything from your cognitive clarity to your immune resilience. We will expose how modern industrial life has corrupted this internal sea, and why the mainstream medical establishment continues to ignore the gut-origin of psychiatric and chronic illnesses in favour of profitable, symptom-masking drugs.

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The Biology — How It Works

The gut-brain axis is not a metaphorical concept; it is a physical, hard-wired communication network that functions as a bidirectional superhighway. This system integrates the central nervous system (CNS), the neuroendocrine system, and the immune system with the gut microbiota.

The Vagus Nerve: The Great Communicator

At the heart of this connection lies the Vagus Nerve (Cranial Nerve X). Often referred to as the "wandering nerve," the vagus originates in the medulla oblongata of the brainstem and extends down to the lowest reaches of the abdomen. It is the primary component of the parasympathetic nervous system, responsible for the "rest and digest" state. Crucially, approximately 80-90% of the fibres in the vagus nerve are afferent, meaning they carry signals *from* the gut *to* the brain, not the other way around.

The gut is literally telling the brain how to feel. The vagus nerve senses the metabolites produced by bacteria in the intestinal lumen—such as short-chain fatty acids (SCFAs) and neurotransmitters—and translates these chemical signals into electrical impulses that alter brain chemistry in real-time.

The Enteric Nervous System (ENS): The Second Brain

The gut contains its own independent nervous system, the Enteric Nervous System (ENS), comprising over 100 million neurons—more than are found in the spinal cord. The ENS can function entirely independently of the brain, managing the complex mechanics of peristalsis and enzyme secretion. However, its most profound role is as a massive endocrine organ.

The Microbiome as a Virtual Organ

The human microbiome consists of roughly 39 trillion microbial cells, outnumbering human cells in the body. These organisms act as a "virtual organ" that performs metabolic functions we cannot perform ourselves. They ferment dietary fibre into butyrate, a critical SCFA that provides energy to the cells lining the colon (colonocytes) and maintains the integrity of the Blood-Brain Barrier (BBB). When the microbiome is in a state of eubiosis (balance), the humours are harmonious. When it shifts into dysbiosis, the resulting "toxic humours"—in the form of lipopolysaccharides (LPS) and inflammatory cytokines—flood the system, leading to the very states of "melancholia" and "choler" described by the ancients.

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Mechanisms at the Cellular Level

To understand why the gut determines our mental state, we must look at the specific enzymatic and molecular pathways that bridge the gap between the intestine and the neuron.

The Tryptophan-Kynurenine Pathway

Tryptophan is an essential amino acid and the precursor to serotonin. However, in the presence of systemic inflammation or high levels of the stress hormone cortisol, the body shunts tryptophan away from serotonin production and down the Kynurenine Pathway. This is mediated by the enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO).

Instead of producing "feel-good" serotonin, the body produces quinolinic acid, a potent neurotoxin. Quinolinic acid acts as an NMDA receptor agonist, leading to excitotoxicity (the death of neurons due to over-stimulation). This is the cellular mechanism behind what Hippocrates called "black bile"—a physiological state where the internal chemistry turns toxic, manifesting as profound depression, anxiety, and cognitive decline.

Short-Chain Fatty Acids (SCFAs) and Epigenetics

The fermentation of prebiotic fibres by gut bacteria produces SCFAs, primarily acetate, propionate, and butyrate. These molecules are not just fuel; they are signalling molecules.

• —Butyrate is a known Histone Deacetylase (HDAC) inhibitor. This means it has the power to turn on or off specific genes.
• —In the brain, butyrate increases the expression of Brain-Derived Neurotrophic Factor (BDNF), which is essentially "Miracle-Gro" for neurons, promoting neuroplasticity and the growth of new connections.
• —A lack of butyrate-producing bacteria (like *Faecalibacterium prausnitzii*) leads to a "thinning" of the brain’s resilience, echoing the "phlegmatic" lack of vital force described in humoral theory.

The Role of Lipopolysaccharides (LPS)

LPS are large molecules found in the outer membrane of Gram-negative bacteria. In a healthy gut with tight junctions, LPS remains in the intestinal tract and is excreted. However, when the gut barrier is compromised—a condition known as intestinal permeability or "Leaky Gut"—LPS leaks into the bloodstream.

Once in the blood, LPS triggers a systemic immune response by binding to Toll-Like Receptor 4 (TLR4) on immune cells. This initiates a "cytokine storm," producing inflammatory markers like Interleukin-6 (IL-6) and Tumour Necrosis Factor-alpha (TNF-α). These cytokines can cross the blood-brain barrier and "set the brain on fire," leading to neuroinflammation, brain fog, and the irritability characteristic of a "choleric" temperament.

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Environmental Threats and Biological Disruptors

The modern world is an assault on the delicate humoral balance of the gut. We are living in a sea of synthetic compounds that did not exist during the time of Hippocrates, and our biological systems are struggling to adapt.

Glyphosate: The Microbiome Destroyer

Perhaps the most pervasive threat in the UK and globally is glyphosate, the active ingredient in many broad-spectrum herbicides. While the agrochemical industry claims glyphosate is safe for humans because we lack the shikimate pathway (which glyphosate targets in plants), they omit a critical fact: our gut bacteria *do* have the shikimate pathway.

Glyphosate acts as a potent antibiotic, selectively killing beneficial bacteria like *Lactobacillus* while allowing pathogenic strains like *Clostridia* to flourish. This disruption of the "internal garden" leads to a collapse of neurotransmitter production and the chronic "leaking" of toxins into the systemic circulation.

Ultra-Processed Foods (UPFs) and Emulsifiers

The modern British diet is heavily reliant on ultra-processed foods. These products contain emulsifiers such as carboxymethylcellulose and polysorbate 80. Research has shown that these chemicals act like "detergent" for the gut, thinning the protective mucus layer that prevents bacteria from coming into direct contact with the intestinal wall. When this mucus barrier fails, the result is chronic low-grade inflammation—the modern equivalent of an "imbalance of the humours."

Heavy Metals and Microplastics

The UK’s industrial legacy has left our soil and water contaminated with heavy metals like lead, mercury, and cadmium. These metals are potent neurotoxins and gut disruptors. Furthermore, the Environment Agency has recently highlighted the staggering levels of microplastics in UK waterways. These microplastics serve as "Trojan horses," absorbing persistent organic pollutants (POPs) and delivering them directly to the gut lining, where they interfere with hormonal signalling and microbial diversity.

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The Cascade: From Exposure to Disease

The progression from environmental exposure to chronic disease is a predictable biological cascade. It begins in the gut but ends in the breakdown of the entire organism.

Phase 1: Dysbiosis and Barrier Breach

The process starts with the loss of microbial diversity due to antibiotics, poor diet, or toxins. This leads to the overgrowth of opportunistic pathogens. These pathogens produce metabolic byproducts that degrade the mucosal barrier. The protein complexes that hold intestinal cells together—zonulin and occludin—are dismantled, creating gaps in the "fortress wall" of the gut.

Phase 2: Systemic Inflammation (Metabolic Endotoxaemia)

As LPS and undigested food particles enter the bloodstream, the liver becomes overwhelmed. The liver, which the ancients associated with the production of bile (yellow and black), is the body’s primary filter. When the liver cannot keep up with the "toxic humours" flooding in from the gut, it enters a state of oxidative stress. This triggers a systemic inflammatory response, or metabolic endotoxaemia.

Phase 3: The "Leaky Brain"

The blood-brain barrier (BBB) is designed to protect the most sensitive organ from toxins. However, the same inflammatory cytokines (IL-6, TNF-α) that opened the gut barrier also act on the BBB. Once the BBB is "leaky," toxins and inflammatory cells enter the brain parenchyma.

Phase 4: Chronic Neurodegeneration and Mental Illness

In the brain, these triggers activate microglia—the brain’s resident immune cells. Chronic microglial activation leads to the destruction of healthy neurons and the inhibition of neurogenesis. This is the biological reality behind the "Four Humours" gone wrong:

• —Chronic Inflammation = The Choleric (Aggressive/Anxious) state.
• —Tryptophan Depletion/Quinolinic Acid = The Melancholic (Depressive) state.
• —Hypometabolism/Mitochondrial Dysfunction = The Phlegmatic (Lethargic) state.

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What the Mainstream Narrative Omits

The UK’s current medical model, largely dictated by the MHRA (Medicines and Healthcare products Regulatory Agency) and the NHS, remains fixated on the "Central Nervous System" as an isolated entity. This is a reductionist error of catastrophic proportions.

The Myth of the "Chemical Imbalance"

For decades, patients have been told that depression is simply a "chemical imbalance" of serotonin in the brain, requiring SSRIs (Selective Serotonin Reuptake Inhibitors). However, this narrative fails to ask *why* the serotonin is low in the first place. By ignoring the gut as the primary site of serotonin production, the mainstream narrative sells a "top-down" solution to a "bottom-up" problem. SSRIs do nothing to address the gut dysbiosis or neuroinflammation that caused the depletion of neurotransmitters.

The Suppression of Nutritional Psychiatry

There is a wealth of evidence showing that dietary intervention can be more effective than pharmacology for mental health. The SMILES Trial, for instance, demonstrated that a modified Mediterranean diet significantly reduced clinical depression. Yet, the NHS continues to prioritise pharmaceutical intervention over nutritional and environmental medicine. Why? Because you cannot patent sauerkraut or sunlight.

The Failure to Regulate Environmental Toxins

The mainstream narrative also omits the role of environmental toxicity. The Environment Agency and the FSA often operate on "acceptable limits" for toxins like glyphosate or fluoride. However, these limits do not account for the synergistic toxicity—the "cocktail effect"—of being exposed to hundreds of different chemicals simultaneously, all of which converge on the gut microbiome.

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The UK Context

In the United Kingdom, we face a unique set of challenges regarding the gut-brain axis. From the quality of our tap water to the structure of our healthcare system, the "humoral balance" of the British public is under siege.

Water Fluoridation and Chlorine

Large swathes of the UK have fluoridated water, and almost all UK tap water is treated with chlorine. While chlorine is necessary to kill waterborne pathogens, it does not stop being an antimicrobial once you swallow it. Chronic exposure to chlorinated water acts as a "micro-dose" antibiotic, constantly pruning the beneficial bacteria in the gut. Fluoride, meanwhile, has been linked to the calcification of the pineal gland and interference with thyroid function, further disrupting the endocrine component of the gut-brain axis.

The "Stiff Upper Lip" and the Vagus Nerve

British culture has historically valued the "stiff upper lip"—the suppression of emotion and the maintenance of high stress levels. From a biological perspective, chronic stress (high cortisol) is the "off switch" for the Vagus nerve. When the UK population lives in a state of constant sympathetic dominance (fight or flight), their "rest and digest" system atrophies. This leads to the high prevalence of Irritable Bowel Syndrome (IBS) and Small Intestinal Bacterial Overgrowth (SIBO) seen across the country.

The Crisis in UK Soil Quality

Post-war intensive farming in the UK has led to a depletion of essential minerals in our soil, particularly magnesium, selenium, and zinc. Magnesium is a co-factor for over 300 enzymatic reactions, including those that synthesise neurotransmitters. Selenium is critical for gut mucosal integrity. If the minerals aren't in the soil, they aren't in the food; and if they aren't in the food, our "humours" cannot be synthesised.

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Protective Measures and Recovery Protocols

If Hippocrates was right and all disease begins in the gut, then all healing must also begin there. Restoring the balance of the humours requires a comprehensive approach to "weeding, seeding, and feeding" the internal environment.

1. Eliminate the Disruptors

The first step in any recovery protocol is the removal of biological insults.

• —Filter Your Water: Use high-quality filtration (such as reverse osmosis or gravity-fed systems with fluoride-reduction filters) to remove chlorine, fluoride, and heavy metals.
• —Go Organic Where Possible: To avoid glyphosate and other pesticides, prioritise organic produce, especially for the "Dirty Dozen" (crops with the highest pesticide residues).
• —Avoid Emulsifiers: Read labels meticulously. Avoid anything containing carboxymethylcellulose, polysorbate 80, or carrageenan.

2. Restore Microbial Diversity (Seeding)

We must re-introduce the beneficial "humours" (microbes) that modern life has stripped away.

• —Fermented Foods: Traditional British foods like real ale and unpasteurised cheeses were once sources of probiotics. Today, we must look to kefir, unpasteurised sauerkraut, and kimchi. These provide live strains of *Lactobacillus* and *Bifidobacterium*.
• —Soil-Based Organisms (SBOs): Our ancestors were exposed to "dirt." Taking SBO probiotics (like *Bacillus coagulans*) can help re-educate the immune system.

3. Provide the Raw Materials (Feeding)

• —Prebiotic Fibre: Feed the butyrate-producers with diverse fibres like inulin (found in chicory root and garlic), resistant starch (cooked and cooled potatoes/rice), and polyphenols (dark berries, green tea).
• —Bone Broth: Rich in glycine, proline, and glutamine, bone broth is essential for repairing the "leaky" intestinal lining and supporting the synthesis of collagen.

4. Vagus Nerve Stimulation (VNS)

To reconnect the gut and the brain, we must tone the Vagus nerve.

• —Cold Exposure: Splashing the face with ice-cold water or taking cold showers stimulates the Vagus nerve and reduces systemic inflammation.
• —Diaphragmatic Breathing: Slow, deep breathing (5 seconds in, 5 seconds out) signals to the brain that the body is safe, allowing the ENS to function optimally.
• —Humming and Gårgling: The Vagus nerve passes through the vocal cords; the vibration of humming or vigorous gargling can physically stimulate the nerve.

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Summary: Key Takeaways

The ancient Greeks did not have the tools to see the microbiome, but they had the intuition to see its effects. The "Four Humours" were a sophisticated metaphorical framework for understanding the internal biochemical environment. As we move further into the 21st century, it is becoming clear that the future of medicine lies in this ancient past.

• —The Gut is the Primary Driver: Mental health is not "all in your head"; it is "all in your gut." The brain is often the final organ to manifest symptoms of a systemic digestive crisis.
• —The Vagus Nerve is the Bridge: Communication between the gut and brain is constant and largely driven by the gut's state.
• —Inflammation is the "Black Bile": Modern "melancholia" (depression) is frequently a result of neuroinflammation triggered by a leaky gut and toxic metabolites like quinolinic acid.
• —Environmental Toxins are Real: Glyphosate, UPFs, and heavy metals are the primary "pollutants" of the modern humours, and they must be actively avoided.
• —The Solution is Biological, Not Just Chemical: True health comes from restoring the ecological balance of the microbiome and the structural integrity of the gut barrier.

At INNERSTANDING, we urge you to stop viewing your body as a collection of isolated parts and start seeing it as an integrated ecosystem. Hippocrates was right: to heal the mind, you must first heal the gut. The "primitive" ancients may have known more about our biological reality than the modern pharmaceutical complex ever will. It is time to return to the source.