The Gut-Immune Axis (Part 1): What Medical School Got Wrong About Your Gut

You’ve been told to take your vitamins. Get enough sleep. But what if one of the most important systems in your body — your immune system — is more connected to your gut than anyone ever told you?

The science is extraordinary. It is also still evolving — and that matters. This series will tell you what the research genuinely supports, where the evidence is promising but incomplete, and where popular wellness claims have run ahead of the science. Because knowing the difference is exactly what good medicine looks like.

”My blood tests are normal. So why do I feel like this?”

I see patients like Sarah every week.

She sat in my clinic looking exhausted. She was catching every cold that went through her office. She felt bloated after meals, her skin was acting up, and a persistent brain fog made her 3 PM meetings feel impossible.

She had seen three doctors before me. They had run the standard blood panels — thyroid, iron, vitamin D, inflammatory markers. Everything came back “normal.”

“You’re just stressed,” she was told. “Try to get more rest.”

Sarah wasn’t making it up. Her symptoms were real. But her previous doctors were looking in the wrong place. They were looking at her blood, at her thyroid, at her stress levels. Nobody had asked her about her gut.

For decades, medicine treated the gut like a sophisticated plumbing system. Input, process, output. That was essentially it.

But what science now shows is that Sarah’s symptoms — the frequent colds, the fatigue, the skin issues — were not separate problems. They were different alarms going off in the same building. And the control room for that building? Her gut microbiome.

This is not a story of medicine getting everything wrong. It is a story of science doing what it is supposed to do — discovering new things and updating its understanding.

The plumbing myth: what they taught us in medical school

I went through years of medical training. And when it came to the gut, the teaching was remarkably simple.

The gut digests food. It absorbs nutrients. Anything the body cannot use gets excreted.

There was no real focus on the trillions of bacteria living in our intestines. No mention of how the gut’s lining might be in constant communication with the brain. So I am not exaggerating when I say medicine is still catching up here.

What We Were Taught	What Research Now Suggests
The gut digests food and excretes waste	The gut houses a large proportion of the body’s immune cells — well established [1]
Bacteria are pathogens to be eliminated	Gut bacteria are essential partners in immunity and metabolism — well established [2]
Serotonin is purely a brain chemical	Most of the body’s serotonin is made in the gut — well established [3]
Mental health is a brain-only problem	Gut-brain communication is real and bidirectional — established; clinical implications still being studied [4]
Skin and energy issues are unrelated to the gut	Associations exist, but causation in humans is not yet fully established, and skin conditions have multiple contributing factors [5]

This is not a story of medicine getting everything wrong. It is a story of science doing what it is supposed to do — discovering new things and updating its understanding.

You are more bacteria than human

Let’s start with a fact that tends to stop people mid-sentence.

You carry approximately 38 trillion bacteria inside your body — roughly comparable to the number of your own human cells. This community of microorganisms is called your gut microbiome, and it is as unique to you as your fingerprint.

For a long time, we thought these bacteria were just passengers hitching a ride. We now know they are active participants in your health. They help train your immune system from birth, produce compounds that repair your gut lining, and communicate with your brain via the vagus nerve.

What is important to understand, though, is that the microbiome is not a single, uniform thing. It is an ecosystem of hundreds of different species, and the relationships between those species, your diet, your immune system, and your health are genuinely complex. The science is advancing rapidly, but it would be misleading to suggest we have a complete picture yet.

The 70% fact that changes everything

If you only remember one thing from this article, make it this:

A large proportion — estimated at 70 to 80 percent — of the body’s immune cells are located in and around the gut wall. [6]

This system has a name: the Gut-Associated Lymphoid Tissue, or GALT. It is the largest immune structure in the human body, and most people have never heard of it.

The GALT contains T-cells, B-cells, macrophages, and a group called ILC3s (Innate Lymphoid Cells type 3) — cells that help regulate the immune response in the gut, deciding what is a harmless food particle and what is a genuine threat. [6] When this system works well, your immune response is calibrated and effective. When it is disrupted, the immune system can become overactive or misdirected — a pattern seen in inflammatory bowel disease, food allergies, and some autoimmune conditions.

Your gut wall: the thinnest line of defence

The barrier separating your bloodstream from the bacteria in your gut is, in most places, just one cell thick. A single layer of epithelial cells, held together by tight junctions, selectively absorbs nutrients while keeping bacteria and toxins out.

When that barrier is damaged — by chronic stress, ultra-processed food, alcohol, or certain medications — those tight junctions can loosen. This increased intestinal permeability (sometimes called “leaky gut”) has been observed in association with conditions including inflammatory bowel disease, type 2 diabetes, and some autoimmune conditions. [7]

It is worth being precise here: intestinal permeability is a real, measurable phenomenon in specific clinical conditions. However, the popular wellness concept of “leaky gut” as a single root cause of all chronic illness is an oversimplification. The research shows associations, and the causal relationships — whether permeability causes these conditions or results from them — are still being worked out. What is clear is that a healthy gut lining matters, and that diet and lifestyle have a meaningful influence on it.

Serotonin and the gut: a nuanced picture

You may have read that “90 percent of serotonin is made in the gut.” This is broadly true — the majority of the body’s serotonin is indeed produced by specialised cells in the gut lining, called enterochromaffin cells, in response to the bacteria living alongside them. [3]

But this fact deserves important context.

Gut serotonin and brain serotonin are not interchangeable in function, and this distinction matters. Gut serotonin primarily regulates intestinal movement — coordinating the muscular contractions that move food through the digestive tract. It does not cross the blood-brain barrier in significant amounts, and it does not directly influence mood in the way that brain serotonin does. The two pools of serotonin are largely separate systems.

The connection between gut health and mental health is real, but it is indirect and considerably more complex than the “gut makes serotonin, therefore gut fixes mood” shorthand that circulates widely online. What gut bacteria actually do is influence the production of precursor molecules and signalling compounds that modulate the brain’s own serotonin pathways — via the vagus nerve, the immune system, and other routes. The gut-brain axis is a genuine, bidirectional communication network. But it is a network of signals, not a simple pipeline. [4]

What this means practically: supporting a healthy gut microbiome is plausibly associated with better mood regulation, and there is a growing body of research exploring this relationship. But the clinical evidence for gut interventions as treatments for depression, anxiety, or other mental health conditions is still preliminary. It would be an overstatement — and potentially harmful — to suggest that improving your diet will resolve mental health conditions that require proper clinical assessment and treatment.

What we can say with confidence — and what needs more research

In the spirit of honest, science-backed communication, here is a clear-eyed summary of where the evidence stands:

Claim	Evidence Level
The gut houses most of the body’s immune cells (GALT)	Well established [1]
Gut bacteria are essential for immune system development	Well established [2]
Diet diversity increases microbiome diversity	Well established [8]
Fermented foods increase microbiome diversity	Well established (Stanford, 2021) [9]
Gut serotonin production is influenced by bacteria	Well established [3]
Gut health influences mood and mental health	Promising and growing — not yet clinically proven for most conditions [4]
Gut interventions improve brain fog or skin conditions	Plausible associations — causation not yet firmly established [5]
“Leaky gut” as a root cause of all chronic illness	Oversimplified — permeability is real in specific conditions, but not a universal explanation

This is not a reason to dismiss gut health. It is a reason to engage with it thoughtfully — which is exactly what The Soulful GP is here to help you do.

5 signs your gut may need attention

These are not diagnostic criteria — always speak to your doctor about any health concerns. But these are signs that research associates with gut microbiome disruption, and they are worth paying attention to:

1. You get sick frequently. Frequent infections may reflect suboptimal immune function. The gut-immune relationship is well established, but frequent illness has many causes — so speak to a doctor to exclude other issues. [1]

2. You feel bloated regularly. Persistent bloating after meals can indicate bacterial imbalance, food intolerances, or motility issues. As one of the more directly gut-related symptoms on this list, persistent bloating is worth a conversation with your GP. [7]

3. You experience brain fog. Some research associates microbiome disruption with cognitive symptoms like poor concentration and mental fatigue. The science is still developing — this is an area of active study, not settled fact. [5]

4. Your skin is reactive. Associations between gut microbiome disruption and conditions like eczema and acne are an active research literature. However, these are associations — the causal mechanisms in humans are not yet fully established, and skin conditions have many contributing factors. [5]

5. Your energy is consistently low. Persistent fatigue is one of the most non-specific symptoms in medicine. Gut inflammation is one possible contributing factor, but low energy can also reflect anaemia, thyroid conditions, sleep disorders, depression, and many other causes. Always speak to your doctor before attributing persistent fatigue to gut health alone.

Coming up next: what you can actually do

The most empowering part of this science is that your microbiome is not fixed. Unlike your human DNA, it changes every day based on what you eat, how you sleep, and how you manage stress.

In Part 2 of this series, we look at exactly what the evidence says you can do to support your gut — with honest guidance on what works, what does not, and where the science still needs more research.

This article is for educational purposes only and does not constitute medical advice. Always consult your GP or healthcare provider regarding any health concerns.

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