Why Your Gut Is Running Your Immune System — and What to Do When It Isn't Working
Every week I see the same patient. She's exhausted in a way sleep doesn't fix, her digestion is never quite right, and she keeps catching whatever goes round. Her bloods come back normal. She's been told to eat well and reduce stress. And she's sitting in front of me saying: "I just feel like I'm missing something." She's right. Here's what it is.
I want to start with a number. 70%. That's the approximate proportion of your immune cells that live in and around your gut. Not in your bloodstream, not in your lymph nodes — in your gut. And I think it's one of the most important numbers in functional medicine, because it changes everything about how you think about symptoms.
When most people think about immune health, they think about not catching colds. About bouncing back quickly from illness. About having a "strong" system that keeps bugs at bay. And those things matter. But they're about 10% of the full picture. Because your immune system isn't just a defence mechanism — it's a regulatory system that touches almost every aspect of how you feel every single day.
And it's running, in large part, from your gut.
The Gut-Immune Axis: What It Actually Is
The scientific term is gut-associated lymphoid tissue — GALT. It's the area in and around your gut lining where immune cells are trained to distinguish between things that belong in your body and things that don't. Friends versus threats. Food versus pathogens. Your own cells versus invaders.
This training is not a one-time event. It's constant, ongoing, and completely dependent on the health of your gut environment. When your gut is functioning well — diverse microbiome, intact lining, low inflammation — your immune system gets calibrated appropriately. It responds to genuine threats and tolerates what it should tolerate.
When your gut is not functioning well, that calibration breaks down.
““You can have a profoundly unhealthy gut and feel it entirely as fatigue, low immunity, joint pain or brain fog — with no obvious gut symptoms at all.””
This is the piece that surprises most of my patients. They assume gut problems come with gut symptoms — bloating, IBS, digestive discomfort. And sometimes they do. But just as often, a gut that is genuinely struggling will announce itself as fatigue after eating, brain fog, skin conditions, multiplying food sensitivities, recurrent infections, or a diagnosed autoimmune condition that nobody has ever connected back to the gut.
The gut doesn't always announce itself loudly. That's what makes it so easy to miss.
The Six Main Disruptors
In clinic, when I look at what's breaking the gut-immune relationship, the same six factors come up repeatedly. Not all of them in every patient — but understanding which ones are in play for you is where useful work begins.
Chronic stress.Cortisol — your primary stress hormone — physically damages the gut lining. It increases intestinal permeability and disrupts the microbiome. Stress doesn't just make you feel worse. It measurably degrades the gut environment.
Antibiotic use.Necessary sometimes, and genuinely disruptive to the microbiome every time. A single course can alter gut bacterial diversity for months if not actively supported during and after.
Ultra-processed foods.These don't just lack nutritional value — they actively feed pathogenic bacteria, promote inflammation, and damage the gut lining. The Western diet is one of the most consistent drivers of gut dysbiosis we have.
Poor sleep.Much of the repair and regeneration of the gut lining happens overnight. Consistently poor sleep quality — not just short sleep — compromises that repair process. The gut-circadian rhythm relationship is real and underappreciated.
Alcohol.Toxic to the gut lining and to the microbiome. Even moderate consumption disrupts the bacterial balance in ways that take time to restore.
NSAIDs taken regularly.Non-steroidal anti-inflammatories like ibuprofen, taken routinely, damage the gut lining. The irony of taking something anti-inflammatory that is itself pro-inflammatory at the gut level is not lost on me.
WHAT I SEE IN CLINIC
The patient I think about most when I talk about this is someone who came to me with Hashimoto's thyroiditis. Her specialist was managing her thyroid medication. Her energy was still terrible. Her immunity was fragile — she was catching everything, recovering slowly.
Nobody had asked about her gut. But when we looked, the picture was clear: significant dysbiosis, markers of increased intestinal permeability, an immune system that had been dysregulated for years. Her gut wasn't separate from her thyroid condition — it was, in all likelihood, part of the story behind how it developed.
We spent six months working on the gut picture. Her energy changed. Her infection rate dropped. Her antibody levels — which her specialist had never expected to move — started coming down.
Her gut was her immune system's problem. And her immune system had been her gut's problem. Neither was being addressed in isolation.
What You Feel When the Relationship Is Under Stress
I want to be specific here, because "gut-immune axis dysfunction" sounds clinical and abstract, and the experience of it is neither. It's exhaustion. It's a brain that won't clear. It's skin that keeps flaring. It's the feeling of a body that's always slightly fighting something you can't identify.
In clinic, the signs that tell me the gut-immune relationship is under strain include:
Fatigue that is disproportionate to what you've done, and that doesn't resolve with rest
Getting every illness that circulates — low-grade immune susceptibility
Slow recovery from infections — taking much longer than other people to fully clear
Food sensitivities that seem to be multiplying over time
Skin conditions — eczema, acne, rosacea, psoriasis — the gut-skin axis is real and well established
Joint pain or stiffness with no clear structural cause
Bloating that nobody can explain, even after dietary changes
Autoimmune conditions that flare with stress, illness, or certain foods
Brain fog — the gut-brain axis means gut inflammation frequently expresses itself as cognitive symptoms
This list matters because it tells you that you don't need to have dramatic digestive symptoms to have a gut that's affecting your immune function. The gut-immune relationship shows up in every system of the body.
What Functional Medicine Actually Does About It
In my practice, when I see these patterns, I use a four-part approach — sometimes called the 4R protocol — that addresses the gut systematically rather than trying single interventions that rarely work in isolation.
Remove the main inflammatory triggers — the dietary factors, food sensitivities, and environmental inputs that are actively driving gut inflammation. This is targeted and individualised, not generic elimination for its own sake.
Replace what's missing — digestive enzymes, stomach acid support, and the nutritional elements needed for gut repair. Many people with gut dysfunction are not producing adequate digestive secretions, and that's a fixable problem.
Reinoculate with beneficial bacteria — diverse dietary fibre, fermented foods (sauerkraut, kimchi, kefir, yoghurt), and targeted probiotic support where appropriate. The microbiome is the gut's working environment, and it needs active cultivation.
Repair the gut lining itself — L-glutamine, zinc carnosine, collagen, omega-3 fatty acids. These are the raw materials the gut lining needs to restore its integrity. You can do all the other steps and still see incomplete results if the lining isn't supported.
This isn't a quick fix. But it is a real one. And unlike most approaches, it addresses the underlying problem rather than managing the symptoms downstream of it.
The Stress Connection — Why You Can't Address the Gut Without Addressing This Too
I want to say something about stress, because I've spent years changing how I talk about it in clinic — and this week's content on Instagram explored this in the context of my own practice.
I used to tell patients to reduce stress. And I meant well. But I hadn't yet understood the full mechanistic picture of what chronic stress actually does to the gut, and therefore to the immune system.
Cortisol — the primary stress hormone — directly increases intestinal permeability. It disrupts the microbiome composition. It suppresses immune regulation. Chronic stress doesn't just make you feel more anxious. It physically degrades the gut environment that your immune system depends on.
This is why when someone comes to me with fatigue, recurrent infections, digestive issues and low mood all at once — and they're clearly under sustained pressure — I no longer treat those as separate concerns. The stress-gut-immune triad is one of the most common patterns I see. And it requires addressing all three corners simultaneously, not picking one to work on while the others continue to pull in the opposite direction.
““The stress-gut-immune triad is the most common pattern I see in clinic. The order of intervention matters as much as the intervention itself.””
Where to Start
If this is resonating, I want to give you something practical rather than leaving you with a lot of information and no clear next step.
The place I recommend starting — before anything else — is understanding your own picture. Not in a vague "eat more vegetables and sleep better" sense. In a specific, functional medicine sense. What are the factors in your particular situation that are most likely driving the symptoms you're experiencing? That's the question that changes everything.
The Optimal You 7 Day Reset is built around this. It's a week-long protocol that addresses gut support, blood sugar stability, inflammation reduction, sleep quality and stress physiology together — not as separate things, but as the interconnected system they actually are. It's where most of my patients start before we do anything more specific, because a grounded baseline week tells us so much about what's going on.
And if you want the full picture — the gut chapter of Optimal You covers the science, the clinical patterns I see, and the framework I use in detail. Because understanding why things are connected is just as important as knowing what to do about them.
Ready to start?
The Optimal You 7 Day Reset is the starting point. One week, built around the functional medicine framework I use in clinic — adapted for you to do at home.
