Everything you need to know about the gut-brain axis

There are many factors in our lives that affect how we feel, friendships, relationships, work, hobbies, even the weather. The list is endless. But one area a lot of people forget is the gut. Simply put, if your gut is experiencing any sort of dysbiosis then that can and often is reflected in your mood. Brain fog, fatigue, low mood, an inability to focus, irritability, these are all signs the gut is struggling, and when the gut is impacted the brain gets affected.

Now it is important to note that the gut is one of many aspects of health that can influence mood. Hormone health sits right alongside it, and remembering that everything is linked in the body is key. But the gut-brain connection is one of the most powerful and most overlooked. 

Let's dive in.

How the gut speaks to the brain

Here's something most people don't realise... your gut sends more information to your brain than your brain sends back. This isn't a one-way street, it's a full communication network running around the clock. But the gut is doing most of the talking.

• The most direct line is the vagus nerve, a long cranial nerve that runs from the brainstem all the way down to the gut. Roughly 80% of its fibres are afferent, meaning they carry information upward. Your gut is constantly reporting on the state of digestion, inflammation, and microbial activity, and the brain is listening to every bit of it. When vagal tone is healthy, you feel calm, grounded, and emotionally resilient. When it's impaired, often because the gut is inflamed or under stress, the nervous system loses that baseline of safety and tips more easily into anxiety or low mood.
• Beyond the vagus nerve, the gut communicates through neurotransmitters produced in the gut wall. Around 90% of the body's serotonin is made in the gut, not in the brain. The gut also produces dopamine, GABA, and other signalling molecules that influence how we feel, how motivated we are, and how well we handle stress. These compounds don't all cross the blood-brain barrier directly, but they influence brain chemistry through immune signalling, vagal tone, and hormonal cascades.
• Then there are immune messengers, cytokines, released when the gut detects a threat. These circulate through the bloodstream and can activate inflammatory pathways in the brain itself. Hormones produced in the gut, like ghrelin and peptide YY, further modulate appetite, energy, and mood. And on top of all of this, your gut bacteria produce their own metabolic by-products, short-chain fatty acids, gases, and other compounds, that feed directly into this signalling web.

Your microbiome is not just bacteria, it is an ecosystem interpreting the outside world. Food, microbes, stress, it takes all of it in and translates it into chemical signals. If that ecosystem is diverse in beneficial bacteria and stable, it sends signals of safety to the brain. If it's imbalanced, it sends signals of threat. When it's compromised, your energetic boundary feels compromised too, anxiety, hypervigilance, emotional fragility, even how you carry yourself socially is affected. The gut is not just processing food. It is one of the largest sensory organs in the body, and it has a direct line to the brain.

What goes wrong

When the gut barrier is compromised, what we often refer to as leaky gut, the tight junctions between intestinal cells loosen, allowing substances that should stay inside the digestive tract to cross into the bloodstream. This includes partially digested food particles, bacterial fragments, and toxins. The immune system recognises these as threats and mounts a response, creating chronic low-grade inflammation that ripples through the entire body, brain included.

• Bacterial, fungal, or parasitic overgrowths make this worse. These organisms compete for nutrients, disrupt bile and digestion, and release metabolites that irritate the nervous system. Many produce compounds that directly increase anxiety, irritability, fatigue, and depressive symptoms. In these cases, mood issues are a downstream effect of chronic immune activation rather than psychological stress alone.
• When the gut is inflamed, signalling becomes impaired. Even "normal" blood serotonin levels don't guarantee proper brain signalling if gut inflammation is present.
• Endotoxins place the body in a constant low-grade stress response. When the gut barrier is compromised, these toxins enter circulation and trigger inflammatory and adrenaline-driven signalling in the brain. The result can feel like persistent anxiety, low resilience, emotional flatness, or burnout. Reducing endotoxin load often leads to rapid improvements in mood and calmness.
• Poor digestion also destabilises blood sugar, and this is a big one. When glucose dips, the body compensates with adrenaline and cortisol spikes that feel like anxiety or irritability. Impaired digestion increases lactic acid and stress respiration, lowering CO₂ tolerance and pushing the brain into a threat state. This is one reason people can feel anxious without any obvious emotional trigger, the gut is driving the experience, not the circumstances.

Your microbiome as a garden

Thinking of your microbiome as a garden is a great way to envision what you want from your gut. A thriving garden has a wide variety of plants, good quality soil, and nothing invasive choking out the things you actually want growing. Your gut works the same way.

If you have an invasive plant in the garden it will crowd out the healthy ones, stealing their supply of nutrients from the soil. The same goes for the soil itself, if it doesn't get the food it needs from decomposing organic matter then it cannot provide those nutrients back to the plants. Everything depends on everything else. Pull one thread and the whole system shifts.

In the gut, the "soil" is your intestinal lining and the environment it provides, and the "plants" are the trillions of bacteria, fungi, and other microorganisms that live there. When the soil is healthy and the ecosystem is diverse, the garden flourishes. When it's depleted of nutrients or overrun, things start to break down. And just like a real garden, it takes time and consistent care to rebuild. You cannot rush it. You have to feed the soil, remove the weeds, and create the conditions for the right things to grow back.

Diversity of beneficial gut bacteria is what we are aiming for, alongside a sealed gut, free of overgrowths or pathogens and frequent bowel movements, at least once a day.

A healthy, diverse microbiome helps regulate mood by producing calming compounds, supporting neurotransmitter synthesis, and keeping inflammation low. Balanced gut bacteria generate short-chain fatty acids that fuel brain cells and stabilise the nervous system, while also modulating histamine and stress responses. When the microbiome is disrupted, signalling shifts toward inflammation and overstimulation, often presenting as anxiety, low mood, or emotional volatility rather than obvious digestive symptoms. You can have a disrupted microbiome and not feel it in your stomach at all, you feel it in your head.

Here's where it gets really interesting... certain gut microbes influence reward pathways, food preferences, risk tolerance, and even social behaviour through dopamine and serotonin modulation. Research increasingly suggests that mood and personality traits can shift when microbial populations change. This is why gut healing sometimes feels like a return to self with baseline neurochemistry is being restored. People often describe it as feeling like themselves again for the first time in years, and that is not a placebo. That is biology.

Brain fog is often a gut issue

Brain fog is frequently gut-driven rather than cognitive. Impaired digestion, microbial imbalance, and inflammation reduce nutrient delivery to the brain and increase inflammatory signalling that slows neural processing. Endotoxins and unstable blood sugar further impair attention and memory.

A lot of people experiencing brain fog assume something is wrong with their brain, they worry about cognitive decline, they question their intelligence, they feel frustrated that they cannot concentrate the way they used to. But in many cases the brain is fine. It is just not getting what it needs. The fuel is inconsistent, the inflammation is high, and the signalling environment is noisy. When gut inflammation falls and metabolic fuel becomes consistent, mental clarity, focus, and motivation often return without any direct cognitive intervention. The brain was capable all along, it was the gut holding it back.

Anxiety and depression linked to the gut?

Anxiety and depression are often downstream expressions of gut dysbiosis rather than primary brain disorders. When digestion is impaired, the gut signals threat through inflammation, endotoxins, blood sugar instability, and disrupted neurotransmitter production. The nervous system picks up on these signals and shifts into a state of chronic stress or shutdown. It is doing exactly what it is designed to do, responding to danger. The problem is that the danger is coming from inside the body.

Endotoxins and inflammatory cytokines activate brain pathways associated with fear, low motivation, and emotional withdrawal. Poor absorption of key nutrients limits serotonin, dopamine, and GABA signalling, the very chemicals that keep us calm, motivated, and emotionally stable. Without adequate raw materials, the brain simply cannot produce what it needs to function well.

From an evolutionary perspective this response is protective. The brain reduces risk-taking and energy output when the gut detects danger. It pulls you inward, makes you less social, less motivated, more cautious. In a survival context that makes sense, if the body is fighting something internally, conserving energy and avoiding risk is a smart move. But in modern life it just feels like depression.

The anxiety side works similarly. When the gut is inflamed and endotoxins are circulating, the body stays in a heightened state of alert. Adrenaline runs higher, cortisol stays elevated, and the nervous system becomes hyper-reactive to stimuli that wouldn't normally bother you. Small things feel overwhelming. Social situations feel draining. Sleep becomes difficult because the body doesn't feel safe enough to fully switch off. People often chase the psychological explanation for this, they look for the thought pattern or the life event that triggered it, when in reality the trigger is physiological. The gut is sounding an alarm and the brain is responding accordingly.

This is not to say that every case of anxiety or depression is purely gut-driven. Life events, trauma, and hormones all play a role. But for a significant number of people, the gut is the missing piece. When gut inflammation falls and microbiome balance improves, anxiety often softens and depressive symptoms lift. Not because the mind was "fixed," but because the original gut-derived stress signal was removed. The brain was never broken, it was responding to a body under stress.

How to lift the fog

Improving mood through gut health starts with reducing inflammation, supporting digestion, and stabilising blood sugar. Prioritise regular meals with adequate protein, carbohydrates, and Fats, focusing on organic whole foods as much as possible. Turning to foods such as bone broth and bone in meats which are rich in collagen and gelatine to help seal the gut.

Support stomach acid and bile flow, ensure good mineral and electrolyte intake, and avoid unnecessary gut irritants such as alcohol, ultra processed foods, pesticide sprayed foods and excessive grain intake. Address overgrowths carefully, lower endotoxin burden, and focus on nourishment before restriction.

In many cases, calming the gut is enough to calm the mind. As gut health improves, inflammation falls, blood sugar steadies, vagal tone increases, and mood often lifts before digestion itself fully normalises.

Occasional, intentional fasting such as 24 hours once a month can support gut rest and healing but frequent intermittent or prolonged fasting often worsens mood, metabolic health. Regular fasting raises stress hormones. For many people, consistent nourishment supports gut repair and emotional stability far more effectively than daily restriction. Turning to occasional fasts only once the body has the nutrients it needs to do the healing it needs to. Forcing a nutrient deprived body to fast regular can backfire.

For a full gut healing roadmap, we wrote a guide. 

The bottom line

Your gut is not just digesting food, it is shaping how you think, how you feel, and how you show up in the world. The conventional approach wants to treat mood as a brain problem and hand you a prescription. But for a significant number of people, the answer isn't in their head. It's in their gut.

The people who see the biggest shifts are the ones who take a comprehensive approach: reducing inflammation, healing the gut barrier, restoring microbial diversity, stabilising blood sugar, and giving their body consistent nourishment instead of restriction. This isn't complicated, but it does require patience. Fix the gut, feed the brain, and give the body what it needs. The mood will often follow.