THE GUT–BRAIN CONNECTION

By now you’ve probably heard that “90% of your serotonin is made in your gut,” and if you’re anything like me and interested in optimising your state of joy, not just your physical wellbeing, you have probably gone down a bit of a rabbit hole with that one.

Because surely if your gut is responsible for that much serotonin, it has to matter for how you feel?

Yes, it’s true that 90% of the body’s serotonin is produced in the gut, done so largely by specialised cells in the digestive tract. But gut serotonin does not simply travel up into your brain and make you happy.

Serotonin is not quite the whole story

The gut and the brain are separated by the blood-brain barrier, so it is not quite as simple as eating something, producing more serotonin in your bowel and suddenly feeling better.

The real story is more interesting…

Your gut microbiome influences tryptophan metabolism. Tryptophan is an essential amino acid and a building block involved in serotonin production. Your gut microbes, immune system, stress load and overall gut environment can all influence where that tryptophan gets directed and what messages get sent back to the brain (PMID: 25078296; PMID: 33804088).

So when I say your gut health can impact your mood, we are talking about a real biological system involving immune signals, inflammation, stress hormones, microbial metabolites, nutrient status and the way your nervous system reads safety.

Honestly, I find it so fascinating, and it goes deeper than this… it’s not just about serotonin production. The bacterial strains we find in our gut can influence our mood and mental health too.

Irish researchers found a specific probiotic strain called Bifidobacterium longum 1714, which is in [Gutsi Gut Feelings](INSERT AFFILIATE LINK).

In a small double-blind, placebo-controlled crossover study, healthy adults took this specific strain for four weeks before being put through a socially evaluated cold-stressor. Which is basically a very scientific way of saying they had to put their hand in freezing water while being watched and assessed.

Compared with placebo, the group taking 1714 had a lower cortisol response, lower self-reported anxiety during the stressor, lower perceived daily stress and a small improvement in visual memory (PMID: 27801892).

Whilst we can’t conclude from this that one strain fixes anxiety or will improve mood in everyone, it does go to show just how much our bugs can impact us beyond digestion.

This is exactly why the gut needs to be part of the conversation when it comes to mental health. Anxiety, low mood, brain fog, feeling overwhelmed, feeling like you are always wired but exhausted… None of these experiences are only happening ‘in your head.’ Your brain is a huge part of the picture, but it’s time we started paying attention to our ‘second brain’ or ‘other brain’ as it’s now being called. 

I’ve witnessed this first hand, not just with myself but with my clients…

In some of my first intensive gut protocols with clients, I watched women make shifts that impacted their microbiome health so deeply they were eventually able to come off SSRIs and prescribed medication for their mental health.

Your microbiome and food really can be that powerful. This is not medical advice to ditch your SSRIs, medication can be incredibly supportive and, for some people, lifesaving. But it is an incredible demonstration of what can happen when you address symptoms beyond the brain.

For a long time, we have spoken about mental health as though it all lives from the neck up: go to therapy, change your thoughts, meditate, take the medication. Which are all very important tools.

But when the body underneath your brain is inflamed, undernourished, unable to digest properly, struggling to clear its bowels and constantly getting stress signals, there is a much bigger conversation to be had.

The group chat you cannot leave

Hopefully by now you’re starting to see, your gut is not just there to digest your food.

It actually has its own nervous system, called the enteric nervous system, and stays in constant communication with the brain in your head. One of the biggest communication highways is the vagus nerve, which runs between your brain and your gut, passing messages both ways all day long (PMID: 31460832).

This is why you get butterflies before a big conversation. Why you can feel nauseous when you are anxious. Why your appetite disappears when life feels like too much. Why you can be constipated for three days after a stressful week, or suddenly need the bathroom every five minutes before a presentation.

Your gut and your mind are in a group chat, and when one starts spiralling, the other is rarely far behind.

When your body thinks you are being chased by a tiger

The HPA axis is one of the main reasons stress has such a huge impact on your gut. HPA stands for hypothalamic-pituitary-adrenal axis. I know, very sexy for all my fellow science wellness nerds in the room.

Think of it as the body’s internal stress-alarm system.

Your hypothalamus picks up on a threat and sends out the first signal. Your pituitary passes the message along. Your adrenal glands release cortisol and other stress hormones to make sure you have enough energy, alertness and resources to deal with the danger.

In a real emergency, this is brilliant. You do not need your body using a huge amount of energy to digest lunch if you are running from a tiger.

The issue is that most of us are not being chased by tigers anymore. We are being chased by a calendar with no white space, a phone that never stops lighting up, family pressure, financial stress, under-eating all day, over-training, too much caffeine, too little sleep, and the mental load of remembering everyone else’s life as well as our own.

Your body does not always distinguish between an actual physical threat and a life that simply never gives it a chance to come down.

When that HPA axis is firing constantly, it can change gut motility, digestive secretions, blood flow, immune activity and intestinal permeability. That is why stress can create constipation in one person, diarrhoea in another, and a very puffy, painful, reactive gut in someone else (PMID: 29276734; PMID: 38353184).

This is how anxiety can become such a full-body experience. It is not just racing thoughts. It can be the nausea before you leave the house, the tight chest, the appetite changes, the poor sleep, the bowel changes, the feeling that you are tired but cannot switch off. Your mind feels stressed, your body responds as though there is danger, and then the physical symptoms can give your mind even more evidence that something is wrong.

Then the gut starts sending its own messages back up. Welcome to the group chat.

This is often why food isn’t “the problem”…

I cannot tell you the amount of times I’ve had women come to me on low-FODMAP or elimination diets, or having spent years without a certain food group because they’re trying to figure out their food sensitivities…

They will tell me, “I can be constipated for a week, then loose the next. I’m bloated after everything. I have tried cutting dairy, gluten, sugar, coffee, alcohol, tried low FODMAPs, and I still cannot work out why my gut is so unhappy.”

Sometimes there is a food trigger. Sometimes there is an overgrowth, an infection, inflammation, poor digestion, a lack of fibre, years of antibiotics, or a gut that genuinely needs more support.

But more often than not, there is another piece of the story sitting right there in front of us.

Their nervous system is wired.

This is why IBS is so interesting.

Whilst it is classified as a disorder of gut-brain interaction, aka the conversation between the brain and the gut has become disrupted, it is very rarely treated like one. Which is why when you go to the doctor, I can almost guarantee that you’ll be put on a low-FODMAP diet indefinitely and not given any other direction from there. Not once has a client told me their physician explained it could be linked to their stress or nervous system.

You see, the gut can become more sensitive, motility can change, stress can alter bowel habits, pain, urgency and bloating. The more the symptoms flare, the more worried the woman becomes about food, eating and leaving the house. Then the gut gets even more stressed, creating a nasty cycle of IBS flares and restriction (PMID: 39148020; PMID: 37469740).

This is why I do not love the blanket advice to simply cut more foods out when a woman has gut symptoms.

For the woman with IBS, the woman who is alternating between constipation and loose stools, the woman who has a Notes app full of every meal that made her bloated, food may absolutely be part of the picture. We want to understand her individual triggers, whether there is inflammation, whether her gut microbiome has been disrupted, whether she has an overgrowth or infection, and whether she has been unknowingly eating in a way her body does not tolerate well.

But the nervous system needs to be part of the investigation too.

Because if every meal feels risky, every symptom sends her into a spiral, and she is running on high alert all day, her gut has very little reason to settle. The brain is telling the gut that life is unsafe. The gut changes its function accordingly. Then the symptoms give the brain even more evidence that something is wrong.

This is why you cannot food-eliminate your way out of IBS.

It requires a whole-body treatment plan. It will mean some food changes whilst you heal and rewire the microbiome, probably some supplement and probiotic support, for others it may mean more motility support is required, potentially further testing, and ultimately figuring out how the nervous system has impacted the gut, and vice versa. It is also teaching the nervous system that it can come back down. The brain and the gut are both involved, lemme dive a bit deeper…

Your microbiome is a rainforest

Your microbiome is the whole community of bacteria, fungi, viruses and other microbes living in and on your body. The largest neighbourhood is in your large intestine.

I want you to think about the Amazon rainforest.

A rainforest is not one kind of tree. It has layers. Tall trees, shrubs, roots, insects, birds, animals, fungi, water systems, decomposers and thousands of tiny jobs happening at once. Every part influences the conditions that allow the next part to thrive.

Your microbiome works in a similar way.

We have different species, different layers, different strains and different functions. Some help break down fibre. Some produce short-chain fatty acids. Some support the gut lining. Some help regulate immune activity. Some become a problem when the environment shifts and they are given too much space to take over.

This is where I will use the language of “bad bugs,” because it helps make the conversation make sense.

I am not saying every bacterium is morally good or bad, obviously. But when there are opportunistic species, overgrowths, infections or an imbalance in the gut, the wrong crowd can start running the show… and just like a rainforest, what thrives depends on the conditions.

Food is an important part of the conversation here.

We have a symbiotic relationship with our microbes, meaning they benefit from us in the fact they have a place to live, and we benefit from them in that they produce things like post-biotics that reduce inflammation and help us thrive in our day-to-day life.

However, which microbes are abundant is hugely dependent on diet…

A high-added-sugar, low-fibre diet can create an environment that favours sugar-loving microbes and reduces some of the bacteria that produce beneficial short-chain fatty acids, including butyrate. Those short-chain fatty acids help support your gut lining, immune system and the messages travelling between your gut and brain (PMID: 26011307; PMID: 41325059).

Now, a few squares of chocolate here and there will not ruin your microbiome. We are not here to create any fear around food. What we are focused on is what you do the majority of the time.

If the beneficial microbes never get fed because there is very little fibre, colour, variety or plant matter coming in, they do not have much to work with. If the rainforest is regularly being hit with high amounts of ultra-processed food, alcohol, erratic eating, stress and poor sleep, the ecosystem can start to look very different.

Poor soil quality, poor food quality, poor food choices and increased stressors create nutrient deficiencies and compromised wellbeing.

That is the picture, and your gut is not going to keep that picture to itself.

The messages travelling back upstairs

There are a few ways an unhappy gut can affect how you feel upstairs, aka in your brain and therefore impact the mind. One is inflammation.

When the gut lining becomes more compromised, or the immune system is being constantly activated, inflammatory messengers can move through the body and become part of the gut-brain conversation. Bacterial components, including lipopolysaccharide, can also trigger immune activity when they cross a more permeable gut barrier. This can affect the HPA axis, the vagus nerve and the wider stress response (PMID: 38353184).

Another is through the metabolites your gut bacteria produce.

When beneficial bacteria ferment fibre, they produce short-chain fatty acids. You will hear people talk about butyrate a lot in the gut-health space, and for good reason. It helps nourish the cells lining your colon, supports the gut barrier and has roles in immune and brain signalling (PMID: 35066114). Which is why supporting your gut can improve far more than your digestion.

When we improve your microbiome by supporting more regular bowel movements and digestion, reducing inflammatory load, addressing infections or overgrowths where they are present, increasing fibre slowly and strategically, improving meal regularity, stabilising blood sugar and giving the body the nutrients it needs, we change the terrain.

We start restoring the rainforest…

We are giving the gut a different environment to work with. We are helping the beneficial microbes do their jobs. We are supporting the gut lining. We are reducing the constant stream of ‘something is wrong’ messages travelling through the system. We are creating a mutually beneficial, symbiotic environment. We are also giving the brain fewer reasons to stay on high alert.

I think this is the part that changes the conversation…

Mental health does not just live in your mind

Supporting your mental health is not just about mindset work or telling yourself to think more positively. It can be therapy, medication, community, sleep, movement, nervous-system regulation, nourishment, blood-sugar stability and yes, looking at what is happening in your gut.

None of these things replace each other, they work together and for some women, finally understanding that their anxiety, low mood, overwhelm or brain fog may have a physical piece to it is incredibly validating. 

It means we can stop looking at mental health as though someone has failed to cope or just needs to think differently, and start asking what else their body might be asking for.

This is not a one-size-fits-all ‘take a probiotic and eat sauerkraut’ situation either.

Probiotic research for anxiety and depression is promising, but it is also wildly strain-specific and still evolving. We cannot throw the same probiotic at every woman with anxiety and expect the same outcome (PMID: 39731509; PMID: 39245752).

Finally, the work needs context…

A woman who has bloating because she is eating too fast, living on coffee and skipping lunch needs a different plan from a woman with constipation, histamine symptoms, high stress, a history of antibiotics, endometriosis, low stomach acid or a confirmed gut infection. A woman who is deeply depressed needs the appropriate mental-health support around her while we address the physical pieces that may be making it harder for her to feel like herself.

That is what root-cause work looks like to me.
We do not chase one symptom, we look at the whole ecosystem.

Your body is always talking to you. The gut-brain connection simply gives us another way to understand what it has been trying to say.

Key takeaways

• Your gut is not just where food gets digested, it has its own nervous system, talks to your brain all day long, and has a genuine role in how your body experiences stress, safety, mood and mental clarity.
• Yes, most of the body’s serotonin is made in the gut, but the gut-brain connection goes far beyond serotonin. We are talking about your microbiome, inflammation, cortisol, immune activity, the vagus nerve, nutrient status and the way your body responds to stress.
• If you are constantly bloated, constipated, running to the bathroom, reacting to food or stuck in the restrict-and-flare cycle, food may be part of the picture. But it is rarely the only part of the picture. Your nervous system deserves to be investigated too.
• IBS is not ‘all in your head,’ or simply an intolerance to low FODMAP foods, but it is a disorder of gut-brain interaction. For many women, the real work is supporting the gut while also helping the body feel safe enough to settle.
• Your microbiome is a rainforest. The food you eat, your fibre intake, stress, sleep, alcohol, medication history, meal regularity and lifestyle all affect what gets to thrive there. We want to create an environment where the beneficial bugs have what they need to do their jobs.
• There is no one probiotic, food or supplement that fixes every gut or every anxious mind. This work needs context, curiosity and a plan that looks at the whole ecosystem, not just the loudest symptom.

Because when you start supporting the gut, you are not just working towards less bloating or better bowel movements. You are creating the conditions for your whole body to function, cope and feel better including your mind! 

References

• Allen AP, Hutch W, Borre YE, et al. Bifidobacterium longum 1714 as a translational psychobiotic: modulation of stress, electrophysiology and neurocognition in healthy volunteers. Translational Psychiatry. 2016. PMID: 27801892.
• Cryan JF, O’Riordan KJ, Cowan CSM, et al. The Microbiota-Gut-Brain Axis. Physiological Reviews. 2019. PMID: 31460832.
• Foster JA, Rinaman L, Cryan JF. Stress & the gut-brain axis: Regulation by the microbiome. Neurobiology of Stress. 2017. PMID: 29276734.
• Del Ponte A, et al. Stressed to the Core: Inflammation and Intestinal Permeability Link Stress-Related Gut Microbiota Shifts to Mental Health Outcomes. 2024. PMID: 38353184.
• Saha L, et al. Molecular signalling during cross talk between gut brain axis regulation and progression of irritable bowel syndrome: A comprehensive review. 2023. PMID: 37469740.
• Silva YP, Bernardi A, Frozza RL. Short-chain fatty acids: Microbial metabolites for gut-brain axis signalling. 2022. PMID: 35066114.
• Cronin P, et al. Review article: dietary fibre-microbiota interactions. Alimentary Pharmacology & Therapeutics. 2015. PMID: 26011307.
• Tappy L, et al. Added sugars, gut microbiota, and host health. 2025. PMID: 41325059.
• O’Mahony SM, Clarke G, Borre YE, Dinan TG, Cryan JF. Serotonin, tryptophan metabolism and the brain-gut-microbiome axis. Behavioural Brain Research. 2015. PMID: 25078296.
• Agus A, Clément K, Sokol H. Tryptophan metabolism and gut-brain homeostasis. 2021. PMID: 33804088.
• Slyepchenko A, et al. Effects of prebiotics and probiotics on symptoms of depression and anxiety in clinically diagnosed samples: systematic review and meta-analysis of randomised controlled trials. 2025. PMID: 39731509.
• Chen Y, et al. Strain-specific effects of probiotics on depression and anxiety: a meta-analysis. Gut Pathogens. 2024. PMID: 39245752.