The Gut-Brain Axis: How Your Gut Controls Your Hormones, Mood, and Nervous System
The gut-brain axis is one of the most transformative concepts to emerge from health research in the past two decades. It explains why gut problems cause anxiety, why stress causes digestive symptoms, why hormonal imbalances are so often accompanied by gut dysfunction, and why treating the gut is foundational in clinical nutrition.
What Is the Gut-Brain Axis?
The gut-brain axis refers to the bidirectional communication network between the gastrointestinal tract and the central nervous system. This network operates via the vagus nerve, the enteric nervous system (sometimes called the "second brain"), the immune system, hormonal signalling, and the gut microbiome itself [1].
The gut contains approximately 500 million neurons, produces over 90 percent of the body's serotonin, and is host to trillions of microorganisms that collectively influence neurotransmitter production, immune regulation, hormonal metabolism, and inflammation.
The Gut and Hormones: The Oestrobolome
The oestrobolome is a subset of the gut microbiome responsible for metabolising oestrogen. Gut bacteria produce an enzyme called beta-glucuronidase, which can reactivate conjugated (deactivated) oestrogen in the colon, allowing it to be reabsorbed into circulation rather than excreted [2].
When the oestrobolome is disrupted by dysbiosis, antibiotics, low fibre intake, or gut inflammation, oestrogen recirculates inappropriately. This contributes to oestrogen dominance, worsens PMDD, and may amplify histamine intolerance (since oestrogen stimulates mast cells to release histamine). Restoring oestrobolome diversity is a direct therapeutic target for hormonal health in women.
The Gut and Serotonin
Approximately 90 to 95 percent of the body's serotonin is produced by enterochromaffin cells in the gut wall, not the brain. While gut-derived serotonin does not cross the blood-brain barrier, it influences gut motility, intestinal secretion, and signals back to the brain via the vagus nerve [3].
Gut dysbiosis, inflammation, and intestinal permeability all disrupt gut serotonin signalling. This is clinically significant for women with PMDD, depression, and anxiety, where serotonin regulation is already compromised.
The Gut and the Nervous System
The vagus nerve is the primary communication highway of the gut-brain axis. Approximately 80 percent of vagal fibres transmit signals from gut to brain, meaning the gut is constantly sending information upward about its state, including stress, inflammation, nutrient status, and microbial composition.
Poor vagal tone (reduced gut-to-brain signalling efficiency) is associated with reduced stress resilience, anxiety, and impaired digestion. Nervous system regulation strategies that improve vagal tone, including deep diaphragmatic breathing, cold exposure, humming, and social connection, directly improve gut function as a result.
Leaky Gut, Inflammation, and the Gut-Brain Connection
Intestinal permeability, sometimes called leaky gut, allows bacterial fragments (lipopolysaccharides, or LPS) to translocate into the bloodstream. LPS is highly inflammatory and activates the immune system, driving systemic low-grade inflammation. This inflammation crosses the blood-brain barrier (via inflammatory cytokines), contributing to neuroinflammation, brain fog, depression, and anxiety [4].
For women with MTHFR or COMT variants, this inflammatory burden further compromises neurotransmitter synthesis and clearance, amplifying the mental health impact.
Signs That Your Gut-Brain Axis Is Dysregulated
• Digestive symptoms (bloating, constipation, diarrhoea, IBS-type patterns) that worsen with stress
• Mood changes correlated with gut flares
• Brain fog that improves when gut function improves
• Anxiety or depression alongside gut issues
• Hormonal symptoms (PMDD, heavy periods, oestrogen dominance) alongside gut dysfunction
• Food intolerances that have developed or worsened over time
Nutritional Strategies to Support the Gut-Brain Axis
Fibre Diversity
Gut microbial diversity is directly linked to fibre variety. Aim for 30 different plant foods per week, including vegetables, fruits, legumes, wholegrains, nuts, and seeds. Different bacterial species feed on different prebiotic fibres, and diversity is the single most important driver of a healthy oestrobolome and neurotransmitter-producing microbiome.
Fermented Foods
A randomised controlled trial published in Cell found that a high-fermented food diet increased microbiome diversity and reduced inflammatory markers over 10 weeks, outperforming a high-fibre diet alone [5]. Daily inclusion of kefir, yoghurt, sauerkraut, kimchi, or kombucha (where histamine-tolerated) is the most accessible approach.
Omega-3 Fatty Acids
Omega-3 fatty acids support gut barrier integrity, reduce gut and neuroinflammation, and support vagal tone. At least 2g EPA and DHA combined daily from fatty fish or supplementation is clinically appropriate for women with gut-brain axis dysfunction.
Polyphenols
Polyphenols from berries, green tea, dark chocolate, olive oil, and herbs are selectively fermented by beneficial gut bacteria, acting as prebiotics and supporting microbial diversity. They also directly reduce gut inflammation.
Glutamine and Gut Barrier Support
L-glutamine is the primary fuel source for enterocytes (gut lining cells) and supports repair of intestinal permeability. Zinc carnosine and vitamin A also support mucosal barrier integrity.
Eliminate Gut Inflammatory Drivers
Reducing ultra-processed foods, excess sugar, alcohol, and dietary triggers of gut inflammation is as important as adding beneficial nutrients. The microbiome responds rapidly to dietary change, both positively and negatively.
Testing the Gut
Comprehensive stool testing (GI MAP or similar) provides clinical insight into microbial composition, dysbiosis, inflammation markers, intestinal permeability, and pathogenic organisms. See the full article on functional testing for what gut testing can reveal and when it is appropriate.
Suspect your gut is driving your hormonal or mood symptoms?
Book a Clinical Case Assessment with Kirstie to explore the gut-hormone connection and what a clinical nutrition approach could look like for you.
References
• Cryan JF, Dinan TG. Mind-altering microorganisms: the impact of the gut microbiota on brain and behaviour. Nature Reviews Neuroscience. 2012;13(10):701-712. PubMed
• Kwa M, et al. The intestinal microbiome and estrogen receptor-positive female breast cancer. Journal of the National Cancer Institute. 2016;108(8):djw029. PubMed
• Yano JM, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161(2):264-276. PubMed
• Dinan TG, Cryan JF. Gut instincts: microbiota as a key regulator of brain development, ageing and neurodegeneration. Journal of Physiology. 2017;595(2):489-503. PubMed
• Wastyk HC, et al. Gut-microbiota-targeted diets modulate human immune status. Cell. 2021;184(16):4137-4153. PubMed
