The Gut-Brain Connection: What It Is and Why It Matters
The idea that the gut and brain are connected is not new, but the science behind it has grown considerably in recent years. The gut-brain axis refers to the two-way communication network between the gut and the brain, and it influences far more than digestion. Mood, stress responses, cognitive function, and even how we experience pain can all be affected by what is happening in the gut. This post breaks down what the gut-brain axis is, how it works, and what nutrition has to do with it.
The gut-brain axis is the bidirectional communication system that links the gastrointestinal tract and the central nervous system. Signals travel in both directions, meaning the brain influences gut function and the gut influences brain function. This communication happens through several pathways, including the nervous system, the immune system, the endocrine system, and the gut microbiome.1,2
The primary physical connection is the vagus nerve, one of the longest nerves in the body. It runs from the brainstem down to the abdomen and carries signals in both directions. Approximately 80 to 90 percent of the fibres in the vagus nerve carry information from the gut to the brain, rather than the other way around, which gives some sense of how much the gut is communicating upward.1
The gut has its own nervous system, called the enteric nervous system. It contains approximately 500 million neurons and is capable of functioning independently of the brain, which is why it is sometimes referred to as the "second brain." It regulates gut motility, secretion, and blood flow, and it communicates constantly with the central nervous system.1,2
The enteric nervous system also plays a role in neurotransmitter production. Approximately 90 to 95 percent of the body's serotonin is produced in the gut, primarily by specialised cells in the intestinal lining. Serotonin in the gut regulates bowel movements and gut motility, and while gut-derived serotonin does not cross the blood-brain barrier directly, it influences signalling along the gut-brain axis in ways that are still being studied.1,3
The gut microbiome, the trillions of microorganisms living in the gastrointestinal tract, is an active participant in the gut-brain axis. Gut bacteria influence brain function through several mechanisms, including the production of neurotransmitters and their precursors, the regulation of inflammatory pathways, and the production of short-chain fatty acids (SCFAs) from dietary fibre. SCFAs support the integrity of the gut lining, modulate immune function, and have been shown to influence brain signalling.1,2
Research has found associations between gut microbiome composition and conditions including anxiety and depression, though this is a complex and developing field and causality is difficult to establish. What is clear is that the microbiome is not a passive bystander in mental health, and supporting microbiome diversity through diet is a reasonable and evidence-informed approach.1,2
The gut-brain connection works in both directions, and stress is a clear example of the brain influencing the gut. When the brain perceives stress, it activates the hypothalamic-pituitary-adrenal (HPA) axis, triggering the release of cortisol and other stress hormones. This can affect gut motility, increase intestinal permeability, alter the composition of the gut microbiome, and heighten gut sensitivity. This is why many people experience gut symptoms during periods of stress, and why conditions like irritable bowel syndrome often involve both gut and psychological components.1,2
The relationship is bidirectional. Chronic gut symptoms can also contribute to stress, anxiety, and reduced quality of life, further influencing the brain end of the axis.
One approach that works directly with the gut-brain axis in the context of IBS is gut-directed hypnotherapy. Research has shown it to be an effective intervention for IBS symptom management, with evidence suggesting it can reduce abdominal pain, bloating, and overall symptom severity by targeting the brain's processing of gut signals and reducing visceral hypersensitivity.4 Nerva is a digitally delivered gut-directed hypnotherapy programme developed in collaboration with Monash University researchers, making this approach accessible outside of a clinical setting and worth considering as part of a broader IBS management plan.
It is also worth noting that the gut-brain axis helps explain why the expectation of a negative reaction to a food can, in itself, produce real symptoms. When the brain anticipates that a food will cause discomfort, it can activate a conditioned physiological response through the central nervous system and HPA axis. Signals travel via the gut-brain axis to the enteric nervous system, altering gut motility and sensitivity even in the absence of a direct physiological trigger from the food. This is known as the nocebo effect, and it is documented in the context of food-related symptoms and IBS.1,2 It is one reason why understanding the gut-brain axis can be a genuinely useful lens when investigating suspected food intolerances, as not all reported reactions reflect a direct physiological response to the food itself.
Diet is one of the most significant modifiable influences on the gut microbiome, and through the microbiome, on the gut-brain axis. What is eaten shapes the composition and diversity of the gut microbiome, influences the production of SCFAs and neurotransmitter precursors, and affects gut integrity and inflammation levels, all of which feed back into the gut-brain communication network.1,2
Mental health is complex, and nutrition is one tool among many. Supporting gut health through diet can be a meaningful part of an overall management plan, alongside other approaches such as psychological support, movement, sleep, and medical care where relevant.
There are several evidence-informed dietary approaches that can support a healthy gut-brain axis. How these apply to you individually depends on your diet, health history, and goals. A registered dietitian can help identify what is most relevant for your situation.
Diverse plant fibre. Feeds a wide range of beneficial gut bacteria and supports SCFA production. Aiming for variety across the week is more useful than focusing on any single food.
Fermented foods (yoghurt, kefir, kimchi, sauerkraut). Support microbiome diversity and may help modulate the gut-brain axis, with some research linking fermented food intake to improved mood outcomes.2
Omega-3 fatty acids (oily fish, flaxseed, walnuts). Anti-inflammatory and support brain health. There is also emerging evidence for their role in gut microbiome composition.
Tryptophan-rich foods (eggs, dairy, turkey, legumes, nuts and seeds). Tryptophan is the dietary precursor to serotonin. While gut serotonin and brain serotonin are regulated differently, adequate dietary tryptophan is relevant to overall serotonin synthesis.
Polyphenol-rich foods (berries, dark chocolate, olive oil, tea, coffee). Polyphenols support beneficial gut bacteria and have antioxidant and anti-inflammatory properties that may benefit gut-brain signalling.1
Want Support With Gut Health and Nutrition?
Gut health is connected to far more than digestion. If you are looking for personalised, evidence-based nutrition support, Nula Nutrition provides registered dietitian consultations for women's health in Calgary and online across Alberta, including Canmore, Banff, Edmonton, and Cochrane.
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Written by Madison Hows RD APD
References
1. Cryan JF, et al. The microbiota-gut-brain axis. Physiological Reviews. 2019;99(4):1877–2013. PMID: 31460832
2. Carabotti M, et al. The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems. Annals of Gastroenterology. 2015;28(2):203–209. PMC4367209
3. Yano JM, et al. Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis. Cell. 2015;161(2):264–276. PMID: 25860609
4. Peters SL, et al. Randomised clinical trial: the efficacy of gut-directed hypnotherapy is similar to that of the low FODMAP diet for the treatment of irritable bowel syndrome. Alimentary Pharmacology and Therapeutics. 2016;44(5):447–459. PMID: 27397583
