The gut was once considered a digestive organ. Something that processed food, absorbed nutrients, eliminated waste. Useful, certainly. But not particularly relevant to the mind.
That understanding has changed dramatically in the past two decades. The gut is now understood to be a second brain — housing over 100 million neurons, producing the majority of the body’s serotonin, and communicating continuously with the central nervous system through a network of pathways so complex and bidirectional that researchers have begun to ask a question that would have seemed absurd twenty years ago: is what lives in our gut shaping how we think, feel, and perceive reality?
For people navigating psychosis and schizophrenia, the implications of that question are significant.
The gut-brain axis
The gut and the brain are in constant conversation. The primary channel of that conversation is the vagus nerve — the longest nerve in the body, running from the brainstem down through the heart and lungs and into the digestive tract. Approximately 80 to 90 percent of the information flowing along the vagus nerve travels upward, from gut to brain, not the other way around.
This means that what is happening in the gut — the state of the microbiome, the integrity of the gut lining, the presence or absence of inflammation — is continuously influencing brain function. Mood, cognition, stress response, sleep, the capacity for regulation: all of these are downstream of gut health in ways that are only now beginning to be mapped with precision.
Stephen Porges’ Polyvagal Theory helps us understand why this matters so much for mental health. The vagus nerve is the primary mediator of the body’s capacity for safety and social engagement. When gut inflammation disrupts vagal tone, the nervous system’s capacity to access states of regulation, connection, and calm is directly compromised. The body becomes more reactive, more threat-sensitive, less able to distinguish present safety from past danger.
What the research shows in schizophrenia
A growing body of research has identified significant differences in the gut microbiome of people diagnosed with schizophrenia compared to those without the diagnosis. These differences are not minor variations — they point to systematic alterations in the microbial communities that regulate inflammation, produce neurotransmitters, and maintain the integrity of the gut lining.
Continue exploring
Food, rhythm, and the nervous system
Chapter 14 of Wisdom of Psychosis explores food, gut health, and daily rhythm as medicine in depth — including practical protocols for integrating these approaches into daily life.
A 2019 study published in Science Advances transplanted gut microbiota from people with schizophrenia into germ-free mice. The mice subsequently displayed behavioural changes consistent with schizophrenia — including hyperactivity and disruptions in sensorimotor gating, the brain’s mechanism for filtering irrelevant stimuli. This is one of the most direct demonstrations yet that the gut microbiome plays a causal, not merely correlational, role in psychotic symptomatology.
Other research has found that people with schizophrenia show elevated levels of inflammatory markers in the blood — suggesting that systemic inflammation, potentially originating in the gut, is a consistent feature of the condition. This connects to the broader inflammatory hypothesis of psychosis, which proposes that chronic inflammation in the brain and body creates conditions in which psychotic experience becomes more likely.
“The microbiome is not a passive passenger in the gut. It is an active participant in the regulation of brain function, immunity, and behaviour.”
— Emeran Mayer, The Mind-Gut Connection
Leaky gut and the blood-brain barrier
One of the most clinically significant findings in this area concerns intestinal permeability — what is sometimes called leaky gut. In healthy gut function, the lining of the intestine acts as a selective barrier, allowing nutrients through while keeping larger molecules, bacteria, and inflammatory compounds out of the bloodstream.
When that barrier is compromised — through chronic stress, poor diet, antibiotic use, or dysbiosis (an imbalance in the microbial community) — inflammatory compounds can pass into the bloodstream and, from there, potentially across the blood-brain barrier. The result is neuroinflammation: inflammation inside the brain itself.
Research has found elevated markers of intestinal permeability in people with schizophrenia, and some studies have found that the degree of gut permeability correlates with the severity of symptoms. This is not yet a settled picture — the research is ongoing and the mechanisms are complex. But the signal is consistent enough to take seriously as a clinical consideration.
The role of diet
If the microbiome plays a significant role in psychotic experience, then what we eat — which shapes the microbiome more than almost any other factor — becomes a legitimate area of clinical attention. This is not a radical claim. It is, if anything, a return to something every traditional healing system has always known: food is medicine.
The research on diet and schizophrenia is still developing, but several findings are worth noting. People with schizophrenia show higher rates of metabolic syndrome, insulin resistance, and nutritional deficiencies — including deficiencies in omega-3 fatty acids, B vitamins, zinc, and magnesium — than the general population. Some of these deficiencies predate psychiatric treatment and are not explained by medication alone.
Dietary patterns associated with reduced inflammation — high in vegetables, legumes, whole grains, and fermented foods, low in ultra-processed foods and refined sugars — are associated with better mental health outcomes across a range of conditions. The mechanisms are multiple: reduced inflammatory load, improved microbiome diversity, better regulation of blood sugar, enhanced production of short-chain fatty acids that support the gut lining and vagal tone.
A Mediterranean-style dietary pattern has received the most research attention in this context. A landmark randomised controlled trial by Jacka and colleagues — the SMILES trial — demonstrated that dietary intervention alone significantly reduced depressive symptoms compared to social support alone. Similar work in psychosis is underway, and the early results are consistent with what common sense might suggest: the brain functions better when the body is nourished.
Fermented foods and the microbiome
Of all the dietary interventions studied for microbiome health, fermented foods occupy a particular place. Traditionally fermented foods — kefir, yoghurt, sauerkraut, kimchi, miso, tempeh, kombucha — introduce live bacterial cultures directly into the gut and have been shown to increase microbiome diversity and reduce markers of inflammation.
A 2021 randomised controlled trial by Wastyk and colleagues at Stanford compared a high-fibre diet to a high-fermented-food diet over ten weeks. The fermented food group showed significant increases in microbiome diversity and significant reductions in nineteen inflammatory proteins — including markers associated with psychotic disorders. The fibre group showed no equivalent benefit on inflammation, suggesting that fermentation specifically — not simply dietary quality — is the active ingredient.
Traditional food cultures understood this without the research. Every indigenous food tradition in the world includes fermented preparations — developed over generations of empirical observation about what kept communities healthy. This is the original pharmacy: the accumulated wisdom of cultures that paid close attention to the relationship between food, community, and mental health.
What this means clinically
The gut-microbiome research does not suggest that schizophrenia is simply a digestive problem, or that diet alone is a sufficient treatment for psychosis. The picture is more complex than that, and intellectual honesty requires saying so.
What it does suggest is that the gut is a legitimate domain of clinical attention in psychotic sensitivities — not an alternative to other treatment, but an additional layer of the picture that has been largely ignored. Several practical implications follow:
- Dietary assessment should be a standard part of mental health care. What someone eats — and what they are deficient in — is clinically relevant information in psychosis, as it is in every other chronic condition.
- Anti-inflammatory dietary approaches are low-risk, evidence-supported, and accessible. They do not replace medication or therapy, but they address a biological substrate that medication does not reach.
- Fermented foods are a simple, affordable, and culturally resonant intervention with meaningful evidence behind them. The barrier to entry is low. The potential benefit is real.
- Gut health is not separate from stress, trauma, and nervous system regulation. Chronic stress degrades the gut lining. Trauma disrupts the microbiome. The gut and the nervous system are in continuous dialogue — which means that working with one invariably affects the other.
In the work I do
Food is one of the original medicines I take most seriously in clinical practice — not because I am a nutritionist or a dietitian, but because I have seen what happens when people who have been living on processed food, stimulants, and irregular meals begin to eat in a way that nourishes rather than depletes.
It is not dramatic. It is quiet and cumulative. Mood stabilises slightly. Sleep improves slightly. The nervous system becomes fractionally less reactive. And those small shifts compound over time into something that feels, to the person living it, like genuine change.
This is not magic. It is biology. And it is available to almost everyone — regardless of diagnosis, medication status, or clinical history. It is, in the truest sense, an original medicine.
Further reading
Mayer, E. (2016). The Mind-Gut Connection. Harper Wave.
Wastyk, H.C. et al. (2021). Gut-microbiota-targeted diets modulate human immune status. Cell, 184(16), 4137–4153.
Jacka, F.N. et al. (2017). A randomised controlled trial of dietary improvement for adults with major depression (the SMILES trial). BMC Medicine, 15(1), 23.
Zhu, F. et al. (2019). Metagenome-wide association of gut microbiome features for schizophrenia. Nature Communications, 10, 3875.
Severance, E.G. et al. (2013). Gastrointestinal inflammation and associated immune activation in schizophrenia. Schizophrenia Research, 138(1), 48–53.
