Introduction
The gut is far more than a digestive organ. Housing over 38 trillion microorganisms — bacteria, fungi, viruses, and archaea — the human gastrointestinal tract functions as an endocrine organ, an immune organ, and a second nervous system. What happens in your gut influences your immune responses, your mood and behaviour, your metabolic health, and even your risk of neurodegenerative disease.
This is gut health at its most fundamental level.
The Gut-Brain Axis and Its Influence on Mood and Cognition
The enteric nervous system — the network of neurons embedded in the gastrointestinal tract — contains approximately 500 million neurons. For context, that is more neurons than are found in the spinal cord. This is why the gut is sometimes called the "second brain."
The gut-brain axis is the bidirectional communication highway between the gastrointestinal tract and the central nervous system. This communication occurs through multiple channels:
The vagus nerve: The primary neural route between gut and brain. Approximately 80% of vagal signals travel from gut to brain (not brain to gut), meaning the gut is constantly sending information upward about its environment.
Neurotransmitters: Approximately 95% of the body's serotonin — the neurotransmitter most associated with mood regulation — is produced in the gut. Gut bacteria directly influence serotonin production by stimulating enterochromaffin cells. Dopamine, GABA, and other neurotransmitters are also produced and influenced by the microbiome.
Immune signals: The gut-associated lymphoid tissue (GALT) houses approximately 70% of the body's immune cells. Gut bacterial communities communicate with these immune cells and the broader systemic immune response through cytokines and other signalling molecules.
Short-chain fatty acids: Butyrate and other SCFAs produced by fibre fermentation cross the blood-brain barrier and directly influence brain function, neuroinflammation, and the integrity of the gut-brain connection.
The clinical implications are significant. Research consistently finds associations between dysbiosis (an imbalanced microbiome) and increased rates of depression, anxiety, and cognitive impairment. Conversely, interventions that improve microbiome composition — dietary changes, probiotic supplementation, fermented food intake — show measurable effects on mood and cognitive function in clinical trials.
How Gut Bacteria Regulate Immune Responses
The gut microbiome plays a foundational role in developing and calibrating the immune system from birth. Early microbial colonisation of the gut "educates" immune cells to distinguish between harmless environmental antigens and genuine threats — a process critical for preventing autoimmune conditions and allergies.
In adults, the microbiome continues to regulate immune function through several mechanisms:
Competitive exclusion: Beneficial bacteria compete with pathogenic organisms for physical space and nutritional resources, preventing the establishment of infections.
Immune modulation: Specific bacterial strains — particularly Bifidobacterium and Lactobacillus species — train regulatory T cells that prevent excessive immune reactions (including autoimmune and allergic responses) while maintaining effective responses to genuine pathogens.
Barrier integrity: A healthy microbiome maintains the integrity of the intestinal lining, preventing "leaky gut" — the translocation of bacterial components (particularly lipopolysaccharides from gram-negative bacteria) into the bloodstream, which triggers systemic inflammation.
Dysbiosis is associated with reduced immune competence, increased susceptibility to infections (including respiratory infections in athletes), and a higher risk of inflammatory and autoimmune conditions.
The Impact of Ultra-Processed Foods on Microbiome Diversity
Ultra-processed foods (UPFs) — defined as industrial formulations containing additives not found in domestic cooking (emulsifiers, preservatives, colourings, artificial sweeteners, modified starches) — now account for over 50% of caloric intake in the UK and US.
The impact on the microbiome is well-documented and largely negative:
Emulsifiers: Polysorbate 80 and carboxymethylcellulose, used in numerous processed foods to improve texture and shelf life, have been shown in animal and human studies to directly alter microbiome composition, reduce microbial diversity, and impair gut barrier integrity — promoting low-grade chronic inflammation.
Artificial sweeteners: Saccharin, sucralose, and aspartame — previously believed to be metabolically inert — have been shown to alter gut microbiome composition and impair glucose tolerance in human subjects, with effects that vary significantly between individuals depending on their baseline microbiome.
Low fibre content: UPFs are almost universally fibre-poor. Without fibre substrate, beneficial bacteria populations decline. This is perhaps the most significant microbiome-related consequence of high UPF intake.
High sugar content: High sugar intake promotes the growth of potentially harmful bacteria and yeasts, while suppressing populations of beneficial Bifidobacterium and Lactobacillus.
Top Evidence-Backed Dietary and Lifestyle Habits for a Healthy Gut
Eat 30+ plant foods per week: Research by Tim Spector and the British Gut Project found that people consuming 30+ different plant foods per week had significantly greater microbiome diversity than those consuming fewer than 10 — regardless of omnivore vs. vegetarian status. Plants include vegetables, fruits, legumes, whole grains, nuts, seeds, herbs, and spices.
Include fermented foods daily: The Stanford fermented food study (Wastyk et al., 2021) found that a diet high in fermented foods increased microbiome diversity and reduced inflammatory markers significantly — more effectively than a high-fibre diet in the short term. Yoghurt, kefir, kimchi, sauerkraut, miso, and kombucha are the most accessible options.
Minimise ultra-processed food: Prioritising whole and minimally processed foods is the most impactful single dietary change for microbiome health. This does not require perfection — it requires making whole foods the default and UPFs the exception.
Exercise regularly: Physical activity is an independent driver of microbiome diversity. Athletes and regularly active individuals consistently show greater microbial diversity and higher abundance of beneficial species compared to sedentary counterparts.
Prioritise sleep and manage stress: Both chronic stress and inadequate sleep directly alter the microbiome via the gut-brain axis, suppressing beneficial species and promoting dysbiosis. These lifestyle factors are not separate from gut health — they are central to it.
Stay hydrated: Adequate water intake supports gut motility and maintains the mucus layer that protects and nourishes intestinal bacteria.
Your gut microbiome is not static. It responds — for better or worse — to every dietary and lifestyle choice you make. The evidence for what it responds well to is increasingly clear, and almost entirely accessible to everyone.