The Ecosystem Inside You
The human body contains approximately 37 trillion human cells. It also contains somewhere in the region of 38 trillion microbial cells — bacteria, viruses, fungi, and other microorganisms that live primarily in the digestive tract. This community, collectively called the microbiome, has been the subject of one of the most significant expansions in biomedical research of the past two decades.
What researchers have found is that the microbiome is not a passive passenger in the body. It actively participates in digestion, immune function, hormone regulation, and — most surprisingly — mental health. The implications are still being worked out, but the broad picture is already clear enough to be worth understanding.
What the Microbiome Actually Does
The gut microbiome performs functions that the human body cannot perform on its own:
- Digestion: Gut bacteria ferment dietary fibre and produce short-chain fatty acids that feed the cells lining the colon and serve as energy sources. Without bacterial fermentation, a significant portion of the nutrients in plant foods would pass through unabsorbed.
- Immune regulation: Approximately 70% of the immune system is located in the gut. The microbiome plays a central role in training immune cells to distinguish between harmful pathogens and harmless substances — a calibration process that begins at birth and continues throughout life. Disruption of this process is linked to increased susceptibility to allergies and autoimmune conditions.
- Neurotransmitter production: The gut produces approximately 95% of the body's serotonin — the neurotransmitter most associated with mood regulation. It also produces GABA, dopamine precursors, and other neuroactive compounds. These are produced and used locally, but their influence extends via the vagus nerve and the bloodstream to the brain.
- Vitamin synthesis: Certain gut bacteria synthesise B vitamins and vitamin K, contributing to nutritional status in ways that are independent of dietary intake.
The Gut-Brain Axis
The connection between the gut and the brain — now called the gut-brain axis — is bidirectional. The brain influences gut function through the autonomic nervous system, which is why anxiety produces digestive symptoms. The gut, in turn, communicates with the brain through the vagus nerve, immune signalling, and the hormones and neurotransmitters produced by gut bacteria.
Research in this area has moved rapidly. Animal studies have shown that germ-free mice — raised without any gut microbiome — display abnormal stress responses and anxiety-like behaviours that can be partially corrected by introducing specific bacterial strains. Human studies have linked microbiome diversity to depression, anxiety, and cognitive function, though establishing causation in human populations is more complex.
A landmark 2019 study published in Nature Microbiology analysed data from over 1,000 participants and found that two bacterial genera — Coprococcus and Dialister — were consistently depleted in people with depression, even after controlling for antidepressant use. The association does not prove causation, but it points in a direction that is driving significant ongoing research.
What Disrupts the Microbiome
Several factors are well established as disruptive to microbiome diversity and health:
- Antibiotics: Necessary and life-saving when required, but they reduce microbial diversity significantly. Recovery of a healthy microbiome after a course of antibiotics typically takes weeks to months and may not fully restore the pre-antibiotic state.
- A low-fibre diet: Gut bacteria feed primarily on dietary fibre. A diet low in vegetables, legumes, and whole grains starves the microbial communities that depend on fermentable substrates, reducing diversity over time.
- Ultra-processed food: Several food additives common in processed foods — certain emulsifiers, artificial sweeteners, and preservatives — have been shown in both animal and human studies to negatively affect microbiome composition.
- Chronic stress: Sustained psychological stress alters gut motility and permeability, and changes the composition of the microbiome in ways that may further amplify the stress response — a cycle with compounding effects.
- Birth and infant feeding practices: Caesarean birth and formula feeding both affect the initial microbial colonisation of the infant gut, with potential long-term implications for immune development and health that are still being studied.
What Actually Improves Microbiome Health
The evidence for specific interventions is more robust in some areas than others:
- Dietary diversity: The single strongest predictor of microbiome diversity in population studies is the number of different plant foods consumed per week. A target of 30 different plant-based foods per week — used in the American Gut Project — is associated with significantly higher microbiome diversity than diets with fewer than 10. This includes vegetables, fruits, legumes, nuts, seeds, herbs, and spices.
- Fermented foods: A 2021 Stanford study found that a diet high in fermented foods — yoghurt, kefir, kimchi, sauerkraut, kombucha — increased microbiome diversity and reduced markers of inflammation more effectively than a high-fibre diet alone. These effects were measurable after just 10 weeks.
- Prebiotic foods: Foods that feed beneficial gut bacteria — garlic, onions, leeks, asparagus, Jerusalem artichokes, bananas, oats — support the growth of specific bacterial populations associated with positive health outcomes.
- Minimising unnecessary antibiotic use: Taking antibiotics only when genuinely necessary, and discussing the narrowest-spectrum option with your doctor when they are needed, preserves microbiome health over time.
The State of Probiotic Evidence
Probiotic supplements — concentrated preparations of live bacteria — are widely marketed for gut and general health. The evidence is more nuanced than the marketing suggests.
For specific conditions, the evidence is reasonably strong: certain Lactobacillus and Bifidobacterium strains have demonstrated efficacy for antibiotic-associated diarrhoea, irritable bowel syndrome, and some inflammatory bowel conditions. For general wellness in healthy adults, the evidence is thinner and strain-specific — not all probiotic products contain strains with clinical trial support, and the quality and viability of commercial products varies considerably.
Fermented foods, which deliver live microorganisms in a food matrix alongside prebiotics and other nutrients, are currently considered by many microbiome researchers to be more reliably beneficial than supplements for most healthy people.
The Bottom Line
The microbiome is not a fringe concept — it is a central part of human physiology that has been underappreciated until recently. The practical implications are accessible: eat a diverse, largely whole-food diet rich in plants and fermented foods, minimise unnecessary antibiotic use, and manage stress. These are not novel recommendations, but understanding that they support one of your body's most important ecosystems adds a layer of reasoning that makes them more compelling than abstract health advice.
