What the microbiome does
The gut microbiome performs functions that the human genome cannot perform alone, functions so central to health that they were once attributed to human biology rather than microbial symbionts. These include: synthesising vitamins K2, B12, B7 (biotin), and folate; producing short-chain fatty acids (SCFAs), butyrate, propionate, and acetate, that feed gut lining cells, reduce systemic inflammation, and regulate immune function; converting inactive dietary compounds into bioactive forms; metabolising bile acids in ways that regulate fat digestion, cholesterol homeostasis, and glucose metabolism; directly training and modulating the gut-associated immune system (GALT); and producing neurotransmitter precursors including 95% of the body's serotonin and significant quantities of GABA and dopamine precursors.
The microbiome is not fixed at birth. It is continuously shaped by diet, medication use, stress, sleep quality, physical activity, and environmental exposures, throughout life. This means it is readily disrupted by modern lifestyle patterns, and readily responsive to restoration.
Key bacterial species and their roles
Dysbiosis, when the microbiome goes wrong
Dysbiosis describes the state of microbial imbalance, reduced diversity, overgrowth of potentially harmful species, or loss of key beneficial species, that produces the downstream consequences seen across chronic disease. Dysbiosis is not a diagnosis; it is a spectrum. Its consequences depend on which species are lost, which overgrow, and which functional pathways are disrupted as a result.
The microbiome's role across chronic conditions
The following conditions all have documented microbiome alterations as primary or significant contributing factors, most directly IBS, where gut bacterial composition is one of the best-established drivers of symptom severity and pattern. In each case, the microbiome alterations are not simply correlations, they participate directly in the disease mechanism through the pathways described above.
How microbiome restoration works in clinical practice
Microbiome restoration is not simply the consumption of probiotic supplements, though targeted probiotic support has an appropriate role. It is a structured approach to changing the ecological environment of the gut to favour beneficial species and discourage pathogenic overgrowth.
Prebiotic fibre is the most important dietary intervention, fermentable fibres (inulin, FOS, resistant starch, pectin) feed the Faecalibacterium, Akkermansia, and Bifidobacterium species that underpin gut health. A diet with fewer than 20g of fibre per day tends to produce microbiome impoverishment; 30–40g per day, from diverse plant sources, is the minimum for meaningful microbiome support.
Dietary diversity drives microbiome diversity, the number of different plant species consumed per week is a stronger predictor of microbiome diversity than any individual food. Consuming 30 or more different plant foods per week, including herbs, spices, nuts, seeds, and legumes, produces measurably greater microbiome diversity than diets with fewer species even at equal fibre intake.
Polyphenols, the plant compounds in colourful fruits, vegetables, olive oil, dark chocolate, and green tea, selectively feed beneficial microbiome species and produce direct anti-inflammatory and barrier-protective effects in the gut independently of their fibre content.
Targeted probiotic support, specific strains with clinical evidence for the specific condition being addressed, provides direct supplementation of depleted species where dietary restoration alone is insufficient or too slow. The specific strain matters: Lactobacillus rhamnosus GG for gut barrier repair differs meaningfully from Lactobacillus acidophilus for lactose intolerance.