The Scale of the Thing
The human gut contains approximately 38 trillion bacteria — roughly equal to the total number of human cells in the body. These bacteria, along with archaea, fungi, and viruses, constitute the gut microbiome: a complex, metabolically active community that has co-evolved with humans over millions of years.
The microbiome is not uniformly distributed throughout the digestive tract. The stomach and small intestine are relatively sparsely populated (the acidic and antimicrobial environment there limits bacterial growth). The colon, by contrast, is densely colonised — bacterial concentrations reach 10^11 to 10^12 cells per millilitre, making it one of the most densely inhabited environments on Earth.
What the Microbiome Does
The gut microbiome performs functions that the human genome simply does not encode. Human cells possess approximately 20,000 genes; the collective bacterial genome (the microbiome) contains an estimated 3.3 million unique genes. This expanded genetic capacity allows the microbiome to metabolise dietary compounds, synthesise vitamins, train the immune system, and maintain the integrity of the gut barrier.
Fermentation of dietary fiber into short-chain fatty acids (SCFAs) is among the most important microbiome functions. Butyrate — produced primarily by Faecalibacterium prausnitzii, Roseburia intestinalis, and other firmicutes — is the primary energy source for colonocytes (the cells lining the colon). Butyrate also has anti-inflammatory properties and plays a role in regulating colonocyte proliferation. Low butyrate production is associated with colorectal cancer, inflammatory bowel disease, and metabolic disorders.
The microbiome plays a direct role in immune development and regulation. Approximately 70% of the immune system is associated with the gut. Microbial products train regulatory T-cells, shape the balance between inflammatory and anti-inflammatory immune responses, and maintain the gut barrier that prevents bacterial translocation into the bloodstream.
The gut-brain axis — the bidirectional communication network between the gut and the central nervous system — is significantly mediated by the microbiome. Gut bacteria produce or regulate the production of serotonin (95% of the body's serotonin is produced in the gut), dopamine precursors, gamma-aminobutyric acid (GABA), and numerous other neuroactive compounds. Disruptions in the gut microbiome have been associated with depression, anxiety, and neurodevelopmental conditions, though causal relationships remain under active investigation.
What Shapes Microbiome Composition
Diet is the single most modifiable determinant of microbiome composition. Dietary changes can produce measurable shifts in bacterial populations within 48–72 hours. A diet high in fermentable fiber (vegetables, legumes, whole grains, fruits) supports diverse microbial communities and high SCFA production. A diet high in ultra-processed food, saturated fat, and low in fiber is consistently associated with reduced microbiome diversity and lower populations of beneficial species.
The diversity of plant foods consumed appears to be particularly important. Research by the American Gut Project found that consuming more than 30 different plant foods per week was associated with significantly higher microbiome diversity than consuming fewer than 10. Diversity of plant consumption matters independently of total fiber intake — different plant compounds (polyphenols, specific fiber types, phytochemicals) selectively feed different bacterial species.
Antibiotic use has one of the most significant acute impacts on the microbiome. A single course of broad-spectrum antibiotics can reduce bacterial diversity by 25–50% and alter community composition for months to years. This is not an argument against using antibiotics when medically necessary — it is an argument for awareness of their impact and for taking steps to support microbiome recovery afterwards.
Other factors that influence microbiome composition include birth method (vaginal birth exposes infants to Lactobacillus from the birth canal; caesarean birth substitutes skin and environmental bacteria), infant feeding (breast milk contains human milk oligosaccharides that selectively feed Bifidobacterium), sleep quality, exercise, chronic stress, and geographic location.
Prebiotics and Probiotics: What the Evidence Actually Shows
Prebiotics are dietary compounds that selectively feed beneficial gut bacteria. The most thoroughly studied prebiotics are inulin and fructooligosaccharides (FOS), found in chicory root, garlic, onion, Jerusalem artichoke, and many other plant foods. These compounds preferentially stimulate Bifidobacterium and Lactobacillus populations. The evidence for prebiotic supplementation improving gut microbiome markers is reasonably consistent, though effect sizes vary.
Probiotics are live microorganisms that, when consumed in adequate amounts, confer health benefits. The evidence for probiotics is highly strain-specific and indication-specific — what works for one condition in one population may not work for another. The best-supported applications include antibiotic-associated diarrhoea (Lactobacillus rhamnosus GG and Saccharomyces boulardii have the most evidence), prevention of Clostridioides difficile infection, and some IBS symptom management.
Fermented foods — yogurt, kefir, kimchi, sauerkraut, miso, tempeh — provide live bacteria alongside prebiotic substrates in a food matrix. A 2021 study in Cell found that a high-fermented food diet increased microbiome diversity and decreased inflammatory markers over a 10-week period, with effects that exceeded those of a high-fiber diet alone in the same timeframe.
Signs of a Microbiome in Trouble
Dysbiosis — disruption of the gut microbial community — does not have a precise clinical definition, but research has identified patterns associated with poor health outcomes. Reduced overall diversity, loss of specific keystone species (particularly butyrate-producing bacteria), and overgrowth of potentially pathogenic species are patterns associated with IBS, IBD, metabolic syndrome, obesity, and some autoimmune conditions.
Symptoms that may suggest gut microbiome disruption include persistent bloating and gas, altered bowel habits, fatigue not explained by other causes, food intolerances that have developed or worsened, frequent infections, and skin conditions like eczema or psoriasis with concurrent gut symptoms. These symptoms are non-specific and have many potential causes, but a conversation with your doctor about gut health is always worthwhile.
Practical Steps
The evidence converges on a relatively consistent set of dietary patterns associated with microbiome health: eat a diverse range of plant foods (targeting 30+ different plants per week); prioritise fermentable fiber from vegetables, legumes, whole grains, and fruit; include fermented foods regularly; minimise ultra-processed food and added sugar; use antibiotics only when necessary and support recovery with fiber-rich foods afterwards; and get adequate sleep and regular physical activity, both of which have direct effects on microbiome composition.
None of this is particularly revolutionary advice. The connection to gut microbiome science, however, provides a mechanistic explanation for why these patterns support health — and why the choices made at every meal have consequences that extend well beyond immediate satiety.