There are roughly thirty-eight trillion bacterial cells living in the human body — a number that matches or possibly exceeds the total count of human cells — and most of them are packed into the gastrointestinal tract, forming an ecosystem of staggering complexity. The gut microbiome, as this community of microorganisms is known, is far more than a collection of passive hitchhikers along for the digestive ride. It functions as a metabolically active organ, shaping immune function, neurological development, metabolic health, and psychological wellbeing in ways that researchers are only beginning to untangle.

The microbiome starts taking shape at birth. Babies delivered vaginally pick up their mother’s vaginal microbiota, while those born by caesarean section are instead colonised by skin microorganisms. From there, breastfeeding or formula, the introduction of solid foods, antibiotic exposure, environment, and diet all leave their mark. By around age three, a child’s gut microbiome has settled into something resembling an adult’s — though it never really stops changing. Illness, medication, aging, and what we eat continue to reshape it throughout life. The microbiome of an elderly person in an institutional setting tends to be dramatically less diverse than that of a healthy younger adult eating a varied diet, and that reduced diversity carries real consequences: greater vulnerability to infection, inflammation, and metabolic dysfunction.

Then there’s the gut-brain axis — and few areas of biomedical research are generating more excitement right now. This is the bidirectional communication network connecting the gastrointestinal tract to the central nervous system through the vagus nerve, the immune system, and a stream of neuroactive compounds. The gut produces around ninety percent of the body’s serotonin, the neurotransmitter most closely associated with mood regulation. Gut bacteria also manufacture dozens of other compounds with direct neurological effects, including gamma-aminobutyric acid and short-chain fatty acids. In animal studies, transferring gut microbiota from anxious mice to germ-free mice has actually transmitted anxiety-like behaviour. Human studies have found correlations between microbiome composition and rates of depression, anxiety, and autism spectrum disorder — though whether those associations are causal remains an open and hotly debated question.

Dietary fibre is the microbiome’s primary fuel source, which makes the collapse of fibre consumption across developed nations over the past century particularly troubling. Pre-industrial populations are estimated to have eaten between fifty and one hundred grams of fibre a day. The typical Western diet today delivers somewhere between fifteen and twenty. The bacteria responsible for producing short-chain fatty acids — compounds that nourish the intestinal lining, reduce inflammation, regulate blood sugar, and appear to protect against colorectal cancer — depend entirely on dietary fibre to survive and function. Without it, they struggle. The most evidence-backed strategy for supporting microbiome health, for now, is also one of the simplest: eat more vegetables, legumes, whole grains, and fermented foods. Feed the bacteria that are, in turn, feeding you.