Abstract
<jats:p>The human gut microbiota is now recognised as a dynamic ecosystem essential to host metabolism, immunity, growth, and neurodevelopment. Advances in sequencing and multi-omics have revealed its vast genetic and functional diversity, illustrating how microbial metabolites, including short-chain fatty acids (SCFA), bile acid (BA) derivatives, vitamins, and neuroactive compounds, modulate host physiology through metabolic, endocrine, immune, and gut–brain pathways. Microbiota composition shifts across the lifespan, shaped by genetics, diet, geography, medication exposure, and early-life environmental factors. When the gut microbiota is disrupted, it can weaken the gut barrier, drive inflammation, and disturb metabolic and brain-related signaling, contributing to a wide range of diseases. Early childhood represents a particularly sensitive window, as microbial immaturity and reduced diversity are linked to impaired growth. Despite rapid advances, key challenges remain, including establishing causal relationships, standardising analytical methods, and integrating longitudinal, multi-omics data across diverse populations. This review consolidates current evidence on gut microbiota composition, function, and health effects to clarify how microbial activity shapes human health and development.</jats:p>