The gut microbiota is a dynamic ecosystem shaped by dietary factors, lifestyle factors, environmental exposures, and host characteristics, with important implications for metabolism, immune regulation, intestinal barrier function, and host-microbiota interactions. This narrative review examines how dietary patterns, specific food components, physical activity, sleep and circadian rhythms, aging, and medication exposure modulate gut microbial composition and function, with particular emphasis on interindividual variability and personalized nutrition. A literature search was conducted in Scopus, ScienceDirect, PubMed, and Google Scholar for publications from 2000 to 2026, with emphasis on recent peer-reviewed evidence. Current evidence indicates that dietary patterns and individual food components can modify microbial community structure, metabolic pathways, and bioactive metabolite production, although responses vary substantially across individuals. Physical activity may modify microbial ecology, intestinal physiology, barrier function, and microbial metabolism, whereas sleep disruption and circadian misalignment may alter microbial rhythmicity and host metabolic regulation. Aging and antibiotic exposure are associated with context-dependent changes in microbial composition, function, resilience, and, with antibiotics, the gut resistome. Across these determinants, interindividual variability emerges as a central feature driven by host characteristics, habitual diet, lifestyle, medication history, baseline microbiota, and microbial functional capacity. Personalized nutrition therefore represents a promising but evolving framework in which microbiome data may be integrated with host and environmental characteristics to predict differential responses. Future research should prioritize longitudinal human studies, standardized methodologies, multi-omics integration, and prospective validation of microbiome-informed interventions to establish clinically meaningful benefits.
