Gut Microbial Function and Immune Aging: An Evidence-Graded Framework Linking Microbial Metabolites to Healthspan and Age-Related Disease

This narrative review examines how the metabolic output of the intestinal microbiota, rather than its taxonomic composition alone, relates to immune aging and to clinically meaningful geroscience outcomes. We organise the evidence around four questions: which microbial features are reproducibly associated with older age, which host pathways plausibly mediate their effects, how strong the evidence is for each mechanistic step, and what this implies for intervention. Three points structure the synthesis. First, the microbiota of older adults is not a single "dysbiotic" state: much of the reported difference between younger and older cohorts is attributable to frailty, multimorbidity, polypharmacy, residential setting, diet, physical activity, bowel transit and geography rather than to chronological age, and healthy-longevity phenotypes frequently diverge from the frail-elderly phenotype. Second, the dominant mediators-short-chain fatty acids (SCFAs), bile acids, tryptophan and polyphenol derivatives, polyamines and trimethylamine N-oxide (TMAO)-are context-dependent rather than uniformly protective or harmful, their host effects varying with local versus systemic concentration, receptor distribution, immune state, hepatic and renal clearance, and measurement platform. Third, most human data are cross-sectional; causal inference rests largely on rodent transfer experiments whose translational relevance to human healthspan(definition) is unresolved. We therefore label the evidence type supporting each major claim, compare cardiometabolic, neurodegenerative, hepatic, musculoskeletal and oncological domains, and appraise microbiota-targeted interventions with attention to strain, dose and formulation specificity, replication failure, endpoint heterogeneity, and safety in frail or immunocompromised older adults. We conclude by proposing an evidence-graded framework mapping defined microbial functions onto discrete features of immune aging and onto validated geroscience endpoints, and by identifying the designs required to move from association to intervention. The causal and clinical evidence base remains limited, and changes in microbiome composition should not be equated with gains in healthspan.