The Gut Microbiome in Aging
The composition of the gut microbiome changes with age for reasons that are not fully explored, but likely involve the progressive failure of the immune system to remove problem microbes. Inflammatory microbial species grow in number while microbial species producing metabolites necessary to tissue function are reduced in number. There is considerable evidence for this change of composition to contribute to degenerative aging, but in humans that evidence is all correlational. From a scientific perspective there is thus a great deal of work yet to be accomplished to obtain a firm map of the problem and how exactly the problem contributes to specific conditions and aging more generally. Nonetheless, even prior to that point it is sensible to try to further develop and run clinical trials for approaches to rejuvenation of the gut microbiome such as flagellin immunization and fecal microbiota transplantation.
Age-related changes in gut microbiota metabolism are heterogeneous across populations and pathways, and their relationships with immunometabolism and organ phenotypes should be interpreted in the context of health status and disease. Microbial abundance or the concentration of a single metabolite does not, by itself, capture microbial production, host exposure, or effects in target tissues. This narrative review summarizes upstream influences, including gastrointestinal physiology, dietary substrates, comorbidities, and medications, and discusses the origins and host processing of major metabolites, their immunometabolic effects, and associated organ phenotypes.
Studies in some older populations report lower fecal short-chain fatty acid concentrations. Microbiota-derived indole derivatives and host-synthesized, microbiota-modified bile acids show molecule- and population-specific patterns. Circulating concentrations of some host-microbe co-metabolites are higher in selected older or disease populations, but are also shaped by host biotransformation and clearance. Human evidence is predominantly observational and links metabolic features to health status or clinical phenotypes, whereas animal and cell studies support effects on barrier function, inflammation, and cellular metabolism under specific experimental conditions. Personalized nutrition, metabolite supplementation, selective targeting of microbial enzymes, and multi-omics-based responder stratification are promising research directions; prospective evidence that these strategies improve functional outcomes in older adults remains limited.