Microbiome-Derived Metabolic Signaling in the Gut–Skin Axis: Implications for Nutricosmetics and Healthy Skin Aging

The gut–skin axis represents a bidirectional communication network linking the gut microbiota, microbial metabolism, immune regulation, and skin physiology. Natural bioactives derived from dietary plants, mushrooms, fermented foods, marine resources, and functional ingredients may influence skin health through both direct nutritional effects and host–microbiome metabolic interactions. This review provides a microbiome-centered, systems-level perspective on the relationships among dietary bioactives, gut microbial biotransformation, microbiome-derived metabolites, and skin aging-associated physiology. Particular attention is given to polyphenols, polysaccharides, carotenoids, peptides, fatty acids, probiotics, postbiotics, and fermented bioactive compounds as modulators of microbial composition and metabolic activity. Microbial metabolites, including short-chain fatty acids (SCFAs), indole derivatives, urolithins, phenolic metabolites, bile acid derivatives, and lipid-associated metabolites, are discussed as potential mediators linking diet and microbial metabolism with skin physiology. Their relevance to oxidative stress, inflammation, mitochondrial function, epidermal barrier homeostasis, extracellular matrix remodeling, and cellular senescence is critically evaluated according to the available level of evidence. Human intervention studies provide preliminary clinical support for selected probiotic and oral nutricosmetic interventions; however, direct evidence establishing microbiome-mediated effects on skin outcomes remains limited. Interindividual differences in microbial metabolic capacity may further contribute to heterogeneous responses to dietary interventions. Overall, this evidence-centered appraisal reveals a substantial gap between mechanistic plausibility and demonstrated human causality, emphasizing the need for studies that directly test microbiome mediation using integrated microbiome, metabolite, and skin-specific outcome measurements.