Exercise physiology is evolving from an organ-based framework toward a systems-level understanding, where molecular interactions between muscle, brain, and the gut microbiome critically influence performance and health. This review systematically examines the genetic, molecular, and cellular bases of this triad, with a focus on translational insights for disease prevention and human optimization. A systematic search of PubMed, Embase, and Web of Science was conducted up to October 2023 to identify studies exploring molecular pathways linking skeletal muscle, cognitive/affective function, and gut microbiota in exercise contexts. Inclusion criteria were original research articles investigating at least two components of the muscle-brain-gut axis. Exclusion criteria included non-English articles, conference abstracts, and studies without molecular data. The PRISMA 2020 guidelines were followed. The search strategy is detailed in Supplementary Material. Evidence was categorized into Grades 1 through 4 based on methodological rigor, omics integration, reproducibility, and translational relevance to human physiology and disease models. Analysis included 154 studies encompassing 987 molecular associations. Among these, 59 associations (Grades 1-2) provided robust evidence for genetically and functionally validated pathways, including myokine-mediated (e.g., irisin, BDNF) and microbially derived metabolites (e.g., SCFAs, tryptophan derivatives) that modulate neuroplasticity, mitochondrial function, inflammation, and HPA axis activity. Psychobiological factors influenced microbial composition, illustrating bidirectional gut-brain-muscle signaling. Most associations (n = 952) were limited by methodological variability or insufficient mechanistic depth. The integration of multi-omics platforms (metagenomics, metabolomics, proteomics) emerges as a key tool for personalized exercise interventions and biomarker discovery. This review synthesizes molecular evidence for the muscle-gut-brain axis as an integrative determinant of exercise responsiveness and disease resilience. We highlight genetic and metabolic pathways with diagnostic and therapeutic potential, aligning with the development of molecular tools for precision medicine. Future interdisciplinary research should leverage artificial intelligence and longitudinal omics to translate these mechanisms into targeted strategies for performance enhancement and disease prevention.
