Exercise-Mediated Molecular Mechanisms of Antidepressant Effects: The Role of the Microbiota–Gut–Brain Axis

Depression is a prevalent and complex neuropsychiatric disorder that substantially impairs quality of life worldwide. Current pharmacological and psychotherapeutic interventions are often limited by inadequate efficacy and restricted accessibility. Exercise has emerged as a potent non-pharmacological alternative, exhibiting antidepressant effects comparable to conventional treatments, with the microbiota–gut–brain (MGB) axis increasingly recognized as a key mediator. Despite growing interest, existing research articles largely describe associations without mapping the molecular pathways from exercise-altered microbes to neurobiological changes in depression. This review addresses these gaps by comprehensively dissecting the molecular cascades from microbial remodeling to synaptic and systemic adaptations, thereby providing a mechanistic roadmap absent in current syntheses. We first delineate how gut dysbiosis contributes to depression via endocrine, neural, and immune pathways. Subsequently, we highlight the ability of exercise to reverse these pathological states by restoring microbial diversity, reinforcing intestinal barrier function, and modulating the production of microbial metabolites. These exercise-driven changes collectively attenuate systemic and neuroinflammation, enhance brain-derived neurotrophic factor signaling, augment serotonin synthesis, and dampen hypothalamic–pituitary–adrenal axis hyperactivity. By reinstating intestinal homeostasis, exercise acts as a central regulator of the MGB axis and represents a promising therapeutic strategy. This integrative framework offers novel insights into depression pathogenesis and opens avenues for multifaceted intervention.