Major depressive disorder is increasingly recognized as a metabolic-immune disorder in which chronic inflammation diverts tryptophan (Trp) metabolism toward the kynurenine pathway (KP), reducing serot Show more
Major depressive disorder is increasingly recognized as a metabolic-immune disorder in which chronic inflammation diverts tryptophan (Trp) metabolism toward the kynurenine pathway (KP), reducing serotonin synthesis and producing neurotoxic metabolites such as quinolinic acid (QA). Elevated kynurenine (KYN)/Trp ratios and an altered QA/kynurenic acid (KYNA) balance have been consistently reported in depressed individuals, implicating the KP as a key therapeutic target. Exercise provides a unique, translationally relevant intervention: unlike pharmacological agents acting directly on neurotransmission, contracting skeletal muscle acts as a "kynurenine sink" by inducing kynurenine aminotransferases that convert circulating KYN into neuroprotective KYNA, thereby reducing brain KYN uptake and mitigating excitotoxicity. Clinical studies and meta-analyses confirm that aerobic, resistance, and high-intensity training produce antidepressant effects comparable to pharmacotherapy, while also improving cognition, fatigue tolerance, and cardiometabolic function. Beyond KP remodeling, exercise-induced myokines (irisin, IL-6, BDNF, apelin, FGF21) and adipokines (adiponectin, leptin modulators) coordinate systemic anti-inflammatory and neurotrophic adaptations that enhance resilience and brain plasticity. Furthermore, pharmacological "exercise mimetics" and metabolic modulators, such as PPAR agonists, AMPK activators, NAD Show less