Depressive disorders are highly heterogeneous syndromes characterized not only by depressed mood but also by cognitive impairment, sleep–circadian rhythm disturbances, altered appetite, somatic discomfort, and metabolic or gastrointestinal comorbidities. In recent years, the microbiota–gut–brain axis (MGBA) has been increasingly recognized as an integrative biological framework linking abnormalities in mood regulation, immune responses, endocrine function, metabolism, and neuroplasticity. This r