2026 Volume 2 Issue 1 Pages 137-153
Background: Major Depressive Disorder (MDD) is recurrent and disabling, and it frequently coexists wit h functional gastrointestinal disorders such as Irritable Bowel Syndrome (IBS) and with altered bowel habi ts. The two conditions are bidirectionally related, and depression may be associated with roughly twice the risk of comorbid or incident IBS, so gastrointestinal and mood symptoms routinely intersect in practice. Int erest in the gut-brain axis is high, yet existing reviews emphasize molecular mechanisms and offer limited guidance for assessing bowel function and diet in outpatient care.
Objective: This review critically integrates basic, observational, and interventional studies, meta-analyses, and guidelines, and considers the gastrointestinal adverse effects of antidepressants and their bidirectional interactions with the gut microbiome, to propose a feasible outpatient framework for assessment and suppo rt.
Main Text: The gut-brain axis has been suggested to operate through vagal, immune-inflammatory, hypot halamic-pituitary-adrenal, short-chain fatty acid, tryptophan‒kynurenine pathway, and bile-acid routes, but most evidence derives from animal and cross-sectional work, so human causality remains unclear. Observ ational data fairly consistently link healthy dietary patterns with lower depression risk and ultra-processed foods with higher risk, though correlation does not imply causation. Dietitian-led Mediterranean-style inter ventions (SMILES, AMMEND) may improve depression, whereas a preventive trial (MooDFOOD) was n egative and omega-3 supplementation shows limited benefit. Psychobiotics appear promising but use heter ogeneous strains, and faecal microbiota transplantation remains experimental. Selective serotonin reuptake inhibitors may slightly increase gastrointestinal bleeding, which rises with concomitant non-steroidal anti-i nflammatory drugs.
Conclusion: Assessing and supporting bowel function and diet in outpatient practice is feasible and clinica lly meaningful, using brief screening, collaborative care, motivational interviewing, and stepwise dietary m odification adapting Japanese cuisine. However, dietary and microbiome interventions complement rather t han replace standard pharmacotherapy and psychotherapy; evidence is still limited, microbiome-targeted te sting cannot yet be widely recommended, and cautious application distinguishing correlation from causatio n is required.
