The Immune Mind: Linking Dietary Patterns, Microbiota, and Psychological Health

Background/Objectives: Nutritional patterns influence the gut-brain axis and immune signaling with potential consequences for depression and anxiety. We conducted a review focused on clinically meaningful psychiatric outcomes (symptom severity/diagnosis) to synthesize recent evidence (2020-2025) on Mediterranean-style dietary interventions; ultra-processed food (UPF) exposure; and psychobiotic/prebiotic strategies, integrating mechanistic insights relevant to practice. Methods: Searches in PubMed/MEDLINE, Scopus, and Web of Science (January 2020-October 2025) combined terms for diet, Mediterranean diet (MD), UPF, microbiota, probiotics, psychobiotics, depression, and anxiety. Eligible designs were randomized/controlled trials (RCTs), prospective cohorts, and systematic reviews/meta-analyses reporting clinical psychiatric outcomes in adults. We prioritized high-quality quantitative syntheses and recent RCTs; data were extracted into a prespecified matrix and synthesized narratively. Results: Recent systematic reviews/meta-analyses support that MD interventions reduce depressive symptoms in adults with major or subthreshold depression, although large, long-term, multicenter RCTs remain a gap. Exposure to UPF is consistently associated with higher risk of common mental disorders and depressive outcomes in large prospective cohorts. Psychobiotics (specific probiotic strains and prebiotics) show small-to-moderate benefits on depressive symptoms across clinical and nonclinical samples, with heterogeneity in strains, dosing, and duration. Mechanistic reviews implicate microbiota-derived metabolites (short-chain fatty acids) and immune-inflammatory signaling (including tryptophan-kynurenine pathways) as plausible mediators. Conclusions: Clinically, emphasizing Mediterranean-style dietary patterns, reducing UPF intake, and considering targeted psychobiotics may complement standard psychiatric care for depression. Future work should prioritize adequately powered, longer RCTs with standardized dietary protocols and microbiome-informed stratification to clarify responders and mechanisms.