“Attacking” the Gut–Brain Axis with Psychobiotics: An Umbrella Review of Depressive and Anxiety Symptoms

Background/Objectives: This umbrella review critically evaluates the available evidence on psychobiotics for depressive and anxiety symptoms, emphasizing methodological quality, consistency of findings, and persistent gaps in the literature. Methods: A comprehensive search was conducted across PubMed/MEDLINE, Scopus, Web of Science, SciELO, Cochrane, and EBSCO (May–June 2025) to identify systematic reviews with meta-analyses of randomized controlled trials examining probiotic, prebiotic, and synbiotic interventions in adults with depressive and/or anxiety symptoms or diagnoses. Two reviewers independently screened studies, extracted data, and evaluated methodological quality using AMSTAR-2. Additional bibliometric, conceptual, and psychometric features were mapped, including geographical origin, publication timeline, scale distribution, and citation-based connectivity. Results: Thirty systematic reviews and meta-analyses were included. Methodological quality was predominantly moderate, low, or critically low in 76.6% of reviews. Probiotic interventions demonstrated consistent benefits for MDD (SMD = −0.50 [95% CI: −0.58 to −0.42], p = 0.0001). However, findings for anxiety were markedly inconsistent, despite the modest improvements in specific subgroups (SMD = −0.19 [95% CI: −0.28 to −0.10]; p < 0.01). Prebiotics for MDD interventions showed limited positive results (SMD = −0.25 [95% CI: −0.47 to −0.03]; p = 0.03). For anxiety, the effects are inconclusive (SMD = −0.07 [95% CI: −0.30 to 0.10]; p = 0.18). Evidence for synbiotics was scarce. Citation-mapping revealed a fragmented and unevenly connected evidence base. Conclusions: The current evidence suggests that probiotics may confer beneficial effects on depressive and anxiety symptoms; however, the same cannot be said for prebiotics and synbiotics. Evidence for the efficacy of prebiotics and synbiotics to treat depression and anxiety is still insufficient or heterogeneous. Registration: CRD420251164884.