Abstract

Background: Serious mental illnesses (SMI), including schizophrenia and bipolar disorder, are associated with substantial morbidity and reduced quality of life. Growing understanding of communication between the gut microbiome and brain has generated interest in microbiome-targeted interventions as potential adjunctive treatments. However, evidence across SMI has not been comprehensively mapped.
Aim: To map the existing evidence on gut–brain axis interventions for the treatment of SMI in humans and identify gaps for future research.
Methods: A scoping review was conducted in accordance with JBI methodology and PRISMA-ScR guidance. MEDLINE, Embase, PsycINFO and CINAHL were searched for peer-reviewed human studies published in English between 2016 and 2026. Two reviewers independently screened records and assessed full texts for eligibility.
Results: Of 787 records identified through database searching, 528 underwent title and abstract screening, 21 underwent full-text assessment, and 18 studies were included. Probiotics were the most frequently investigated intervention (55.6%), while synbiotics, dietary interventions, probiotic plus dietary fibre, prebiotics and faecal microbiota transplantation were less studied. Schizophrenia-spectrum populations dominated the evidence base, and only 3 of 18 studies included follow-up.
Conclusion: Research on gut–brain axis interventions for SMI is emerging but concentrated on probiotics and schizophrenia-spectrum disorders. Future studies should broaden the interventions and SMI populations investigated and incorporate longer-term follow-up to strengthen the evidence for their potential use as adjunctive treatments.