A Dutch research team has designed a crossover trial testing whether an anti-inflammatory dietary pattern called the BrAIN diet can improve functioning across four brain disorders: bipolar disorder, schizophrenia spectrum disorders, Parkinson's disease, and Alzheimer's disease. The 100-participant study uses 12-week intervention periods with home-delivered food boxes and dietitian counseling, measuring outcomes including global functioning, cognition, gut microbiome composition, and inflammatory markers. This transdiagnostic approach represents a significant shift in psychiatric and neurological research, moving beyond disorder-specific treatments toward interventions targeting shared biological mechanisms. The gut-brain axis has emerged as a promising therapeutic target, with mounting evidence linking microbiome dysregulation and systemic inflammation to multiple brain disorders. If effective, dietary interventions could offer accessible, low-risk adjunct treatments for conditions that traditionally rely heavily on medications with significant side effects. However, this is a protocol paper describing study design rather than presenting results, and the open-label design may introduce bias. The crossover methodology with extended washout periods should help control for individual differences. As a preprint awaiting peer review, the methodology and eventual findings will require validation through the peer-review process.