Diet and synbiotics in the gut–immune–brain axis: implications for gastrointestinal and psychological health

The gut microbiota participates in gut–immune–brain communication through metabolic, immune, neural and endocrine pathways. Dietary patterns and microbiota-directed interventions, including prebiotics, probiotics and synbiotics, may affect these pathways, but biological responses do not necessarily translate into clinical benefit. We examine mechanistic and human evidence, focusing on disorders of gut–brain interaction, particularly irritable bowel syndrome (IBS), and on psychological outcomes. Selected synbiotic trials report improvements in gastrointestinal symptoms, although formulations, populations, comparators and endpoints differ substantially. Evidence for psychological benefit is less consistent and remains limited by small or uncontrolled studies. Human milk oligosaccharides illustrate the potential for selective substrate use, but their clinical value in adults requires further testing. Future trials should specify the intended clinical effect, select appropriate comparators and measure microbial, metabolic and host responses alongside validated symptoms.