The association between gut microbiota and cognitive decline: A systematic review of the literature

• Gut microbiota modulation improves cognition in adults with early impairment. • Diet, probiotics, and faecal microbiota transplantation share mechanistic pathways. • Integrated evidence clarifies how microbiota-targeted strategies support cognitive health. Increasing evidence implicates the gut–brain axis in the pathogenesis of neurodegenerative disorders. Age-related alterations in gut microbiota composition may influence cognitive decline and dementia progression; however, the efficacy of microbiota-targeted interventions in improving cognitive outcomes remains unclear. A systematic review of randomized controlled trials and quasi-experimental studies published up to June 2025 was conducted through comprehensive searches of PubMed, COCHRANE, CINAHL, Web of Science and EMBASE. Eligible studies included participants aged over 45 years with cognitive impairment or at risk of developing dementia and evaluated the cognitive effects of gut microbiota modulation through probiotics, faecal microbiota transplantation, or dietary interventions. Data extraction and quality appraisal were conducted using the JBI Critical Appraisal Checklist. Methodological quality ranged from moderate to high. Data were synthesized narratively and cognitive outcomes were measured using validated assessment tools, including the MMSE, MoCA, and RBANS. Fifteen studies comprising 4,275 participants from Europe, Asia, North America, and the Middle East met the inclusion criteria. Interventions such as Mediterranean and ketogenic diets, probiotic supplementation and faecal microbiota transplantation (FMT) demonstrated improvements in memory, executive function, and global cognition, particularly in individuals with early cognitive impairment. Cognitive benefits were associated with enhanced microbial diversity, increased short-chain fatty acid (SCFA) production, and reduced neuroinflammation. Effects were minimal in advanced Alzheimer’s disease. Gut microbiota modulation emerges as a promising non-pharmacological approach to support cognitive function, especially in prodromal or mild cognitive impairment. Early intervention appears to be key to optimizing outcomes. Further large-scale, longitudinal RCTs are warranted to validate these findings and elucidate underlying mechanisms. This review of 15 trials examines diets, probiotics, and FMT on cognition. These interventions regulate the gut–brain axis through SCFA production, reducing neuroinflammation. Findings indicate improved memory and executive function, specifically in Mild Cognitive Impairment (MCI). Data suggests microbiota modulation is more effective in early stages than in advanced Alzheimer’s disease.