Analysis of 784 Chinese adults aged 70-98 revealed that a modified healthful plant-based diet index (mHPDI) strongly improved global cognition, but only in individuals with non-Prevotella-dominant gut enterotypes (β = 0.34 vs 0.04 for Prevotella-dominant types). The diet altered 15 microbial species and 17 genera, with 12 circulating metabolites—primarily branched-chain amino acids and short-chain fatty acids—mediating 11% of the cognitive benefits. This represents a paradigm shift in nutritional neuroscience, demonstrating that individual gut microbiome composition fundamentally determines dietary efficacy for brain health. The finding explains why plant-based diet studies often show inconsistent cognitive outcomes—the same intervention produces dramatically different results depending on baseline enterotype. For aging adults, this suggests personalized nutrition based on microbiome profiling could optimize cognitive protection. However, the study's limitation to a rural Chinese population raises questions about generalizability to Western populations with different baseline microbiomes and dietary patterns. The mechanistic insight into amino acid and SCFA pathways provides clear targets for microbiome-informed interventions.