Analysis of 915 visits from 349 adults over six years revealed that butyrate-producing gut bacteria consistently correlated with lower carotid-femoral pulse wave velocity, a gold-standard measure of arterial stiffness. Higher microbiome evenness also associated with more flexible arteries, while trimethylamine-producing genes showed the opposite pattern. The study tracked both between-person differences and within-person changes over time using whole genome metagenomic sequencing. This represents the first robust human longitudinal evidence connecting specific microbial metabolic pathways to cardiovascular aging, building on extensive mouse studies. The butyrate connection aligns with emerging research on this short-chain fatty acid's anti-inflammatory properties and its role in endothelial function. However, the observational design cannot establish causation, and the within-person associations were often weaker than between-person differences, suggesting individual microbiome changes may be harder to detect than baseline variations. The identification of specific pathogenic bacteria and metabolic pathways offers concrete targets for interventions. Given that arterial stiffness predicts cardiovascular events and mortality, these findings could guide microbiome-based strategies for healthy aging, potentially through targeted probiotics or dietary interventions that boost butyrate production.