The gut microbiome is increasingly recognized as a modifiable contributor to metabolic, immune, and stress-related physiology, yet many nutritional interventions produce broad microbial shifts that may be poorly tolerated. We investigated the effects of a fermented and pasteurized oat-based preparation (Keystone Postbiotic®) on microbiome composition and selected biomarkers in a 4-week randomized, double-blind, placebo-controlled trial in generally healthy adults.