Higher abundance of Bilophila wadsworthia, a sulfite-reducing bacterium that proliferates on high-fat diets, correlates with reduced volumes in specific brain regions controlling movement and reward processing. Using brain MRI and microbiome profiling across two population cohorts, researchers found this bacterium specifically associated with atrophy in the globus pallidus and nucleus accumbens—core basal ganglia structures. The bacterium also linked to elevated liver enzymes, triglycerides, and inflammatory markers. This represents a significant advancement in gut-brain axis research, providing the first direct evidence linking a specific dietary-responsive microbe to measurable brain structural changes in healthy humans. The findings carry profound implications for understanding how processed food consumption may accelerate neurodegeneration. While the cross-sectional design cannot establish causation, the specificity to basal ganglia regions—areas affected early in Parkinson's disease—suggests B. wadsworthia abundance could serve as an early biomarker for neurodegenerative risk. The metabolic modeling revealing enhanced bile acid metabolism and TMAO pathway modulation provides mechanistic insight into how this microbe might influence brain health through inflammatory and vascular pathways.