Dietary Pattern-Induced Gut Microbiota Differences Are Associated with White Matter Volume Changes in Middle-Aged Female Macaques

Background/Objectives: Western and Mediterranean diets have divergent effects on the brain. The gut microbiome may mediate diet effects, and specific microbes may be particularly significant contributors to these processes. Oscillospira, a genus of gut-dwelling bacteria, has been implicated as a key microbial target. Other peripheral contributors may include short-chain fatty acids (SCFAs), branched-chain amino acids (BCAAs), insulin resistance, and microbial translocation. Methods: We determined the effects of long-term (31 months, ~9 human years) consumption of a Mediterranean or Western-type diet on Oscillospira abundance, fecal SCFAs, plasma BCAAs, soluble CD14 (sCD14), and insulin responses in a randomized trial of 38 middle-aged female cynomolgus macaques (Macaca fascicularis). We determined diet effects and associations between dependent variables. For variables that were affected by diet composition and significantly associated with Oscillospira, we tested whether Oscillospira abundance mediated the effects of diet. Results: The Mediterranean diet resulted in higher Oscillospira (p = 0.004) and SCFAs (acetate p = 0.002; propionate p = 0.049) and lower BCAAs (isoleucine p = 0.035; leucine p = 0.007; valine p < 0.001). The Western diet increased insulin resistance (p = 0.040) and WM loss (p = 0.011). Oscillospira abundance was negatively associated with BCAAs (leucine p = 0.007; valine p = 0.005) and insulin resistance (insulin AUC: p = 0.024; increase in insulin AUC from pretreatment: p = 0.020), with trends for isoleucine (p = 0.066) and sCD14 (p = 0.103). Oscillospira abundance was positively associated with acetate (p = 0.032) and WM volume changes (p = 0.012). Oscillospira abundance significantly mediated the effects of diet on white matter volume changes (p = 0.020) and on insulin resistance (insulin AUC: p = 0.012 at study end; increase in insulin AUC during study: p = 0.020), presenting potential pathways through which diet may influence the brain. Conclusions: These findings suggest that diet-driven differences in Oscillospira are linked to metabolic regulation and white matter integrity, and Oscillospira may mediate the relationships. The results highlight a potential role for diet–microbiome interactions in shaping metabolic and brain aging trajectories.