Physical Activity, Dietary Inflammatory Potential, and Depressive Symptoms Among U.S. Adults: The Potential Contribution of Oral Microbial Dysbiosis

Background/Objectives: Physical activity and dietary inflammatory potential are modifiable correlates of depressive symptoms. We examined whether oral microbial dysbiosis—an imbalance in oral microbial community structure—was associated with an integrated lifestyle profile and depressive symptoms in U.S. adults. Methods: We analyzed 6519 adults aged 20–69 years from NHANES 2009–2012. The Integrated Lifestyle Score combined standardized Dietary Inflammatory Index and leisure-time physical activity; higher scores indicated higher dietary inflammatory potential and lower physical activity. Oral dysbiosis was assessed using a study-specific genus-level log-ratio index. Survey-weighted regression models were fitted, and indirect associations were evaluated using 5000 bootstrap resamples. Results: A less favorable lifestyle profile, characterized by higher dietary inflammatory potential and lower leisure-time physical activity, was associated with higher PHQ-9 scores (β = 0.314, 95% CI 0.113–0.515, p = 0.005), greater odds of clinically relevant depressive symptoms (PHQ-9 ≥ 10; OR = 1.355, 95% CI 1.159–1.584, p < 0.001), and greater oral dysbiosis (β = 0.191, 95% CI 0.050–0.331, p = 0.011). Greater oral dysbiosis was associated with higher PHQ-9 scores (β = 0.101, 95% CI 0.059–0.144, p < 0.001) and greater odds of clinically relevant depressive symptoms (OR = 1.069, 95% CI 1.035–1.105, p < 0.001). The indirect association for continuous PHQ-9 was 0.0188 (bootstrap 95% CI 0.0033–0.0402), representing approximately 6.0% of the total association. Conclusions: A less favorable lifestyle profile was associated with greater depressive symptom burden and oral microbial imbalance. Oral dysbiosis statistically accounted for a small proportion of this association. Given the cross-sectional design and study-specific indices, these findings should be interpreted as exploratory rather than causal.