<b>Nonlinear Association Between Quercetin Intake and Periodontitis in US Adults: A Cross-Sectional Analysis of NHANES</b>

<b><i>Introduction</i></b>: Quercetin, a common dietary flavonol, has been shown in numerous animal studies to possess anti-inflammatory and antioxidative properties. However, its relationship with periodontitis in the general population, particularly regarding potential nonlinear patterns, remains insufficiently explored.<b><i>Methods</i></b>: This cross-sectional study included 2,699 U.S. adults aged ≥30 years from the 2009–2010 National Health and Nutrition Examination Survey (NHANES). Intake of total flavonols and four subclasses (isorhamnetin, kaempferol, myricetin, and quercetin) was estimated based on 24-hour dietary recall. Multivariable logistic regression models were applied to evaluate the associations between flavonol intake (quartiles) and moderate/severe (mod/sev) periodontitis. Nonlinearity was assessed using restricted cubic spline (RCS) regression. Subgroup and sensitivity analyses were performed, and the predictive performance was evaluated using receiver operating characteristic (ROC) curves.<b><i>Results</i></b>: Among total flavonols and their subclasses, only quercetin intake showed a significant association with mod/sev periodontitis. In fully adjusted models, higher quercetin intake was associated with reduced odds of mod/sev periodontitis (Q2 vs Q1: OR = 0.62, 95% CI: 0.39–0.98, <i>P</i> = 0.042; Q3 vs Q1: OR = 0.64, 95% CI: 0.44–0.94, <i>P</i> = 0.026; <i>P</i> for trend = 0.578). Subgroup and sensitivity analyses confirmed the robustness of the findings. RCS regression revealed a significant nonlinear association (<i>P</i> for nonlinearity = 0.003). The inflection points of the RCS curve were aligned with the quartile distribution of quercetin intake. Moreover, quercetin intake demonstrated better predictive ability for mod/sev periodontitis compared to total flavonol and other subclass intakes in ROC analysis.<b><i>Conclusion</i></b>: Quercetin intake shows a nonlinear inverse relationship with mod/sev periodontitis in U.S. adults. These findings suggest that appropriately increasing dietary quercetin may be a promising strategy for periodontal disease prevention.