Synergic Elevation of Systemic Inflammation by the Coexistence of Periodontitis and Diabetes Mellitus: A Nationwide Analysis of Korean Adults

<b>Background/Objectives:</b> This study aimed to evaluate the additive effect of periodontitis and diabetes mellitus on systemic inflammation, measured by high-sensitivity C-reactive protein (hs-CRP), in a nationally representative Korean population. <b>Methods:</b> Data from 3178 adults (≥19 years) in the 2015 Korean National Health and Nutrition Examination Survey were analyzed. Periodontitis was assessed using the Community Periodontal Index (CPI), and diabetes mellitus was defined based on clinical criteria. Participants were classified into four groups according to the presence of periodontitis and diabetes. hs-CRP levels were analyzed by quartiles and ADA/CDC cardiovascular risk categories. ANCOVA and multivariable logistic regression, adjusted for socioeconomic status, oral health and health behaviors, and comorbidities, were used to examine the association between coexisting periodontitis and diabetes and elevated hs-CRP. <b>Results:</b> Mean hs-CRP increased progressively from G1 (1.11 ± 0.49 mg/L) to G4 (2.37 ± 0.38 mg/L). After adjustment, G4 retained the highest concentration (2.31 ± 0.39 mg/L) versus G1 (1.37 ± 0.11 mg/L; <i>p</i> = 0.020). High-risk hs-CRP prevalence (>3.0 mg/L) increased nearly threefold across groups (<i>p</i> < 0.001). Similarly, G4 had increased odds of being in the ADA/CDC high-risk category (>3.0 mg/L) (aOR = 2.73, 95% CI: 1.64-4.54), whereas no significant associations were observed for periodontitis or diabetes alone. <b>Conclusions:</b> The coexistence of periodontitis and diabetes mellitus is significantly associated with elevated systemic inflammation, as measured by hs-CRP, suggesting a synergistic effect beyond the impact of either condition alone.