Systemic inflammation mediates the association between environmental tobacco smoke and depressive symptoms: A cross-sectional study of NHANES 2009–2018

Depression is associated with both environmental tobacco smoke (ETS) and inflammation. However, whether systemic inflammation mediates the ETS-depression relationship is unclear. We analyzed 19,612 participants from the 2009–2018 National Health and Nutrition Examination Survey (representing approximately 206,284,711 USA individuals), utilizing data of depressive symptoms (assessed by Patient Health Questionnaire-9), blood cotinine level (an ETS biomarker), dietary inflammatory index (DII, assessed by 24-h dietary recall) and inflammation, represented by immune-inflammation index (SII) and systemic inflammation response index (SIRI). Weighted multivariable logistic regression showed that a higher blood cotinine level is significantly associated with a higher depressive symptoms risk ( OR  = 1.79, 1.35–2.38). After adjusting for covariates, the effect in smokers ( OR  = 1.220, 95   % CI: 1.140–1.309) is larger than that in non-smokers ( OR  = 1.150, 95   % CI: 1.009–1.318). Compared to the lowest level, depressive symptoms risks in participants with the highest level of SII, SIRI and DII are 19 % ( OR  = 1.19, 1.05–1.35), 15 % ( OR  = 1.15, 1.01–1.31) and 88 % ( OR  = 1.88, 1.48–2.39) higher, respectively. Weighted linear regression demonstrated positive correlations of SII ( β  = 0.004, 0.001–0.006), SIRI ( β  = 0.009, 0.005–0.012) and DII ( β  = 0.213, 0.187–0.240) with blood cotinine level. Restricted cubic splines model showed a linear dose-response relationship between blood cotinine and depressive symptoms ( P non-linear  = 0.410), with decreasing risk for lower DII. And SII and SIRI respectively mediate 0.21 % and 0.1 % of the association between blood cotinine and depressive symptoms. Cross-sectional design, and lack of medication data for depression. Positive association of ETS (blood cotinine) with depressive symptoms risk is partly mediated by systemic inflammation, and anti-inflammatory diet could be beneficial.