The role of C-reactive proteins in tobacco smoke exposure and the risk of depression

This study investigates whether Tobacco smoke exposure (TSE) is associated with C-reactive protein (CRP) and depression and whether CRP plays a mediating role. The data set, including 8,917 adults aged more than 18 years old, was provided by US National Health and Nutrition Examination Survey during 2005-2010 and 2015-2018. A logistic-regression-based mediation analysis was applied to clarify the direct effect of serum cotinine on depression and the indirect effect mediated by CRP. The results indicated that serum cotinine was positively correlated with the risk of depression (Q4 vs. Q1, <i>OR</i>_{Model 1} = 2.72, 95% <i>CI</i>: 1.95-3.82; <i>OR</i>_{Model 2} = 1.62, 95% <i>CI</i>: 1.10, 2.37 <i>OR</i>_{Model 3} = 1.58, 95% <i>CI</i>: 1.07, 2.34). Active smoking was associated with the risk of depression (<i>OR</i>_{Model 2} = 1.69, 95% <i>CI</i>: 1.23, 2.31; <i>OR</i>_{Model 3} = 1.66, 95% <i>CI</i>: 1.21, 2.28), while passive smoking was not significantly associated with the risk of depression after adjusting for covariates (<i>OR</i>_{Model 2} = 1.02, 95% <i>CI</i>: 0.72, 1.44; <i>OR</i>_{Model 3} = 1.02, 95% <i>CI</i>: 0.72, 1.44). Increased serum cotinine level was associated with an increased risk of depression, and the effect (4.95%) can be explained by a significant indirect effect of CRP (<i>OR</i> = 3.80 × 10^-6, 95% <i>CI</i>: 2.81 × 10^-7, 8.52 × 10^-6). The findings suggest that anti-inflammation may be a potential goal for depression intervention among the tobacco-smoke-exposed population.