Associations of self-reported smoking status, serum cotinine level and a composite indicator of smoke exposure with depressive symptoms in middle-aged and elderly U.S. individuals: a cross-sectional study based on the National Health and Nutrition Examination Survey, 2005–2018

BACKGROUND: Depression is a common mental illness among middle-aged and elderly people. However, few studies have explored the relationship between smoke exposure and depression in those aged ≥ 45 years. This study aims to determine the association between the level of smoke exposure and depression in middle-aged and elderly individuals. METHODS: We extracted the data from the National Health and Nutrition Examination Survey (NHANES) 2005-2018, focusing on U.S. participants aged 45 years or older with complete data. Depression was assessed using the Patient Health Questionnaire-9 (PHQ-9), based on Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) criteria. Combusted cigarette smoke exposure was defined in three ways: self-reported smoking status, serum cotinine level, and a composite indicator combining self-reported smoking status, serum cotinine level, use of nicotine-containing products in the last five days, and household smokers. Three multivariable binary logistic regression models (Models 1-3) were established based on the three definitions of smoke exposure, and restricted cubic spline (RCS) analysis was used. RESULTS: This study included 9,571 participants aged ≥ 45 years, with 747 (7.80%) showing depressive symptoms. Model 1 indicated that current smokers were more likely to experience depression (compared to non-smokers, current smokers: OR of being depressed = 2.03 [95% CI: 1.65 ~ 2.49], P < 0.001). Model 2 showed that individuals with serum cotinine levels > 3.08 ng/ml were at increased risk for depression (compared to serum cotinine level < 0.05 ng/ml, serum cotinine level > 3.08 ng/ml: OR = 1.99 [95% CI: 1.63 ~ 2.43], P < 0.001). In Model 3, active smokers were more prone to depression (compared to no smoke exposers, active smokers: OR = 2.26 [95% CI: 1.80 ~ 2.84], P < 0.001). The RCS regression revealed that serum cotinine level and depression showed a linear relationship. CONCLUSIONS: In the U.S. population aged 45 and older, higher levels of smoke exposure were associated with an increased risk of depression. Increasing public attention and caring for mental health of middle-aged and elderly individuals, especially females, and reducing exposure to smoke will minimize the incidence of depression.