Abstract

Objective
Depressive symptoms have been suggested to increase mortality risk but causality remains unproven. Depressive symptoms increase likelihood of smoking which is thus a potential factor modifying the effect of depressive symptoms on mortality. This study aims to assess if the association of depressive symptoms and all-cause mortality is affected by smoking.

Methods
A prospective cohort study in Finnish primary care setting was conducted among 2557 middle-aged cardiovascular disease (CVD) risk persons identified in a population survey. Baseline depressive symptoms were assessed by Beck’s Depression Inventory (BDI) and current smoking by self-report. Data on mortality was obtained from the official statistics. Effect of depressive symptoms and smoking on all-cause mortality after 14-year follow-up was estimated.

Results
Compared to non-depressive non-smokers, the adjusted hazard ratio (HR) for all-cause mortality was 3.10 (95% CI 2.02 to 4.73) and 1.60 (95% CI 1.15 to 2.22) among smoking subjects with and without depressive symptoms, respectively. Compared to the general population, relative survival was higher among non-depressive non-smokers and lower among depressive smokers. Relative standardized mortality ratio (SMR) for all-cause mortality was 1.78 (95% CI 1.31 to 2.44) and 3.79 (95% CI 2.54 to 6.66) among non-depressive and depressive smokers, respectively, compared to non-depressive non-smokers. The HR for all-cause mortality and relative SMR of depressive non-smokers were not increased compared to non-depressive non-smokers.

Conclusion
Current smoking and increased depressive symptoms seem to additively contribute to excess mortality.

Introduction

Depression and subthreshold depressive symptoms have been related to increased mortality risk [1]. This is plausible as depression intervenes with somatic diseases in complex biological pathways [2] and is considered as a risk factor for major causes of mortality, CVD [3] and cancer [4,5]. Depression and depressive symptoms are also associated with other risk factors such as hypertension [6], metabolic disturbances [7,8], sedentary and unhealthy lifestyle including smoking [[9], [10], [11]], and low socioeconomic status [12]. Nevertheless, there is still controversy weather depressive symptoms per se are associated with mortality. Depression can been considered to namely cause excess mortality through its effect on physical health, social factors, and lifestyle [13]. However, two recent meta-analyses concluded that causality remains unproven, in part due to inadequate adjustments for confounding factors [14,15].

Depression and depressive symptoms increase likelihood of smoking [11] and decrease odds to successful quitting [16]. Current smokers have 2 to 3 times higher mortality than never-smokers [17], and although prevalence of smoking has decreased during the past decades, it still accounts for a substantial loss of life-years [18]. This excess mortality has been attributed to at least 26 different disease categories including several cancers, many cardiovascular (CVD) and respiratory diseases, diabetes, renal failure, and some infections [17].

Results

۳٫۱٫ Characteristics of the subjects
At baseline, 20.6% (106/515) of the subjects with increased depressive symptoms smoked and 16.8% (344/2042) were non-depressive smokers (p = 0.047). The probability of smoking increased from BDI score 10 onwards (Fig. 1).

Baseline characteristics of the subjects are presented in Table 1. Subjects with depressive symptoms were more often women, slightly older, and were living alone more often than non-depressive subjects. They also performed less LTPA, had higher mean AUDIT score, higher BMI and larger WC, higher triglyceride levels, higher 2-h glucose, and more glucose disorders than those without depressive symptoms.