Depressive disorder is a common mental illness and is considered a leading cause of major health problems worldwide [1]. Principal clinical manifestation of depression is recurring episodes of depressed mood, wherein each episode is marked by lowering of mood, reduction of energy and decrease in activity [2]. Until very recently, depression has been deemed a purely mental disorder whose clinical consequences fell solely in the hands of psychologists and psychiatrists [3]. But chronic depressive mood can lead not only to a mental illness but also to deteriorated physical health status [4]. Recent epidemiological studies have revealed that several metabolic disorders such as insulin resistance, established diabetes, alcoholism, inflammatory conditions and vascular disorders such as arterial hypertension may be associated with depression [3].

Cardiovascular disease (CVD) refers to those conditions such as coronary heart disease, cerebrovascular disease and peripheral arterial disease that affect the heart and blood vessels [5]. Depression is associated with changes in individuals’ health status that might in turn influence the development and progress of CVD and promote unhealthy behaviors of individuals at risk of developing CVD, such as tentativeness in modifying lifestyle, noncompliance with medical recommendations and predisposition to lifestyle choices like smoking cigarettes [67]. Several studies have shown depression and its associated symptoms to be major risk factors for both development of CVD and death after a myocardial infarction [8]. Depressed patients faced a significantly higher risk of cardiac morbidity and mortality even in an absence of a diagnosis of major depressive disorder (MDD) [9]. The strongest association was observed among the case of severe CVDs; during 6.6 years of follow up, the risk of CVD death and coronary death was found to be high among depressed people regardless of whether they were diagnosed with CVDs or not at the entry [10]. In addition, meta-analysis integrating longitudinal evidences from 21 studies involving over 120,000 subjects also suggested that depression results in a 80–90% increased risk of onset of CVD [11].

Now, although depression is considered as a known risk factor for CVD, as well as an independent predictor of poor prognosis following a cardiac event, the mechanistic relationship between the 2 remains unclear [6]. In addition, although there are many studies investigating the association between depression and CVD, most have scrutinized this relationship on a rather narrower and specific study population such as clinically diagnosed depressive patients, CVD patients or inpatients. By focusing our research on general population of Korea, we inspected the association between depression and CVD risk on a broader population, drawing a contrast in respect to previous researches. Further, we also aimed to seek association of depression with cardiovascular risk factors that were used as components of the formula for calculating CVD risk.

While many studies have shown the association between depression and CVD in the past, most studies were carried out on very specific and select population such as patients who were diagnosed with depression or CVD or who were inpatients. Hence, focusing our research on the data that represents general population of Korea, we delved into the association between depression and CVD risk by comparing the mean CVD risk of normal, MD and MSD groups. Further, by calculating AORs of cardiovascular risk factors that are incorporated in calculating CVD risk in respect to MD group and MSD group compared to normal group, we tried to delineate the association of depression with each component.

Our study revealed that both male and female MSD groups faced a significantly raised mean CVD risk when compared to their normal counterparts. Further, there was a tendency for CVD risk to increase as the level of depression worsened in both men and women. We found that whether depression was diagnosed or not, depressive symptoms were associated with increased potential risk of CVD occurrence in general population of Korea. Also, depression was associated with cardiovascular risk factors such as high BMI, large WC, current smoking, taking antihypertensive medication, increased FBS and taking diabetes medication. Though limited findings on the association between depression and cardiovascular risk factors were shown in our study, we could find other studies that support the relationship between depression and those risk factors.

In this study, the relationship between age and depression is analyzed, looking for effects of maturity, decline, life-cycle stage, survival, and historical trend. The data are from a 1990 sample of 2,031 U.S. adults and a 1985 sample of 809 Illinois adults. The results show that depression reaches its lowest level in the middle aged, at about age 45. The fall of depression in early adulthood and rise in late life mostly reflects life-cycle gains and losses in marriage, employment, and economic well-being. Depression reaches its highest level in adults 80 years old or older, because physical dysfunction and low personal control add to personal and status losses. Malaise from poor health does not create a spurious rise of measured depression in late adulthood. However, some of the differences among age groups in depression reflect higher education in younger generations, and some reflect different rates of survival across demographic groups that also vary in their levels of depression.In this study, the relationship between age and depression is analyzed, looking for effects of maturity, decline, life-cycle stage, survival, and historical trend. The data are from a 1990 sample of 2,031 U.S. adults and a 1985 sample of 809 Illinois adults. The results show that depression reaches its lowest level in the middle aged, at about age 45. The fall of depression in early adulthood and rise in late life mostly reflects life-cycle gains and losses in marriage, employment, and economic well-being. Depression reaches its highest level in adults 80 years old or older, because physical dysfunction and low personal control add to personal and status losses. Malaise from poor health does not create a spurious rise of measured depression in late adulthood. However, some of the differences among age groups in depression reflect higher education in younger generations, and some reflect different rates of survival across demographic groups that also vary in their levels of depression.In a cross-sectional study on nationally representative samples from the 2005–2006 National Health and Nutrition Examination Survey of US, WC was significantly associated with both major depressive symptoms (odds ratio [OR], 1.03; 95% CI, 1.01–1.05) and moderate-to-severe depressive symptoms (OR, 1.02; 95% CI, 1.01–1.04) [25]. In the study using the data from WOBASZ study, a multi-center nationwide study of the Polish population’s health, men and women with depressive symptoms were not physically active than their counterparts without depressive symptoms [26]. In van Gool et al.'s study [27], depressed people were more likely to be smokers (OR, 1.71; 95% CI, 1.17–2.52). In the cohort study of US National Health and Nutrition Examination Survey I, a high level of depression remained an independent predictor of incident hypertension for whites aged 45 to 64 years (relative risk [RR], 1.80; 95% CI, 1.16–2.78) and blacks aged 25 to 64 years (RR, 2.99; 95% CI, 1.41–6.33) [28]. In a meta-analysis on the prevalence of comorbid depression in adults with diabetes, the risk of depression was twice higher in the diabetic group than in the non-diabetic group (OR, 2.0; 95% CI, 1.8–2.2) [29]. A study on the association between serum lipids level and MDD using data from the Netherlands Study of Depression and Anxiety showed that patients diagnosed with MDD exhibited unhealthier lipoprotein profiles such as lower HDL cholesterol level and higher triglyceride level when compared to patients with remitted MDD and individuals from the control group [30]. In addition, 568 participants diagnosed with chronic kidney disease but not requiring maintenance dialysis yet were recruited at a tertiary hospital in Southern Taiwan for a prospective observational cohort study; they were followed up for 4 years, and participants who exhibited a higher level of depressive symptoms had experienced more rapid reduction in GFR [31].

As evinced by findings from our study and aforementioned epidemiological studies, depression was associated with various cardiovascular risk factors. We assumed that this relationship could affect the association between depression and CVD risk because the method of estimating CVD risk was structurally dependent on the individual condition of those risk factors. Also, we could take into account of the possibility that other factors affected by depression were associated with CVD risk. Rottenberg et al. [32] suggested that depression was related to cardiac vagal control. Dysregulation of the autonomic nervous system could lead to development of the metabolic syndrome and CVD [333435]. Knorr et al. suggested that dysregulation of hypothalamic-pituitary-adrenal (HPA) axis was associated with depression [36]. Dysregulation of HPA axis in depression patients might increase cardiovascular mortality [37]. Dowlati et al. [3839] performed a meta-analysis and presented that proinflammatory cytokines, which reported as predictive factors of cardiovascular events, were higher in depressed people. Therefore, we could consider depression as one of the important risk factors of CVD.

This study has an advantage of representativeness since it used the KNHANES. Whereas previous studies concerning the association between depression and CVD were performed on select groups, such as clinically diagnosed patients and matched controls, this study was based on the data representative of Korean population. We showed not only that depression and CVD were associated but also that depression was associated with particular cardiovascular risk factors among general population of Korea.

Despite this strength, there are some limitations in this study. First, this study was a cross-sectional study, as it was based on observational data obtained from the KNHANES. While we can infer possibility of association between depression and CVD risk from the cross-sectional study, causality cannot be guaranteed. We also could not exclude the possibility of reverse causality; we should consider the bidirectional effect among depression, CVD and cardiovascular risk factors. Next, other factors that might affect both depression and CVD, such as family history or socioeconomic status, were not considered. Prospective longitudinal follow-up studies with considering these limitations are needed in the future.

We found that in general population of Korea, CVD risk was higher in MSD group than normal group and there was a tendency of CVD risk to increase as the degree of depression increased. Also, depression was associated with cardiovascular risk factors which were considered to calculate CVD risk and we assumed that this relationship could affect the association between depression and CVD risk. We suggest that depression is associated with increased risk for CVD occurrence in general population of Korea.