The impact of sitting time and physical activity on major depressive disorder in South Korean adults: a cross-sectional study

Major depressive disorder (MDD) is an important public health issue, with a lifetime prevalence of approximately 11–15% worldwide [1]. The World Health Organization (WHO) recently stated that MDD was expected to be the second leading cause of disability and fourth leading contributor to the global burden of disease by 2020 [2]. Moreover, MDD not only contributes to the overall global burden of disease but also leads to suicide in some individuals [3]. In fact, South Korea has the highest suicide rate of those reported by Organization for Economic Co-operation and Development (OECD) countries. The lifetime prevalence of MDD based on DSM-IV was 7.5% [4] while other research presented the prevalence of MDD based on CESD-11 as 11 and 10.6%, respectively [5, 6], which were not high rates of MDD compared to those of OECD countries, and the lowest level of antidepressant consumption in Korea has been recorded as the lowest of these countries [7]. The relatively fewer expressions for depressed mood in Asian populations may have resulted in under-reporting of depressive disorders [8]. These rates could be considered a proxy indicator of the population’s mental health. [9]. To reduce this burden, it is necessary to identify crucial risk factors associated with MDD prevention and management.

Physical activity (PA) is known to be associated with a decreased risk of mortality and morbidity resulting from chronic conditions such as cardiovascular disease (CVD) [10, 11], type 2 diabetes [12‐14], obesity [15, 16], and cancer [17‐19]. Moreover, recent research suggests that PA is related to mental health outcomes [20, 21]. Several recent studies have identified sedentary behavior (SB) as another lifestyle factor associated with poor cardiovascular [22‐24] and metabolic health outcomes [25, 26] and reduced life expectancy [27, 28]. Recently, numerous studies have reported on the relationship between sitting time, PA, and MDD [29], as well as on the effects of interaction between sedentary time and PA which may affect mental well-being and productivity [30], or a specifically, divided PA (domain of PA or Social context of PA) and/or SB (time spent on sitting while watching TV or using computer, sitting for travel or overall sitting time, or sitting around throughout the week) [21, 31] were related to mental health outcomes; however, such effects are still less well-known.

Numerous studies have reported relationships between SB and mental health in older adults [32, 33], disadvantaged women [21], middle-aged women [29], employed adults [31], and patients with schizophrenia [26]; however, the definition of SB included only leisure-time, such as that spent watching TV and/or using a computer or the Internet [34, 35]. Moreover, no studies have examined associations between MDD and PA and overall sitting time including that involved in work, leisure, home-based activities, and transportation. Therefore, the aims of this study were to examine the relationship between sitting-time and MDD and estimate the effects of sitting-time and PA on MDD in a representative South Korean population.

Table 1 shows the participants’ general characteristics. The 4145 participants included 1664 men and 2481 women; of these, 112 (6.7%) men and 312 (12.6%) women reported MDD. Sitting time of ≥8 h/d was reported by 45.6% of participants. Specifically, men tended to sit excessively more than women. The proportion of men (20.01%) who sat for >10 h/d was higher compared to women (16.36%; Additional file 1: Table S1).

Table 2 presents the estimated odds ratios (ORs) from the multiple logistic regression analysis. Risk of MDD in women was twice that observed in men (OR: 2.00, 95% CI: 1.49–2.68). Regarding sitting time, risk of MDD in men who sat for >10 h/d was greater relative to those who sat for <5 h/d (OR: 2.04, 95% CI: 1.12–373). Similarly, risk of MDD in women who sat for ≥8 h/d was greater relative to that of those who sat for <5 h/d (8–10 h/d: OR: 1.59, 95% CI: 1.12–2.27; >10 h/d: OR: 1.64, 95% CI: 1.09–2.45). There was a significant association between linear trends of sitting time and MDD (P = 0.0001). Both men and women who sat for longer periods were significantly more likely to report higher MDD compared to those who sat for <5 h/d (P = 0.0053 and P = 0.0040, respectively). Low level of physical activity was not an independent risk factor for MDD in men or women.

Figure 1 depicts the subgroup analysis in which multiple logistic regression was performed to assess the relationship between sitting time and MDD according to PA. Results showed that men who sat for ≥8 h/d were at significantly high risk of MDD compared to those who sat for <5 h/d (8–10 h/d: OR: 3.04, 95% CI: 1.15–8.01; >10 h/d: OR: 3.43, 95% CI: 1.26–9.35). Women who sat for >10 h/d were at greater risk of MDD compared to those who sat for <5 h/d (OR: 2.27, 95% CI: 1.23–4.21) (Additional file 1: Table S2).

This study is the first to consider evidence for the effect of overall sitting time on risk of MDD in Korea. Using cross-sectional design, our study found that sitting time was positively associated with MDD. Sitting for long periods (>10 h/d) was significantly related to higher risk of MDD in both men and women. Sitting for 8–10 h/d was associated with risk of MDD in both sexes; however, the association was statistically significant only in women. In addition, PA was inversely related to MDD risk, although this association was nonsignificant.

The results are consistent with those of previous studies indicating that longer periods of sitting are related to poor mental health [21, 29, 32, 33, 41] and PA was negatively associated with risk of depression [20, 21]. However, these studies did not examine sex differences in health outcomes related to SB; one study involved both men and women, but did not examine potential interactions according to sex [33], while others involved only women or assessed the relationship between leisure-related sitting time, such that spent watching TV or using a computer, and depressive symptoms [21, 35]. Moreover, a sex difference has previously been observed in mental health outcomes related to sitting time, with findings partially similar to the current results. In the current study, sitting for >7 h/d was associated with greater MDD risk in women relative to that observed in men [31]; however, MDD risk in men who sat for >10 h/d was twice that of men who sat for <5 h/d and higher relative to that of women.

The findings of the current study indicated that there was no interaction between sitting time, PA, and MDD risk; this may have occurred because of the small sample size. However, subgroup analysis was performed because PA is an important factor in the attenuation of depression [29]. Interestingly, the results showed a sex difference. MDD risk in men who did not engage in PA and sat for ≥8 h/d was approximately 3 times higher relative to that of those who did not engage in PA and sat for <5 h/d. In women, MDD risk in those who did not engage in PA and sat for ≥8 h/d was higher relative to that of those who sat for <5 h/d, but the associations between MDD and sitting time and PA were nonsignificant. This indicates that men who did not engage in PA and sat for long periods were likely to experience MDD; Interestingly, risk of MDD in women who engaged in PA and sat for >10 h/d was more than twice that of those who sat for <5 h/d. This could have occurred because half of the women in the study population, such as housewives who engaged in PA via daily chores, were unemployed, and unemployed people were at greater risk of MDD relative to employed individuals (Table 2); therefore, MDD risk in women who engaged in PA could have differed according to sitting time.

The results of the current study are consistent with those of a previous Spanish study [35] that examined the combined effect of sitting time and PA on depression. The results showed that risk of mental disorders in those with high PA levels and short periods of SB was 25% lower relative to that of those with low PA levels and long periods of SB. Moreover, Lucas et al. examined the combined effect of PA and time spent watching TV on depression. Their results suggested that both factors contribute to depression risk, because watching TV typically replaces PA [34]. These studies identified SB as an important correlate of decreased MDD risk when PA levels were low but not when they were high, suggesting that PA could affect the relationship between SB and depression. Even though this was a cross-sectional study and did not explain causal relationships, sitting for long periods and failure to engage in PA increased MDD risk, which is consistent with the findings of previous studies.

Some studies have described potential mechanisms underlying the inverse relationships between PA and sitting-time and MDD. One possible explanation for the positive relationship between sitting time and MDD risk is that sitting time might replace PA [20, 34]. A few studies have suggested that the more time adults spend sitting during activities, such as watching TV, the less time they spend engaging in PA [42]. Numerous studies have shown that PA is not only associated with a reduced risk of negative health outcomes [10‐17] but also involved physiological mechanisms including changes in endorphins, core body temperature, central serotonergic systems, and brain activation involved in emotional regulation [43‐45]. Another possible explanation for the association between sitting for long periods and high risk of depression involves the social withdrawal hypothesis. Psychosocial mechanisms include distraction, enhanced feelings of control and mastery, improved self-esteem and self-efficacy, behavioral activation, and social interaction [46‐48]. For instance, as the frequency with which people watch TV or use a computer or the Internet increases, they become further removed from social interaction, which increases their risk of depression [49]. However, this type of activity must be contrasted with computer or Internet use for work or communication [50, 51]. Nevertheless, physiological or psychological mechanisms could support the relationship between sitting time and MDD risk. South Korea has the highest proportion of households with broadband Internet access (97.2%) [52] and the highest business use of broadband (98.4% of businesses with 10 or more employees) [52] of the OECD countries, along with high levels of mobile penetration and daily mobile use for long periods [53]. It is easy to be exposed to the Internet via computers or mobile phones in South Korea, which could affect the relationships between MDD and sitting-time and PA. As a result, people with depression might have poorer physical health because they were more likely to choose a sedentary lifestyle due to their depressed feeling, fatigue, or evasion of social interaction.

This study showed that sitting for long periods was associated with greater risk of MDD in South Korean adults. The findings accentuated the importance of reducing overall sitting time and increasing PA and suggested that policymakers should develop strategies involving PA, to decrease sitting time and alleviate the burden of depression in terms of fiscal health premiums and social problems.