4.1 Comparison of Demographic Characteristics
Demographic correlations are roughly consistent with previous surveys; however, there are some differences among PA.
A large number of studies have consisted that depression is more common among females; the results of this study support previous studies. This may be related to social factors, including gender discrimination, adversity exposure in childhood or adulthood, sexual abuse, etc (35), as well as biological factors, including exposure to developmental tissue risk factors, biological stress response, gonadal hormone concentrations, etc.
Significant differences were observed in depression among participants of different ages and presented a normal distribution, depression is more common in middle-aged participants. However, in previous studies, participants were mostly restricted, such as only with the elderly (25), university students (20,21), or patients with chronic conditions (36). Although failed to receive sufficient attention in previous studies, there was still evidence supporting that the middle-aged population is a high-risk group for depression (37). This may be due to the continuous downturn of the U.S. economic situation, the rising cost of living, the high unemployment rate and other factors related to greater stress among the middle-aged population. A study suggested that stress is an important risk factor for depression (38), and several other studies support this view (39-41).
Differences were also observed in depression levels among adults of different races, which is not particularly significant (P=0.043). Non-Hispanic whites are still the high incidence group. The depression incidence rates of non-Hispanic blacks and Hispanics are roughly similar, while non-Hispanic Asians are below the average level. The influencing factors of racial/ethnic differences in depression are complex, which may include complex mechanisms such as social, economic, cultural, biological factors and their interactions. The reasons for these differences are not clear yet, but does not reduce the importance of attention to minorities, in which treatment differences still exist (35).
The Highly educated population is less likely to present depression. There were no significant differences in depression between military personnel and non-military personnel and among different educational backgrounds. These two results support previous studies. However, an easily overlooked detail showed that the prevalence of depression among participants with college and above was significantly lower than those with other educational levels, which provides evidence for the perspective that the highly educated population is less likely to present depression. People with higher education tend to occupy more social resources, are better paid and less unemployed, which also leads to lower stress. Although previous studies suggested that the prevalence of depression among the highly educated population, especially master's and doctoral students, is significantly higher than the average level (42), but combined with the higher lifetime incidence rate of depression and long-term social facts, gaining a higher degree is still necessary.
A significant difference was also observed between married and non-married participants; that is, depression among married participants is significantly lower than that of non-married participants. In this regard, we suggest the following explanation: 1) marital status may directly affect depression, happy marriage may improve mental health. Psychological studies showed that intimate actions such as touching, kissing, hugging and intercourse can significantly reduce the incidence of depression (43,44). Intimate actions lead to the greater secretion of positively emotional catecholamines such as dopamine, which have been proved to significantly resist depression (45). 2) Depression may directly affect the marital status; those with depressive syndrome or already diagnosed with depression shall take more effort into attracting the opposite sex as well as maintaining a marriage.
Depression is more common among smokers (who have smoked 100 cigarettes in life), and may be strongly associated with addictions. The results from this study support previous studies; i.e. anxiety and depression were the most common emotional disorders, which have been proved to be important factors for drug, alcohol and tobacco as well as Internet addictions (46,47). However, whether participants’ smoking behavior causes depression or depression made them smoke has not been confirmed, but we prefer the latter view. In addition, depression is more common in obese participants. We believe this may as well be related to the mechanism of addiction. Except for a few patients with congenital obesity [such as leptin deficiency (48)], obesity was believed strongly related to uncontrolled diet (49), and quite a number of common chemicals in food, such as sucrose, sodium chloride, fat and caffeine, are addictive (50-52).
Patients with depression have a stronger tendency to self-injury (53). Another conclusion in this study supports this view, that is, depression is more common among participants with a tendency to self-injury. Depression is not a fatal or disabling disease, but it is an important risk factor leading to suicide and self-injury.
There were significant differences in depression between OPA and LTPA of different intensities, but the effects on depression were opposite: participants with greater intensity of OPA are more likely to present depression, while participants with greater intensity of LTPA were opposite. Previous studies normally believed that depression was significantly related to PA, and the more PA, the better. WHO did not make an upper limit recommendation for PA (18). However, the conclusion of this study found that depression is not only related to PA, but also may be related to the variety of PA. OPA may be a risk factor for depression, and LTPA may be a protective factor. Greater intensity of OPA may be related to greater stress, leading to boredom, anxiety and depression, while recreational PA may enhance positive emotions against stress. In addition, the conclusion of this study also enlightens us that there may be many complex interactions in the interior of PA. different physical activities may have different effects, and some of them could be negative.
4.2 Depression Situation
Middle-aged females who are not married tend to be the most likely to suffer from depression and may be accompanied by worse smoking status and physically less active. Middle-age is a high incidence period of chronic conditions in females (54), among which physical conditions, such as chronic pain, cancer, cardiovascular conditions, diabetes and hormone disorders, are the leading factors of depression in the middle-aged population (55). As presented physical disorders while presented indifferent and fail to commend basic physiological needs independently, one should contact a doctor immediately, for these symptoms may be related to emotional and subjective reactions, which is easy to trigger depression. When females entered the middle-age period, their endocrine situation changes sharply (56), which may lead to neurotransmitter disorders and unstable emotions. Patients with depressive disorder were believed to lack some chemicals called neurotransmitters (57) in their brains, and it is believed that the imbalance between serotonin and nor-epinephrine may lead to depression and/or anxiety. Lacking serotonin and nor-epinephrine often lead to depressive emotions, decreased motivation and changes in appetite and sexuality, which is also factors of depression in the middle-aged female population. On the other hand, emotional factors such as non-marriage and divorce may further deteriorate their depression situation.
In this study, 25.75% of participants were considered to have depressive symptoms, of which moderate to severe depression was 8.82%, which means that treatment is acquired. Differences in the incidence rate of depression were observed in different countries and regions. (58) The representative sample has a high risk of depression, which are therefore a cause for general alarm in U.S. residents. The United States is one of the most developed countries in the world, but its conditions of depression is as well among the highest (5). To our knowledge, we believe that the following three social factors are to be deeply concerned: 1) People are experiencing more stress events. Stress events are required high-intensity centralized attention to deal with. These kinds of stuffs need careful planning, which requires more time and attention than usual, which naturally increases the psychological burden. 2) Abuse and bullying occur frequently. Victims who have experienced some physical and mental abuse, or campus bullying, are often more likely to increase the risk of depression. In particular, one's self-esteem and sense of security could be seriously affected by the long-term insulting and abusing in childhood, or being in an abusive relationship in adulthood and being subjected to the control and malicious attack of one’s partner. Currently, various cases of depression and even suicide events caused by campus bullying has been reported. 3) People may be experiencing more interpersonal disharmony. Studies confirmed that people's happiness is not about their income or social status but the harmony of important interpersonal relationships (59), especially intimate ones. Disharmonious important interpersonal relationships and frequent family conflicts could lead to negative emotions. One cannot live without these important relationships, and they are fixed with problems, leading to long-term emotional distress. If these relationships are not improved and emotions are not adjusted, depression may take its advantage.
4.3 Sedentary Behavior Duration Situation
The factors affecting SB are diverse, including but not limited to personal and social factors. Studies revealed that educational level, age, employment status, gender, BMI, income, smoking status, moderate to vigorous PA, attitude, depressive symptoms and quality of life were related to sedentary behavior (35). Our study has not explored the factors of SB and the impact of the environment on it, which is also the directions of future research. However, we discovered that the average length of SB of participants was 330.61 ± 119.67, which may be due to the wide demographic distribution of the included participants, which led to significant differences.
4.4 Findings Support or Supplement to Previous Studies
This study found in a large representative sample of the adult population in the U.S. that with the length of LTSB, the risk of depression increased, especially the risk of moderate to severe depression. The study found a significant positive correlation between LTSB and depression only in the LTSB population of 720 min and above. In contrast, there was no correlation between SB and depression in other periods, except that SB less than 360 min showed a significant negative correlation with depression. Previous studies have shown that sedentary behavior in all its form was a risk factor for depression in different populations. Lee found that as sitting hours increased, university students' stress, anxiety, and depression significantly increased, and suggested intervention programs that reduce SB and improve PA were necessary (60). Xu found that the total sedentary time and time spent on school assignments on weekends were significantly associated with depression (20). Results from Meta-analyses suggested that mentally passive SB, such as watching television, could increase the risk of depression, interventions that reduce mentally passive SB may prevent depression (61). While screening time-based sedentary behavior was also approved to be associated with depression risk and the effects varied in different populations (62). Other Meta-analyses also contribute to this conclusion (24). Eriksson pointed out that in the 70-years and older population a greater percentage of the day spent sedentary and the longest average length of sedentary bouts increased the risk of depression (63). In addition, this study is also a supplement to previous studies, which confirms that SB, as an independent risk factor, contributes to the occurrence of depression. A previous study on the combined effect of SB and PA showed that although reducing PA could cause an increase in depression symptoms, a combined effect on changes in depression was not observed. Researchers hold doubt as to whether reduced PA or increased SB were driving the changes in psychological function (19). Another study suggested that evidence highlights the importance of maintaining physical activity and reducing sedentary screen-use to promote mental health (64). The conclusion of this study supports previous studies, i.e. SB is still an independent risk factor for depression, excluding the influence of PA and other variables, and adds to the confusion about the impact of depression under the mixed influence of PA and SB in previous studies.
4.5 New Findings from This Study
PA and SB are a pair of relative concepts. Generally, the intensity of activity during the awake time was used as the division basis and took 1.5 MET as the boundary value between SB and PA (16). Once the length of PA increases, it will inevitably lead to the shortening of SB duration. An interesting result showed that SB less than 360 min was negatively correlated with depression. This conclusion had never been mentioned in previous studies. To our knowledge, this conclusion is related to the length of PA: the reduction of SB will inevitably lead to an increase in PA, and there was evidence that the length of PA should also be within a certain time range (65,66). Previous studies have always emphasized the negative psychological effects of insufficient physical activity, but few studies provided evidence that excessive PA can also bring negative psychological effects. Previous studies found that excessive exercise may lead to a series of physical conditions (67,68), and chronic physical symptoms were one of the important factors leading to depression (69). Therefore, we believe that SB is also a resting behavior to some extent and should be within a reasonable period, just like PA and sleep (70). Thus, one of future research directions is to determine the best SB range through large-scale epidemiological investigation and analysis, to supplement various suggestions on PA in previous studies.
4.6 Limitations of This Study
This study has several limitations, including in data selections and reference cites related to this study. Depression situations were changing sharply (6), we performed cross-sectional analysis extracted only one cycle of data, which failed to reflect the changes in depression situations among U.S. residents. The outbreak of corona-virus disease 2019 (COVID-19) pandemic in late 2019 significantly impacted on daily routine. However, the 2017-2018 data we selected excluded entirely the impact of the pandemic on various key factors. Given above circumstances, we propose the following solutions: 1) we will combine data of different cycles and weight them to establish a complete database for further analyses. 2) Based on the fact that the NHANES program suspended field operations in March 2020 due to the pandemic, data collection for the NHANES 2019-2020 cycle was not completed, and the collected data are not nationally representative, it is necessary to conduct another epidemiological survey to explore the impact of SB on depression under the influence of COVID-19 pandemics in the future. There were few studies similar to this study, which was another main limitation. Due to this concern, we summarized the following possibilities: first, current studies were mainly focused on the impact of PA on depression or the harm of sedentary conditions in depressive patients. Second, whether SB leads to depression or patients presented SB due to their depressive syndrome is still uncertain and needs further discussion. Third, increasing PA through interventions is essentially the same as reducing SB. Although the lack of similar studies leads to some limitations in ideas and references it also indicates that this study is unique. The results support that SB is an independent risk factor for depression; thus, in future studies, we will continue to verify the reliability of this conclusion by increasing the sample size as well as switching investigation areas. In addition, this study also had certain limitations in the exclusion of interference factors. Some common risk factors of SB, together with depression related to disabilities and chronic conditions, were not excluded. Due to the insufficient original investigation, data on some factors cannot be directly obtained. Additionally, the number of these samples is limited, which may have an impact on the reliability of study results. Thus, future research designed should avoid these interference factors and rigorously exhibit the impact of SB to public.