4.1 Demographic Results
Demographic correlations were broadly consistent with previous investigations. All factors, except physical activity, were significantly associated with depression.
Higher prevalence of depression in females may be related to multidimensional gender differences. A large number of studies have consisted that depression is more common among females; the results of this study support previous studies. Much has been made in psychology and bio-medicine about the fact that women in the United States and much of the West are diagnosed with depression at about twice the rate as that of men (20). The exact female/male ratios of diagnosis vary among different countries and cultures from 1.6 to 3.1 to 1, but rates of depression are consistently higher among females across all of those cultures (21). This may be related to social factors, including gender discrimination, adversity exposure in childhood or adulthood, sexual abuse, etc (22), and may also be related to brain function and neural mechanisms.
Depression varied significantly across races and was influenced by multiple factors. Differences were also observed in depression among adults of different races, which was very significant (P < 0.001). Asian Americans are the least likely to be depressed, with obviously lower rates of depression than other races (16.42%). Incidence rates in Whites, Blacks and Hispanics are roughly similar, all at levels higher than 25%. The influencing factors of racial/ethnic differences in depression are complicated, 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 (20).
The negative effect of higher education on depression may be related to economic factors. The results of the education data in this study showed that education level is negatively rank-sum related with depression, i.e., higher education levels leads to lower depression levels. This result supports previous studies. Details suggest that participants with high school and higher education had significantly lower prevalence of depression than those with less than high school education level, with a difference of more than ten percentage points, which provides evidence for the point that people with higher education are less likely to experience depression. Highly educated people tend to have better stress responding abilities, and these individuals also possess more social resources, higher incomes, and lower unemployment rates, which also lead to lower stress. Although previous studies have shown that the prevalence of depression is significantly higher than average in highly educated populations, especially among master's and doctoral students (23), combined with the higher lifetime prevalence of depression and long-term social facts, access to education may be a protective factor for depression.
Psychological and biochemical factors may act as mediators of the interaction between marital status and depression. Data from this study showed that people once married but living alone were most likely to suffer from depression, followed by those who had never been married, and those with a co-resident partner were the least likely to suffer from depression. This suggests that the companionship of a partner can be effective in reducing the risk of depression. Psychological studies have shown that intimate behaviors such as touching, kissing, hugging, and physical activities such as sexual intercourse can significantly reduce the incidence of depression (24, 25). These behaviors or activities lead to greater secretion of positive mood-promoting catecholamines such as dopamine, which has been shown to have a antidepressant effect (26). Poor marital status is a risk factor for depression, and some studies have indicated that marital satisfaction decreases over time during the course of long-term depression (27). Therefore, it is essential to find a suitable partner and maintain a good emotional relationship, both for the general public and for patients with depression.
The impoverished are facing higher psychological and living burdens. In the present study we discovered significant differences in depression between the impoverished and moderate income participants. The results suggested that the impoverished were more likely to suffer from depression, with an increase in prevalence of more than 50% compared to the moderate income population (33.64% vs. 21.89%). People with higher income may face less financial and life stress, thus reducing the risk of depression. In contrast, the poor were forced to face living conditions along with other risk factors such as discrimination. In addition, factors like unemployment, life confidence or self-efficacy deficiencies also affect depression through the mediating factor of poverty. Sex difference were also important, some studies have indicated that living in states with higher income inequality in the United States increases the risk of depression in women (28).
Suicidal attempt may be related to increased sense of psychological pain. Depression is one of the main triggers of suicidal behavior (29), result from this study supported previous studies. Moreover, suicidal attempt was also strongly associated with depression symptoms. There are also related studies which indicate that psychological pain is a prominent dimension of depression and has been associated with higher risk of suicidal attempt and suicidal behavior. The sensitivities to psychological and physical pain were increased in depressant populations. Conversely, higher tolerance to physical pain is negatively associated with suicidal behavior (30).
4.2 Vitamin D Deficiency and Depression
The results of this study support previous related researches. Lower vitamin D levels were associated with increased symptoms of depression and anxiety (31). Regression results from the present study suggest that those who obtain moderate vitamin D levels are 76.1% as likely to develop depression than those who are deficient, and that maintaining above-normal levels of serum vitamin D levels is a protective factor against depression which may be an effective means of preventing depressive disorders.
Vitamin D may mediate effects on depression via Ca2 + and 5-HT. Vitamin D works normally to maintain a low acellular Ca2+ level, but when vitamin D levels drop, Ca2+ begin to rise within the cell and this may enhance the onset of depression. In addition, vitamin D acts to reduce inflammation, maintains the synthesis of 5-HT, and induces the expression of DNA demethylases that controls the epigenetic landscape, thus enabling gene transcription to continue to maintain normal neuronal activity and to prevent depression (32).
In addition, solar exposure and low calcium intake are likely to be indirect factors affecting depression. Vitamin D deficiency has been shown to be significantly associated with exposure to sunlight. Major global vitamin D deficiency may be strongly related to public underestimation of the role of sunlight in providing vitamin D requirements for human body; vitamin D is mainly produced through biosynthesis, and few foods naturally contain, while sunlight exposure is practically essential to initiate the synthetic reaction (33). One of the major functions of vitamin D is to promote calcium absorption, however, low calcium intake can disrupt this regulatory mechanism, before exacerbating the consequences of vitamin D deficiency (34).
4.3 Age and Vitamin D
Younger populations may be more susceptible to vitamin D deficiency due to metabolic factors and physical activity. This study showed a significant association between age and vitamin D. Regression models demonstrated that for every 20-year decrease in age, the risk of vitamin D deficiency increased by 37.1%. Evidences indicated that depression in young people had increased in the UK and may be related to the age-specifics characteristics of vitamin D (35). Most previous studies were major focusing on vitamin D deficiency in older adults, children and adolescents, and females, but few studied younger adult populations. There were studies suggested that younger individuals may be more susceptible to vitamin D deficiency, which may be due to better metabolic capacity than in older individuals. One relevant study indicated that vitamin D deficiency were frequently observed in professional athletes, with up to 90% presented deficient (36). Relatively, younger people typically preformed greater physical activity, which may be one of the key reasons why younger people are more prone to vitamin D deficiency. From this point of view, vitamin D deficiency are to be seriously concerned: younger individuals are more likely to be vitamin D deficient, and their physio-psychological need for vitamin D may also be higher.
4.4 Age and Depression
A large number of studies have shown that older people are more likely to suffer from depression, and the results of this study support previous studies. The incident rate of depression increases by 8.5% for every 20-year increase in age, which may be related to a variety of bio-physiological, psychological and sociological factors.
The physiological degeneration that occurs in older adults is associated with depression. It has been shown that decrements in neurological and psycho-physiological functions are closely related to aging (37). Recent studies have shown that depression is associated with pain in older adults (38). Vascular lesions in selected regions of the brain may contribute to a distinctive form of late-life depression, in addition to which endocrine changes have been associated (39). Non-suppression of cortisol is associated with depression (40), which is associated with excessive secretion of corticotropin releasing factor (CRF): CRF are believed to mediate sleep and appetite disturbances, hypoesthesia and psychomotor changes (41), while responsiveness of adrenocorticotropic hormone, cortisol and dehydroepiandrosterone sulfate to CRF increased responsiveness to CRF are considered to be associated with aging (42).
Depression in older populations may be related to psychodynamics due to life experiences. Behavioral, psychodynamic, and cognitive aberrations are considered to be responsible for depression in older populations (39). Depression is associated with emotional abuse and neglect during childhood, as well as behavioral problems and stress from primary social relationships in later adulthood (43). In a meta-analysis, the number of total negative life events and daily hassles were associated with depression in an older population (44). Negative achievement events were also associated with depression in those who place a high value on personal success and control (39). Older individuals experienced more of these aspects, while younger individuals experienced fewer.
Increased social isolation may also be an influencing factor for depression in older populations. Associations between depression and impaired social support in order population has been established for years, impaired social support is associated with poor outcomes of depression in older males, but not in females (39). Related studies have indicated that loneliness may be a key factor for depression (45). After excluded confounding factors, older people who are less socially engaged were found to be more depressed (e.g. elderly people who stopped driving are at greater risk of worsening depressive symptoms) (46), whereas younger people always did better on these dimensions.
4.5 Limitations of This Study
This study has several limitations: a) We have imperfections in excluding confounding factors: the etiology of depression is multi-factorial, including multiple genetic, bio-physiological, environmental and social factors, and this study failed to exclude all influencing factors. b) This study only explored the effects of vitamin D deficiency in adults and did not analyze its effects on adolescents and children, a group that may also be affected by changes in vitamin D. c) This study extracted data from 2013–2018, but the emergence of the COVID-19 pandemic in 2020 raised many issues that directly or indirectly had a greater impact on the mental health of the general population, leading to changes in depression in the present post-epidemic era (19). d) Many related studies have been conducted prior to this study, challenging the innovative nature of the initial selection of this study. However, we have also made new discoveries. In our future design, we will consider avoiding these limitations and further explore the deeper mechanics of vitamin D and aging effecting depression based on the innovative points of this study, including not limited to biochemical and genetic studies.