Introduction
Depression is a common mental disorder characterized by high disability and mortality, with a lifetime prevalence of 20% [
1,
2]. Notably, the number of individuals with depression was estimated to rise by 27.6% globally in 2020 as a result of the coronavirus disease 2019 (COVID-19) pandemic [
3]. Moreover, depression is a major contributor to the global burden of diseases [
4] and is predicted to be the second leading cause of burden of disease by 2030 [
5]. The risk of depression in the general population is increasing along with the lack of response of many patients to antidepressant therapy [
6]. It is, therefore, important to identify the underlying factors associated with depression, which may facilitate the early identification of high-risk population.
A growing body of evidence suggests that an imbalance of trace elements was contributed to the pathogenesis and pathophysiology of multiple mental illness including depression [
7,
8]. Copper (Cu), zinc (Zn), and selenium (Se) are essential trace elements that function as cofactors or structural constituents of a large number of enzymes and other important proteins. Copper is a key component of ceruloplasmin metalloproteinase and copper/zinc superoxide dismutase, which are crucial for the antioxidant defense system [
9]. Copper imbalance may cause oxidative stress and damage neurons, thereby increasing the risk of depression [
10]. Additionally, copper influences depression-related neurotransmitters, such as gamma aminobutyric acid (GABA) and glutamate [
11‐
13]. Our previous study also identified a relationship between serum copper concentrations and neurobiochemical metabolism in depressed patients [
14]. Regarding zinc, it is a modulator of synaptic activity, neuronal metabolism and plasticity; dysregulated zinc homeostasis has been linked to a number of neurological disorders, including depression [
15]. Animal experiments have shown that zinc deficiency could lead to depressive-like behaviors and reduce the antidepressant-like effect, which involved decreased serum zinc concentrations [
16,
17]. Low serum zinc concentrations contributed to elevated serum corticosterone levels in rats with depressive-like behaviors, while hyperactivation of the hypothalamic-pituitary-adrenal (HPA) axis seemed to underlie depression [
16,
18]. Normalizing HPA hyperactivity could improve depressive-like behaviors in chronic unpredictable mild stress (CUMS) rats [
19]. Another critical role for zinc is its anti-oxidative and anti-inflammatory function in the central nervous system [
20,
21]. Moreover, zinc regulates glutamate homeostasis in a concentration-dependent manner, promoting glutamate release at lower concentrations [
22,
23]. It is well known that inflammation, oxidative stress and glutamate homeostasis are mechanistically linked to depression [
12,
24]. Furthermore, zinc may also affect depression through its interactions with monoamine neurotransmitters (such as dopamine, serotonin, or norepinephrine) [
25]. It is known that selenium and selenium-containing proteins (e.g., glutathione peroxidase) are antioxidants [
26,
27]. Similarly, selenium has anti-inflammatory effects [
28]. An experimental animal study has revealed that selenium-containing protein can ameliorate depressive-like behavior by abrogating inflammation and oxidative stress [
29]. Furthermore, a recent meta-analysis suggested an inverse association between dietary selenium intake and depression [
30]. Accordingly, copper, zinc and selenium have been implicated in the pathogenesis of depression.
It is noteworthy that the concentrations of copper [
31], zinc [
32], and selenium [
33] in peripheral blood are correlated with depression. However, studies on the relationship between peripheral blood concentrations of copper, zinc, and selenium and depression are limited, and the results are controversial. Besides, most studies have concentrated on the relationship between copper and zinc concentrations and depression, but few have investigated selenium concentrations. Prior studies found that patients with depression tended to have higher serum copper concentrations and lower serum zinc concentrations compared to healthy controls [
8,
14,
34]. However, other researchers established that serum copper concentrations in depressed patients were equal or lower than in healthy volunteers [
35,
36]. Similarly, no significant difference in serum zinc concentrations was also observed between depressed and non-depressed control subjects in other studies [
37,
38]. With regard to selenium, while two studies have found no association between serum selenium concentrations and depressive symptomology in Iranian adults and Chinese older adults [
39,
40], a study conducted in Australian young adults found a non-linear association [
41]. Recently, a meta-analysis found no difference in the serum selenium concentrations between depressed and non-depressed participants [
33]. There is, however, a limitation to these studies in terms of sample size. Further, our previous preclinical and clinical studies also suggested that copper induced depressive-like behaviors in rats [
42], and that the serum copper concentrations were significantly higher in patients with depression than in normal controls [
14,
43]. Nevertheless, external validation by clinical studies with large sample sizes is required to confirm our findings. In addition, a large sample study demonstrated that dietary copper, zinc, and selenium intake were associated with depression [
44], and the risk of depression varied by intake levels of these trace elements [
30,
45]. However, it is unknown whether the risk of depression also differs by the serum concentrations of these trace elements, as studies have shown that dietary intake of trace elements does not totally reflect their blood concentrations [
46,
47]. This means that even though studies have explored the dietary intake of these trace elements, further research into their serum concentrations is warranted.
To address the aforementioned issues, we conducted a population-based cross-sectional study to investigate the association between the serum concentrations of these trace elements (copper, zinc, selenium) and depressive symptoms in the US adults (≥ 20 years) by utilizing data from the National Health and Nutrition Examination Survey (NHANES) 2011–2016.
Discussion
To our best knowledge, few studies have investigated the associations between serum copper, zinc, and selenium concentrations and depressive symptoms. This is the first study to investigate the association of these serum trace elements with depressive symptoms in US adult population using NHANES data (2011–2012, 2013–2014, and 2015–2016). Our results demonstrated two main findings. First, serum copper concentrations were elevated in US adults with depressive symptoms, and only in obese individuals were the two highest quartiles (Q3 and Q4) of copper concentrations associated with an increased risk of depressive symptoms. Second, lower serum zinc concentrations (Q2) were also consistently and positively associated with depressive symptoms, although no differences were found in serum zinc concentrations between the depressive and non-depressive groups. These findings provide evidence for the relationships of serum copper and zinc concentrations with depressive symptoms.
Our study revealed that the serum copper concentrations in the depressive group were higher than those in the non-depressive group, which was consistent with our earlier finding [
14]. Similarly, a recent meta-analysis based on observational research also established higher serum copper concentrations in patients with depression [
31]. Besides, copper exposure could induce depressive-like behavior in rat model [
42,
52]. A complex mechanism of interaction might exist between serum copper and depressive symptoms. On the one hand, depression has been identified as a pro-inflammatory state [
53]. It activates the inflammatory response system [
54] and may further promote elevated levels of serum copper [
55,
56]. On the other hand, depression is strongly associated with oxidative stress. Evidence implies that patients with depression have excessive levels of reactive oxygen species (ROS), accompanied by elevated superoxide dismutase (SOD) activity [
10]. Besides, copper is not only a component of copper-zinc superoxide dismutase (Cu/Zn-SOD), but also can regulate Cu/Zn-SOD activity [
57]. Meanwhile, copper and its complexes are also known to have antioxidant activities [
58]. Thus, elevated serum copper concentrations may be related to increased antioxidant activity in depressed patients. In addition, our previous investigation showed that the mRNA expression levels of ATPase copper-transporting alpha (ATP7A) decreased in patients with major depression [
59]. ATP7A is known to regulate copper homeostasis, and its abnormal expression may increase serum copper concentrations. In contrast, excessive copper exposure could also alter the levels of many cytokines and cause inflammatory responses [
60]. Notably, peripheral inflammation was found to increase the permeability of the blood-brain barrier (BBB) [
61], resulting in disrupted brain homeostasis and depression. Otherwise, excessive peripheral blood copper could directly destroy BBB [
62], increasing brain copper levels as copper enters the brain mainly through BBB [
63]. In turn, excessive brain copper catalyzed the formation of ROS [
9,
64], increased the neurotoxic effects of oxidative stress, and induced neuronal oxidative damage [
65], which could contribute to depression [
10]. Additionally, copper release was associated with N-methyl-D-aspartate (NMDA) receptor activation [
66]. Our previous research showed that memantine (NMDA receptor antagonist) treatment not only decreased serum copper levels, but also improved the depressive-like behaviors induced by corticosterone and copper [
42]. That is, glutamine activity may partially explain the relationship between elevated serum copper concentrations and depressive symptoms. Copper also binds to serotonin and induces oxidation and structural modification in serotonin, especially at its high concentrations, ultimately resulting in neurotoxicity and serotonergic dysfunction [
67‐
69]. Hence, serotonergic system dysregulation may be related to depressive symptoms induced by high copper concentrations [
70]. High serum copper concentrations may also induce depression by influencing neurobiochemical metabolism [
14].
Moreover, this study revealed that obese individuals with high copper concentrations were more likely to have depressive symptoms. Previous studies have found that obesity was associated with higher depression prevalence [
71,
72] and obesity prevalence was associated with depression severity [
73,
74]. In addition, BMI was positively linked not only to depression [
75] but also to serum copper concentrations [
76], especially in regards to the association of the increased odds of obesity with elevated serum copper concentrations [
77]. Taken together, obesity may be a moderator in the relationship between depression and high copper concentrations. Notably, depression was accompanied by inflammation [
54]; inflammatory processes could also lead to copper accumulation [
78]. Whereas, obese individuals tended to be in an inflammatory state [
79]. Obesity, thus, may induce depression by affecting copper metabolism through inflammation [
71,
80]. Meanwhile, the interaction of inflammatory and oxidative stress may play an important role in this process. Copper, increased by obesity, could directly raise the levels of reactive oxygen species (ROS) and reduced the activities of antioxidant enzymes, further activating the microglial ROS/nuclear factor-kappa B (NF-κB) pathway to secrete inflammatory products, leading to neuroinflammatory response and neuronal apoptosis [
81,
82], thereby inducing depression [
83]. Furthermore, cortisol reactivity mediated the depression-obesity relationship [
84]. The interplay between cortisol and inflammation might also be the underlying mechanism for the relationship between copper and depression in obese subjects [
85]. Thus, it is possible that maintaining relatively low serum copper concentrations in obese populations may be beneficial for reducing the risk of depressive symptoms, but further investigations are needed to provide evidence to this speculation. However, weight control may be more conducive to physical and mental health. Nevertheless, the underlying causal mechanism of depressive symptoms in association with serum copper concentrations in obese subjects is still unclear.
In addition, the current study suggested that depressed subjects appeared to have lower serum zinc concentrations than non-depressed subjects, but this difference was not statistically significant. This observed trend was similar to previous findings of a significant decrease in serum zinc levels in patients with depression [
35,
86,
87]. There are several possible reasons for this unremarkable discrepancy of the current study. First, there was a difference in the study subjects. In the present study, participants with depressive symptoms were not depressed patients; their depressive symptoms might be milder in severity and shorter in duration than patients with depression. A negative correlation was also found between serum zinc concentrations and depressive symptoms severity in the present study. Therefore, we speculated that serum zinc concentrations did not fall significantly because the present study subjects were not patients diagnosed with major depression with major depression with severe depressive symptoms. Second, there might be regional and ethnic differences in subjects between studies. Only US populations were included in the present study. Genetic differences might exist among races, and differences in dietary structure and risk of zinc exposure might exist among regions. Third, the sample size in the present study varied widely between depressive group and non-depressive group.
Furthermore, we noticed that lower serum zinc concentrations were associated with depressive symptoms. This result was robust in all three regression models. Our findings were consistent with the ones of previous studies [
86,
87], which indicated that individuals with lower serum zinc concentrations within the physiologic range may be more susceptible to depressive symptoms. Collectively, lower serum zinc concentrations may be a risk factor of depressive symptoms. Zinc was considered to interact with the serotonin system [
88] and BDNF [
89]. Hence, lower serum zinc concentrations could compromise serotonin and BDNF activity and diminish neurogenesis, which may be the pathophysiology of depression [
25]. Zinc is a modulator of excitatory (glutamate) and inhibitory (GABA) neurotransmitters [
90]: zinc binds to GluN2A subunit via zinc transporter 1 and inhibits N-methyl-d-aspartic acid (NMDA) receptor function [
91]; zinc activates the zinc-sensing receptor GPR39 to regulate glutamate and GABA, maintaining the brain’s excitatory-inhibitory balance [
23]. Consequently, reduced zinc concentrations may trigger glutamate release and elicit neuronal excitotoxicity, which contributes to depression [
92]. Notably, the synergistic interaction among low serum zinc concentrations, GPR39, BDNF, and serotonergic system may be an underlying mechanism of depression [
88,
89]. Additionally, it has been demonstrated that inflammation and oxidative stress are implicated in the pathophysiology of depression [
24,
93]. Zinc deficiency could activate the immune-inflammatory response system. Low serum zinc concentrations were usually accompanied by raised immuno-inflammatory indicators like CD4+/CD8 + T-cell ratio and interleukin 6 in depressed patients [
94,
95]. Zinc deficiency also triggered oxidative stress, further activating oxidant-sensitive transcription factors such as NF-κB and activator protein-1 (AP-1), thereby causing DNA damage and neuronal apoptosis, ultimately leading to depression [
96‐
98]. Therefore, low serum zinc concentrations may cause depression through the interaction of inflammatory cytokines and oxidative products. Moreover, HPA axis hyperactivity has been demonstrated in depression and is associated with decreased serum zinc concentrations [
16,
99]. There is also a close association between cortisol concentrations and immune/inflammatory markers in patients with depression [
100,
101]. Thus, low zinc concentrations may promote depression via the interplay between immune/inflammatory and HPA axis functions. Besides, an animal study found that zinc deficiency caused phospholipid-protein imbalance leading to depression due to the effect on phospholipids and proteins [
102]. Thus, maintaining relatively high serum zinc concentrations within the normal range may be associated with a decreased risk of depressive symptoms. Dietary zinc intake or zinc supplementation may also help improve depressive symptoms risk [
103,
104]. However, the causal mechanisms connecting low serum zinc concentrations and depressive symptoms remain elusive.
The present study did not find group differences in serum selenium concentrations between individuals with and without depressive symptoms, nor an association between serum selenium concentrations and depressive symptoms. These findings were in agreement with previously reported results [
33,
105]. Accordingly, there is insufficient evidence to support an association between selenium status and depressive symptoms, and further studies are needed. Nevertheless, selenium, serving as an antioxidant, can help protect the central nervous system from free radical damage [
106]; selenium supplementation could alleviate depressive symptoms [
107].
Several limitations exist in this study. First, this study was cross-sectional, and, therefore, causal conclusions could not be drawn. Although we provided explanations of the biological mechanisms in the
discussion section, animal trials and prospective cohort studies should be conducted to confirm the causal direction of the relationship between serum copper or zinc levels and depressive symptoms. Second, it is still possible that residual confounding factors (e.g., dietary or physical activity) have influenced our results, although a large number of covariates have been controlled. Third, no multiple comparisons correction was applied to the copper interaction test as the analyses were exploratory in nature. Finally, the PHQ-9 is a depression measurement scale rather than a diagnostic instrument, which can be employed for the assessments of depressive symptoms rather than the diagnosis of depression. We also could not exclude the presence of individuals diagnosed with depression, nor did we know the course of depressive symptoms over time. Future studies could confirm our findings by recruiting large samples of participants with depression as well as using a depression assessment scale with better reliability and validity, such as the Hamilton Depression Rating Scale.