3.1. Main analyses of NHANES
Table 1 delineates the baseline characteristics of the participants, stratified by survey wave. The overall prevalence of emphysema stands at 2.2%, showing a slight upward trend from 2.0% in 2013-2014 to 2.5% in 2015-2016, though this increase lacks statistical significance. The changes in the prevalence rates of chronic tracheitis, COPD, and asthma over the two periods also have no statistical significance. The distribution of serum Se levels across the two periods showed a slight increase at the lowest (<Q1) and medium quantiles (Q1-Q3) and a slight decrease at the highest quantile (>Q3), with differences that are statistically significant. The distribution of serum Cu and Zn levels remained consistent across the two periods, with no significant variations observed.
Table S1 displays the characteristics of participants, comparing those with CRDs to those without. From the perspective of univariate analysis, advanced age, race, level of education, marital status, alcohol consumption, smoking, hypertension, diabetes, income status, and serum Cu levels may be risk factors for emphysema. Conversely, BMI, serum Zn levels, serum Se levels, physical activity, and sex appear to be unassociated with the prevalence of emphysema.
Advanced age, sex, race, educational attainment, marital status, smoking, hypertension, diabetes, serum Cu levels, income status, and BMI may act as risk factors for chronic tracheitis. In contrast, alcohol consumption, serum Zn levels, serum Se levels, and physical activity do not appear to correlate with the incidence of chronic tracheitis.
Advanced age, race, marital status, income status, alcohol consumption, smoking, hypertension, diabetes, and serum Cu levels may be potential risk factors for COPD. Conversely, educational level, BMI, serum Zn levels, serum Se levels, physical activity, and gender appear to be unrelated to the prevalence of COPD.
Advanced age, gender, level of education, marital status, alcohol consumption, smoking, hypertension, BMI, and serum Cu levels may be potential risk factors for asthma. In contrast, race, income status, serum Zn levels, serum Se levels, physical activity, and diabetes appear to be unrelated to asthma prevalence.
Table 2 outlines the findings from a binary logistic regression analysis assessing the impact of trace serum metals on the prevalence of CRDs in the U.S. adult population. Comprehensive adjustments were made to account for sociodemographic factors, behavioral patterns, and health-related covariates in the analysis. The study observed no significant correlation between serum levels of Zn or Se and the likelihood of emphysema. Nevertheless, adults exhibiting the highest quartile of serum Cu concentration (>Q3) were associated with a substantially higher risk — a 283.0% increase — of emphysema compared with those within the lowest quartile (<Q1) (OR: 3.83, 95% CI: 1.70-9.35, p = 0.0019).
In the case of chronic tracheitis, no link was established with serum Zn levels. Conversely, adults with serum Cu levels between the 1st to 3rd quartiles (Q1-Q3) demonstrated an 82.0% heightened risk of chronic tracheitis relative to individuals in the lowest quartile (<Q1) (OR: 1.82, 95% CI: 1.10-3.12, p = 0.0230). Adults within the middle quartiles for serum Se levels (Q1-Q3) showed a 36.0% decreased risk of chronic tracheitis compared to those in the lowest quartile (<Q1) (OR: 0.64, 95% CI: 0.41-0.99, p= 0.0453).
In regard to COPD, while serum Se levels were not indicative of COPD risk, individuals with serum Cu levels in the 1st to 3rd quartiles (Q1-Q3) showed a 114.0% increased risk of COPD (OR: 2.14, 95% CI: 1.09-4.37, p = 0.0307). Moreover, those with serum Cu concentrations in the highest quartile (>Q3) faced a 233.0% increased risk (OR: 3.33, 95% CI: 1.68-6.93, p = 0.0008). Adults within the highest quartile of serum Zn levels (>Q3) were associated with a 91.0% increased risk of COPD compared to counterparts in the lowest quartile (<Q1) (OR: 1.91, 95% CI: 1.05-3.55, p = 0.0374).
As for asthma, the study found no associations between the risk of developing the condition and serum concentrations of Cu, Zn, or Se in the adult population of the United States.
Follow-up analyses applying Restricted Cubic Splines (RCS) were implemented to delineate the dose-response dynamics linking serum trace metal concentrations with the incidence risk of CRDs. Depicted in Figures 2A and 2B, a discernible increase in the risk for both emphysema and COPD was observed corresponding to elevated Cu exposure, with these elevations attaining statistical significance (p = 0.023 for emphysema and p = 0.016 for COPD, respectively). Nevertheless, the evidence currently available is inadequate to assert a nonlinear enhancement of risk in direct proportion to increased Cu exposure. Displayed in Figure 2C is a statistically significant positive association between exposure to Cu and the occurrence of chronic tracheitis (p = 0.027), exhibiting a nonlinear pattern as exposure levels vary (p for nonlinearity = 0.013). Figure 2D indicates that the association between Cu and asthma lacks statistical significance. Moreover, while Figures 2E to 2L showcase the potential interaction effects of Zn, Se, and CRDs within the serum as analyzed by the logistic regression model, these relationships failed to attain statistical significance.
3.3. MR of trace metals and CRDs
We pinpointed SNPs robustly linked with Cu, Zn, and Se exposure, and subsequently excised any linked disequilibrium. Our analysis discerned nine SNPs correlated with Cu levels, four in relation to Zn, and six pertaining to Se, all manifesting synergistic effects with studied health outcomes. It was determined that these SNPs were independent of both the outcomes themselves and potential confounding variables.
Figure 3 presents the results of MR analyses, including random-effects IVW analysis, which indicated that serum Cu were not genetically causal for COPD (OR:1.000, 95% CI: 0.998-1.001, p = 0.544) nor for chronic tracheitis (OR:0.999, 95% CI: 0.998-1.000, p = 0.085). Additional MR approaches, such as MR-Egger, weighted median, simple mode, and weighted mode analyses, consistently supported the inference that Cu does not have a genetic causative role in either COPD or chronic tracheitis (p > 0.05).
Conversely, in Figure 4, reverse MR analysis did not establish a causal effect of chronic tracheitis on Cu exposure, but it did reveal a causal association between COPD (OR: 1.18241E-23, 95% CI: 9.58E-39-1.46E-08, p = 0.003) and Cu exposure. Similarly, Zn levels were found to be genetically causally related to COPD (OR: 1.001, 95% CI: 1.001-1.002, p = 0.014), with reverse MR also indicating a causal link between COPD (OR: 2.718, 95% CI: 2.475-2.985, p < 0.001) and Zn exposure.
Moreover, a genetic causal association was identified between Se and chronic tracheitis (OR: 1.002, 95% CI: 1.000-1.00 3, p = 0.010), with reverse MR supporting a causal relationship between chronic tracheitis (OR: 2.718, 95% CI: 2.360-3.13, p < 0.001) and se exposure. These findings prompt a re-evaluation of the roles that these trace elements may play in respiratory health and disease mechanisms.
3.3. Sensitivity analyses
The Supplementary Tables S2 details the outcomes of logistic regression analyses across three distinct models, examining the correlation between serum trace metal levels and CRDs in U.S. adult populations. Figure 2 illustrates that the trajectories discerned within the RCS plots align well with the magnitude and direction of effect sizes ascertained by the logistic regression analysis.
Regarding inverse MR, we repeated the above steps by exchanging exposures and CRDs in the same database to assess the inverse causality of CRDs with exposures. Figure S1 and S2 show the results of bidirectional MR. As a sensitivity analysis, we used a "leave-one-out" approach to check whether the MR estimates were driven by a single SNP. Figure S3 and S4 illustrates the results.