Infections have long been considered one of the major causes of significant health loss globally. Specifically, respiratory infections are a leading cause of morbidity and mortality annually worldwide (1). Acute upper respiratory tract infections (URTI) are among the most common diagnoses in global primary care due to their high incidence rate, with 17.2 billion cases occurring annually worldwide (2). Lower respiratory tract infections (LRTI) represent the primary infectious factor contributing to mortality and rank as the fifth-leading cause of death globally (3). LRTI was a major cause of illness and death in children, particularly those under five years of age (3). Pneumonia, the most common type of LRTI, accounts for 30% of all respiratory system deaths, according to data from the Organization for Economic Cooperation and Development (4). Infections impose a heavy burden on families and societies. Thus, preventing, diagnosing early, and treating these infections pose significant challenges to public health systems, making it crucial to identify modifiable risk factors for these infections. Today, while some infection-related risk factors, such as BMI (5) and lifetime smoking (6), have been identified, the role of circulating micronutrients in the pathogenesis of infections remains unclear.

Micronutrients in the blood, including various vitamins (such as vitamin A, vitamin B6, vitamin B12, folate, vitamin C, vitamin D, and vitamin E) and trace elements [like copper (Cu), zinc (Zn), selenium (Se), magnesium (Mg)], play a crucial role in the immune system (7). Therefore, their deficiency can increase the likelihood of host infections. Conversely, severe or recurrent infections can also increase the risk of malnutrition. Observational studies have found that deficiencies in vitamins A and C are associated with increased susceptibility to respiratory infections (8). However, some randomized controlled trials have produced inconsistent results, indicating that supplementation with vitamins A and C does not improve the incidence or duration of LRTI (9, 10). Other researchers believe that lower levels of vitamin D are associated with an increased risk and severity of acute respiratory infections (11, 12), yet some studies show contrary evidence, finding no difference in vitamin D status between LRTI patients and control groups (13), and no significant correlation between low levels of vitamin D and a higher rate of influenza (14). It is noteworthy that these observational study results vary greatly, potentially influenced by underlying confounding factors or reverse causality, which is unavoidable in traditional epidemiological research.

Mendelian randomization (MR), utilizing genetic variations as instrumental variables (IVs), effectively circumvents the confounding factors often unmanageable in observational studies and minimizes the likelihood of reverse causation. This approach is widely used to assess causal relationships between risk factors and diseases. In the absence of randomized controlled trials (RCTs) or when initiating new RCTs is not feasible, MR serves as a crucial alternative strategy, offering reliable evidence on the causal links between exposures and disease risks (15).

Given the lack of evidence on the relationship between circulating micronutrients in blood and respiratory infections, we aimed to integrate genome-wide association study (GWAS) resources to evaluate their causal links using the MR approach. We hypothesized that levels of micronutrients are associated with the risk of respiratory tract infections. Specifically, we identified 12 micronutrients [calcium (Ca), beta-carotene, Cu, folate, Fe, Mg, phosphorus, Se, vitamin B6, vitamin C, vitamin D, and Zn] related to infection and assessed their relationship with URTI, LRTI, pneumonia, and their 14 common subtypes, providing feasible strategies for early prevention and improvement of respiratory infection.

The circulating micronutrients had between 1 to 11 instrumental variables, with a median count of 4. These SNPs had F-statistics ranging from 16 to 711, with the median at 49, clearly above the traditional threshold of 10, suggesting a very low probability of weak instrument variables. One SNP for vitamin C (rs13028225) was unavailable in all outcome datasets. Two SNPs for phosphorus (rs1697421, rs9469578) were removed due to incompatible alleles.

This MR research focused on examining the possible connections between 12 circulating micronutrients and respiratory infections. Our findings revealed associations of potential statistical significance with five micronutrients. After multiple correction tests, we established a strong and robust causal link between one micronutrient and respiratory infections. We ruled out the possibility of reverse causation, confirming that the identified micronutrient is a precursor to infection, not a result. Specifically, high levels of genetically predicted circulating Cu in the blood were closely associated with a lower risk of upper respiratory tract infections. This research is the first of its kind to use MR methods to thoroughly examine the causal connections between various micronutrients in the blood and the susceptibility, severity, and subtypes of respiratory infections.

Cu, a key micronutrient, is instrumental in the functioning and maintenance of various immune cells, including Th cells, B cells, neutrophils, NK cells, and macrophages. Previously, there was no clear link between circulating Cu levels in the blood and the risk of human respiratory infections. An RCT revealed that supplementing burn patients with elevated levels of Cu notably decreased their infection risk (41). Another trial demonstrated that the enhancement of Cu supplementation in healthy individuals with Cu levels that were below to within the normal range led to increased interleukin-2 production from blood cells (42), which is essential for natural killer cell cytotoxicity and T-helper cell proliferation. Moreover, recent research has shown that supplementing Cu and zinc may have a positive effect on platelet activation in patients with pulmonary embolism during SARS-CoV-2 infection (43), leading to the proposal of Cu as a potential therapy during COVID-19. This aligns with our findings that high levels of Cu may offer protection against infectious diseases. A previous cellular experiment revealed a possible mechanism, showing that Cu-enriched cell cultures enhanced macrophage phagocytosis and bactericidal capabilities (44). Polarized macrophages produce more anti-inflammatory factors, such as IL-10 and TGF-β, and mitigate inflammation by inhibiting effector T cells (45). Our MR study provides genetic evidence supporting the protective effect of Cu against URTI. Additionally, we observed a suggestive association between Cu levels and a reduced risk of LRTI and acute pharyngitis, although these findings did not meet our stringent statistical criteria. These results suggest that Cu may play a beneficial role in respiratory infections and could be a promising candidate for the prevention and treatment of these conditions. Based on these findings, incorporating assessments of Cu levels into routine health screenings and exploring Cu-based nutritional interventions as part of personalized medical strategies may be beneficial for optimizing the prevention and management of infections.

We also found a suggestive negative correlation between genetically estimated blood vitamin B6 levels and the risk of pneumonia. Although previous research on Vitamin B6 and respiratory infections was sparse, its protective role against diseases was indirectly supported by extensive studies. In previous observational studies, vitamin B6 deficiency has been associated with a variety of autoimmune diseases, including rheumatoid arthritis, multiple sclerosis (46), and type I diabetes (47). Vitamin B complex can treat neuroinflammation after peripheral nerve injury (48), and multiple animal studies may explain the mechanism of this occurrence. Mice deprived of the Vitamin B6 diet showed decreased IL-2 levels and increased IL-4 levels (49). Similarly, decreases in anti-inflammatory cytokines (TGF-β, interleukin-4, interleukin-10, interleukin-11, and interleukin-13) and increases in pro-inflammatory cytokines (interleukin-1, interleukin-6, interleukin-8, interleukin-12, interleukin-15, and interleukin-17) were also found in grass carp deficient in vitamin B6, Activated the NF-κB signaling pathway (50). A recent MR Study also found a suggestive association of increased vitamin B6 levels with reduced risk of ischemic stroke (51). Unfortunately, this finding, like ours, did not pass a strict statistical threshold. In this study, we observed a suggestive association between vitamin B6 levels and reduced risk of pneumonia, indicating its potential application in the prevention and treatment of pneumonia. Larger studies are needed to further confirm our conclusions.

In addition, we observed a suggestive association between genetically predicted increases in folate levels and a reduced risk of viral pneumonia, similar to previous observational studies in which Jacobson found significantly reduced serum folate in patients with viral and mycoplasma pneumoniae infections (52). Sato et al.’s study also proved that folate deficiency may be an independent marker of increased risk of aspiration pneumonia in the elderly (53). Recently, Najafipou’s study also proposed that hypomethylation of ACE-2 gene caused by folate deficiency is an independent risk factor for severe acute respiratory distress syndrome (ARDS) (54). However, our MR Did not find an association between folate and pneumonia, bacterial pneumonia, asthma-related pneumonia, and the severity of pneumonia, and further studies are needed to explore the protective effect of folate on the risk of viral pneumonia.

Concerning vitamin C, earlier observational studies have suggested that vitamin C may reduce the severity and mortality risk in elderly patients with pneumonia (55). However, a recent MR study found no link between circulating vitamin C levels and the risk of developing pneumonia (56). Although our research indicated a nominal positive correlation between circulating vitamin C levels and the incidence of severe lower respiratory tract infections and severe pneumonia, this finding could be a result of multiple testing and did not meet our strict statistical significance threshold.

Interestingly, our research did not establish a causal relationship between the genetically predicted levels of circulating nutrients such as calcium, beta-carotene, vitamin D, iron, magnesium, phosphorus, selenium, and zinc, and the risk of respiratory infections, including both upper and lower tract infections and pneumonia. Some systematic reviews indicate that the effect of micronutrient supplements on infections is minimal. For example, zinc supplementation shows no impact on LRTI in infants (57), and there was no evidence supporting selenium’s role in infection prevention (58). This points to a lack of extensive research in this area. While previous MR studies have found a connection between iron and sepsis (59), we did not find any evidence linking these nutrients to the infections we studied. Earlier MR studies suggested a link between low vitamin D levels in the blood and a higher risk of pneumonia (60), but meta-analyses of vitamin D supplementation experiments contradict this idea (61). These studies may suggest that these micronutrients might not be causative factors in respiratory infections and related outcomes.

There were several strengths in our study. It stands as the inaugural MR study that thoroughly examines the association between 12 different micronutrients and the risk of respiratory infection susceptibility and severity, thus reducing potential confounders present in observational studies. Furthermore, Existing large-scale GWAS data were predominantly available for European ancestry populations, enabling us to access sufficient sample sizes for robust analyses, European ancestry populations tend to have higher genetic homogeneity, reducing heterogeneity in gene–environment interactions. Moreover, we used data from two separate GWAS databases for our three primary outcome analyses (URTI, LRTI, and pneumonia), and performed a meta-merger of these results. This approach lessens the impact of data source discrepancies and bolsters our associative analysis capabilities. Additionally, we conducted extensive sensitivity analyses to confirm the reliability of our findings.

Additionally, our research was subject to certain limitations. First, our MR approach used published aggregated data, which may not account for possible non-linear relationships. Second, our sample was limited to individuals with European ancestry, which restricts wider applicability, though it reduces bias from population stratification. Future research should aim to replicate our findings in diverse populations to ascertain the universality of the associations. Third, Subtle differences in the definition of respiratory infection subtypes across different countries and regions might affect the generalizability and accuracy of the study findings. However, by utilizing outcome data from two distinct databases, we have enhanced the consistency of the results and mitigated the impact of this heterogeneity to some extent. Fourth, although we found no associations between particular micronutrients and respiratory infection risk, we cannot completely discount the possibility that the effects of calcium, beta-carotene, iron, zinc, magnesium, phosphorus, selenium, and vitamin D were too subtle to determine a causal relationship. Therefore, future large-scale genome-wide studies were necessary to further explore the potential impact of micronutrients on infections. Finally, for Cu, our main MR analysis included only two instrumental variables, but additional analyses with relaxed thresholds and more genetic tools yielded results similar to our main analysis. This consistency was further supported by various sensitivity analyses, multivariable MR, and reverse MR, validating our key findings.

Our study suggests that genetically predicted levels of Cu may be causally linked to a reduced risk of URTI. This finding offers new evidence for the prevention and treatment of respiratory infections.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

Ethics approval and consent to participate original GWAS research’s ethics approval and participant permission were available, and only publicly accessible GWAS data were used in this analysis.

ZWei: Conceptualization, Data curation, Formal analysis, Investigation, Methodology, Project administration, Writing – original draft, Writing – review & editing. QX: Data curation, Validation, Writing – review & editing. LL: Data curation, Validation, Writing – review & editing. ZWu: Project administration, Writing – review & editing. ZC: Project administration, Writing – review & editing.