Associations between seminal plasma metal mixture and semen quality: A metabolome-mediated case-control study

Metal exposure constitutes a global public health concern associated with male infertility. This matched case-control study advances mechanistic understanding of how environmental pollutants interact with biological systems to impair human reproduction by investigating multi-metal exposure and seminal plasma metabolic responses. Based on this matched case-control study among 522 males, we assessed the role of untargeted metabolomic profiling of 265 seminal plasma metabolites in the relationship between seminal metals and abnormal semen quality (ASQ). The relationship between metals and ASQ was analyzed using single-exposure models (single-metal and multi-metal logistic regression) and mixed-exposure models including Quantile Gaps-Cumulated (QG-C), weighted quantile sum (WQS) regression, and Bayesian kernel machine regression (BKMR). The findings revealed that metal mixture exposure collectively increased ASQ risk, with seminal plasma Cu demonstrating a significant risk-enhancing effect in both single- and mixed-exposure models, while Fe and Se consistently exhibited protective trends. These associations were robustly supported by sensitivity analyses. In addition, orthogonal partial least squares discriminant analysis (OPLS-DA) identified 74 significant differential metabolites out of a total of 265 metabolites. Among these, 22, 21, and 12 differential metabolites were found to mediate the association between iron, selenium, copper, and the risk of ASQ, respectively. Moreover, 16-glucuronide-estriol, Aspartyl-Valine, Dihydrocoumarin, L-(-)-3-Phenyllactic acid, and trans-cinnamate were significant mediators in the association between iron, selenium, copper and ASQ. This study provides the evidence that seminal plasma metals disrupt male fertility through metabolite-specific pathways, with copper driving damage while iron and selenium exert protection. These findings highlight candidate biomarkers warranting validation in environmental reproductive epidemiology, while suggesting biologically plausible pathways for future intervention studies.