Serum Minerals and Male Age-Related Hypogonadism: Multimodal Evidence Decoding Associations and Intervention Strategies

Background Male age-related hypogonadism involves progressive testosterone decline. Existing evidence on minerals’ role in sex hormone homeostasis remains inconsistent, partly due to limitations in single-element analyses and inadequate age-stratified investigations. Objective This study examined age-specific associations between serum minerals and male testicular function, focusing on iron's dual role in testosterone regulation. We genetically evaluated causal serum iron-total testosterone (TT) relationships while exploring underlying mediators. Methods We analyzed cross-sectional data from 687 age-stratified males in the National Health and Nutrition Examination Survey (NHANES) 2013–2016 using weighted linear regression, restricted cubic splines (RCS), and piecewise regression models. Causal relationships were investigated through univariable and multivariable Mendelian randomization (UVMR/MVMR) analyses, complemented by instrumental variable mediation analysis. Results Cross-sectional analyses demonstrated a positive association between serum iron and TT (β = 0.194–0.244, P = 0.016–0.033) in middle-aged males, with a J-shaped nonlinear relationship (nonlinear P =0.044). Threshold effect analysis identified an inflection point at 12.18 µmol/L. Associations of phosphorus, copper, zinc, and calcium with sex hormones attenuated upon multivariable adjustments in other age strata. UVMR supported iron's causal effect on TT (P = 1.24×10⁻⁵), though this effect diminished in MVMR after adjustment for metabolic factors. Mediation analysis detected bilirubin degradation metabolite C₁₆H₁₈N₂O₅ (2) as a negative mediator (mediation proportion = -68.0%, P = 0.016). Conclusions Serum iron bidirectionally modulates testicular function through redox and metabolic networks. Our findings establish age as an effect modifier in mineral-endocrine interactions, emphasize personalized precision nutrition strategies, and uncover novel bilirubin-mediated regulatory pathways.