Seminal Zinc Levels and Their Nonlinear Relationship with Sperm Concentration: A Chinese Population-Based Study.

Zinc is essential for testicular development, spermatogenesis, sperm protection, and male fertility maintenance. However, existing research on the relationship between zinc and sperm quality parameters shows inconsistent results. This study aimed to evaluate seminal plasma zinc levels and analyze their correlation with sperm parameters. We collected 791 male semen samples, measuring seminal plasma zinc levels via spectrophotometry and assessing the linear relationship between zinc levels and sperm parameters using RCS curves and threshold effect analysis. The results demonstrated an inverted U-shaped association between seminal plasma zinc concentration and both sperm concentration and total sperm count (p for nonlinear < 0.001). When the concentration of zinc in seminal plasma is < 156.54 mg/L, sperm concentration increases with the increase of zinc concentration in seminal plasma (β = 0.19, 95% CI: 0.12-0.26, p < 0.001), but when it is > 156.54 mg/L, although there is no statistically significant difference (β = -0.09, 95% CI: -0.20-0.02, P =0.097), sperm concentration shows a downward trend (β = -0.09, 95% CI: -0.20-0.02, p = 0.097). When the concentration of zinc in seminal plasma reaches 158.13 mg/L, the total sperm count increases significantly (β = 0.71, 95% CI: 0.45-0.96, p < 0.001), but it decreases when it exceeds this level (β = -0.31, 95% CI: -0.73 - 0.11, p = 0.152). Total zinc content demonstrated a positive linear correlation with both sperm concentration and total sperm count (p < 0.001). The findings revealed a nonlinear association between seminal plasma zinc levels and both sperm concentration and total sperm count. Total zinc content exhibited a linear relationship with these seminal parameters. These results highlight the critical need to establish a safe supplementation threshold for seminal plasma zinc.