Obesity has been increasingly linked to impaired male reproductive function, yet the association between body mass index (BMI) and semen quality remains unclear, particularly in large Asian populations. In this retrospective cross-sectional study, we analyzed 11 154 men aged 18–50 years who underwent semen analysis at Nanjing Drum Tower Hospital (Nanjing, China), between January 2024 and January 2025. Semen parameters were assessed according to the World Health Organization Laboratory Manual (6th edition), and sperm kinematics were measured via computer-aided sperm analysis. BMI was modeled both as a continuous variable and as a categorical variable, and associations of BMI with sperm parameters were evaluated using multivariable linear regression, sensitivity analyses, restricted cubic splines, and locally estimated scatterplot smoothing (LOESS) curves. The mean BMI of the participants was 26.3 (standard deviation: 3.8) kg m−2. A higher BMI was associated with modest but consistent reductions in semen volume, total sperm count, sperm concentration, and selected sperm kinematic parameters. No U- or J-shaped associations were observed. The results of the restricted cubic spline analyses suggested only modest nonlinearity in the relationships of BMI with sperm concentration and total sperm count, whereas the associations with semen volume and kinematic parameters were largely linear. The results of the sensitivity analyses were consistent with the primary findings. In the subset analyses, obesity was associated with more severe sperm DNA fragmentation, whereas exploratory hormone analyses in men with obesity indicated that a higher BMI was associated with lower total testosterone levels, higher estradiol levels, and a lower total testosterone/estradiol (TT/E2) ratio, which were also related to a lower semen volume. Overall, a higher BMI was linearly and inversely associated with several semen parameters, although the effect sizes were modest.

Keywords: body mass index; computer-aided sperm analysis; male infertility; obesity; semen quality; sperm kinematics; sperm motility