Although the detrimental impact of abnormal body mass index (BMI) on semen quality is well established in the general population, studies exploring this relationship in infertile men remain limited. This study aimed to investigate the association between BMI and semen quality parameters and to identify potential BMI thresholds linked to altered semen quality. In this cross-sectional study, clinical records from 28 167 men evaluated for infertility prior to assisted reproductive treatment (ART) at Northwest Women’s and Children’s Hospital (Xi’an, China) between January 2019 and December 2023 were analyzed. Participants were categorized by BMI as underweight (<18.5 kg m−2), normal (18.5–24.9 kg m−2), overweight (25.0–29.9 kg m−2), and obese (≥30.0 kg m−2). Conventional semen analysis was performed according to World Health Organization (WHO) guidelines, and a composite measure of “good” semen quality was defined using established reference criteria. Multivariable regression, smoothed curve fitting, and threshold effect analyses were used to examine associations between BMI and semen parameters. Underweight men had lower sperm counts, whereas overweight men showed slightly higher progressive motility. Obese men demonstrated significant reductions in semen volume, sperm concentration, sperm count, motile sperm counts, and the likelihood of achieving good semen quality compared with men of normal weight. Smoothed curve fitting and threshold effect analyses revealed an inverted U-shaped relationship between BMI and overall semen quality, with improvement up to a BMI of 26.0 kg m−2, followed by a decline beyond this point. These findings suggest that maintaining a BMI between 18.5 kg m−2 and 26.0 kg m−2 may optimally support semen quality and reproductive potential in infertile men.

Keywords: assisted reproductive technology; body mass index; male infertility; obesity; semen quality