TOPLINE

Higher body fat percentage is associated with reduced total sperm count and disrupted reproductive hormone levels in young men. Men with high body fat (20%-24%) showed 23% lower total sperm count, while those with very high body fat (> 25%) had 15% lower counts than men with normal body fat percentage (8%-19%).

METHODOLOGY

• Researchers conducted a cross-sectional study using data from the Fetal Programming of Semen Quality (FEPOS) Cohort nested within the Danish National Birth Cohort in Denmark, with 1058 men (19% of 5697 invited) participating at age 18 years and 9 months during 2017-2019.
• Participants had body fat percentage measured by bioelectrical impedance, BMI and waist-to-height ratio calculated, and provided semen and blood samples while self-measuring testicular volume and completing health behavior questionnaires.
• Body fat percentage was categorized as low (< 8%), normal (8%-19%, reference), high (20%-24%), or very high (> 25%) and analyzed as both categories and continuous variables (in standard deviations).
• Outcomes included semen characteristics (volume, concentration, total sperm count, motility, morphology), testicular volume, and reproductive hormones (follicle-stimulating hormone [FSH], luteinizing hormone [LH], sex hormone-binding globulin [SHBG], testosterone, estradiol, and free androgen index [FAI]).
• Analysis used negative binomial regression models to estimate relative differences in outcomes with 95% CIs, adjusted for parental education, maternal pre-pregnancy BMI, maternal smoking, birth weight for gestational age, and participants' smoking, alcohol consumption, and physical activity.

TAKEAWAY

• Compared with normal body fat percentage (8%-19%), high body fat (20%-24%) was associated with 23% lower total sperm count (95% CI, -35% to -9%), and very high body fat (> 25%) with 15% lower count (95% CI, -35% to -13%); per standard deviation increase in body fat, total sperm count decreased by 8% (95% CI, -13% to -2%).
• Higher body fat percentage was associated with lower SHBG and testosterone levels, but higher LH, estradiol, and FAI levels; no consistent associations were observed for sperm motility, morphology, testicular volume, or FSH.
• Body fat percentage showed stronger associations with total sperm count, semen volume, and sperm concentration than BMI and waist-to-height ratio, suggesting it may be a better measure of adiposity.
• Among men with normal BMI (18.5-24.9), body fat percentage showed even stronger associations with total sperm count (13% decrease per standard deviation, 95% CI, -21% to -5%) and normal morphology (7% decrease, 95% CI, -14% to 0%), although associations with reproductive hormones attenuated.

IN PRACTICE

“If our associations reflect causality, they suggest that a higher percentage of adipose tissue might adversely affect semen production and hormonal balance,” wrote the authors of the study.

SOURCE

The study was led by Nis Brix, MD, Department of Public Health, Research Unit for Epidemiology, Aarhus University in Aarhus, Denmark. It was published online in Human Reproduction.

LIMITATIONS

The participation rate was low at 19% in FEPOS, which researchers attempted to address using selection weights. Body fat percentage measured by bioelectrical impedance may involve misclassification despite being a better measure than BMI. The cross-sectional data collection poses a risk for reverse causation, although a sensitivity analysis using BMI at 11 years of age as the exposure indicated minimal risk. The measurement of body fat percentage by bioelectrical impedance is sensitive to hydration status, and measurements were done non-fasting without instructions to abstain from exercise, potentially introducing non-differential measurement error. Wide day-to-day variations in semen characteristics may also affect the results, though this is not expected to introduce systematic error.

DISCLOSURES

This study was supported by the ReproUnion collaborative study, co-financed by the European Union, Interreg V-A ÖKS. The FEPOS project received funding from the Lundbeck Foundation, the Capital Region of Denmark, Medical Doctor Sofus Carl Emil Friis and Spouse Olga Doris Friis’s Grant, Axel Muusfeldt’s Foundation, and A.P. Moller Foundation. Additional funding was provided by the Independent Research Fund Denmark and co-funded by the EU. K.S. Hougaard disclosed having Eli Lilly and Novo Nordisk stocks. The remaining authors reported no conflicts of interest.

This article was created using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.