A study of more than 1,000 young Danish men found that higher body fat percentage is associated with lower total sperm count and altered reproductive hormone levels, according to research published on Aug. 2. The study reported that the association between increased body fat and reduced sperm count was strongest among men who had a normal body mass index (BMI). No association was found between body fat percentage and testicular volume.
The research, conducted as part of the Fetal Programming of Semen Quality (FEPOS) cohort, involved 1,058 Danish men around 19 years old. Body fat percentage was measured using bioelectrical impedance, while BMI and waist-to-height ratio were also recorded for comparison. Participants provided semen and blood samples, self-measured their testicular volume, and completed health and lifestyle questionnaires. Analyses accounted for parental education levels, maternal BMI before pregnancy, maternal smoking during the first trimester, birth weight for gestational age, participants’ own smoking habits, alcohol use, physical activity levels, and sample-collection factors.
Among those with complete data on all covariates—978 participants—75% had normal body fat percentages; 11% had high values; 5% had very high values; and 9% were classified as low. Men with higher body fat percentages tended to have parents with less education and mothers who smoked or had higher BMI before pregnancy. These individuals also showed higher birth weights for gestational age as well as elevated BMI and waist-to-height ratios compared to their peers.
Men classified as having high or very high body fat experienced a reduction in total sperm count by up to 23%, compared to those with normal percentages. Although mean total sperm counts remained above reference limits in this population overall, researchers said such reductions could be clinically relevant for some individuals with borderline semen quality. No clear associations were observed between increased adiposity and either sperm motility or morphology across the full cohort.
Researchers noted that "body fat percentage may better capture adiposity than BMI," since BMI does not distinguish between lean mass and fat mass—a limitation highlighted by stronger associations seen when using direct measures of adiposity rather than composite indices like BMI or waist-to-height ratio. The cross-sectional design means causal relationships cannot be established from these findings alone.
The authors concluded that further prospective studies are needed to clarify whether these observed relationships are causal in nature: "These associations need to be explored in future prospective studies to clarify their temporal relationship and determine whether they are causal."
