A large study of over 1,000 Danish men reports that body fat percentage may be a more accurate indicator of male reproductive health than body mass index, according to findings published on Jul. 29 in Human Reproduction.
Researchers found that higher body fat percentages were linked to lower sperm counts and changes in the balance of male reproductive hormones, including testosterone levels. These associations remained even among men with normal BMIs but elevated body fat percentages.
Dr. Nis Brix, associate professor in the Department of Public Health at Aarhus University and first author of the study, said, "Our study found that men with a high body fat percentage generally had lower sperm counts than men with a normal body fat percentage and they also tended to have lower semen volume and sperm concentration. Importantly, body fat percentage was a better predictor of reduced sperm count than BMI or waist-to-height ratio. We also found that even men with a normal BMI, but a relatively high body fat percentage, had lower sperm counts and poorly formed sperm. This suggests that measuring body fat percentage may identify men at risk of impaired reproductive health who would not be identified based on BMI alone."
The study measured participants' body composition using bioimpedance analysis and collected data from the Fetal Programming of Semen Quality group within the Danish National Birth Cohort. The researchers categorized participants' body fat into four groups: low (less than 8%), normal (8-19%), high (20-24%), and very high (25% or more). Men with high or very high percentages showed average reductions in total sperm count by 23% and 15%, respectively.
Dr. Brix said, "About 95% of people have values within two standard deviations of the average, so a change of one SD represents a meaningful and comparable difference. Using SDs allowed us to compare body fat percentage and BMI on the same scale. Using this approach, we found that a 1-SD increase in body fat percentage among men with a normal weight BMI was associated with a 13% lower sperm count, whereas a 1-SD increase in BMI was associated with only an 8% lower sperm count. This suggests that body fat percentage is a stronger predictor of sperm count than BMI." He added, "For doctors, assessing body fat percentage in selected patients could help identify men at risk of reduced fertility who might otherwise be overlooked based on BMI alone."
The researchers are now collecting further data as part of the BIOSFER project involving Danish and Norwegian participants to replicate these findings.
