Body fat percentage – better than BMI at predicting male fertility?

Body fat percentage may better capture the impact of increased fat levels on male reproductive health, according to a new Danish study.

Male factors are thought to contribute to up to half of all cases of infertility. Previous studies have linked excess body fat, commonly assessed using a formula known as body mass index (BMI), with poorer male fertility. However, BMI is a simple measure of weight relative to height and does not distinguish between fat and muscle mass: as a result, men with the same BMI may have different levels of body fat. With obesity and infertility rates both increasing worldwide, researchers explored whether body fat percentage could better reflect male fertility.

'Our study found that men with a high body fat percentage generally had lower sperm counts than men with a normal body fat percentage. They also tended to have lower semen volume and sperm concentration,' said Dr Nis Brix, associate professor in the department of public health at Aarhus University, Denmark, and first author of the study published in Human Reproduction.

The research included 1058 young Danish men, whose body fat percentage was directly measured using bioelectrical impedance analysis, a technique that relies on how an electrical current moves through the body. The men also provided semen and blood samples, self-assessed their testicular volume and completed lifestyle questionnaires. The researchers investigated how body fat percentage, as well as traditional measures such as BMI and waist-to-height ratio, related to markers of male reproductive health.

Overall, a higher body fat percentage was associated with poorer semen characteristics and altered reproductive hormone profiles, including among some men with a normal BMI.

Men in the high body fat group (defined as 20-24 percent of the body mass) had 23 percent fewer sperm on average, with each 5.7 percent increase in body fat associated with an eight percent reduction in total sperm count. The relationship between body fat percentage and sperm count was stronger than that observed with BMI or waist-to-height ratio.

Increased body fat percentage was also associated with lower sex hormone-binding globulin and testosterone levels, and higher amounts of luteinising hormone and oestradiol. However, no consistent associations were found between body fat percentage and testicular volume, sperm motility or morphology.

According to Dr Brix, these results indicate that body fat percentage may be a better predictor of sperm count compared to traditional measures, and that it could help identify men who have decreased fertility despite their BMI being normal.

'Future studies should confirm our findings in other groups, particularly our findings in men with a normal BMI,' he added. 'They should also investigate why a higher body fat percentage may reduce sperm count. This could help identify men at risk earlier and guide future strategies to improve male fertility.'