Body fat percentage predicts male fertility better than BMI

A man's body fat percentage may be a better indicator of his reproductive health than body mass index (BMI) according to a large study of over 1000 Danish men.

In a study published today (Thursday) in Human Reproduction, one of the world's leading reproductive medicine journals, researchers found that a higher body fat percentage was linked to lower sperm counts, and also associated with a change in the balance of male reproductive hormones, including testosterone levels.

These associations remained even if the men had normal BMIs, but their body fat percentage was higher than normal.

First author of the study, Dr Nis Brix, an associate professor in the Department of Public Health at Aarhus University, Denmark, 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."

Previous studies have shown that overweight and obesity, which are usually assessed using BMI, are associated with impaired reproductive health in men, including worse semen quality and reduced fertility. However, BMI is only an indirect measure of fatty tissues and does not distinguish between fat mass and lean body mass, which can include muscle mass. As a result, men with the same BMI can have markedly different levels of body fat.

In the current study, the researchers measured body fat percentage using a bioimpedance analyser, which sends a weak electric current through the body and the voltage is measured in order to estimate body fat, body water and muscle mass. 

The researchers used data from the Fetal Programming of Semen Quality (FEPOS) group of men, which is a sub-group of the Danish National Birth Cohort and includes 100,000 pairs of mothers and children, recruited between 1996 and 2002. Between March 2017 and December 2019, the researchers invited 5697 young men who had just turned 18 years and 9 months to the study, and 1058 (19%) participated.

As well as body fat percentage, they measured BMI and waist-to-height ratio. The men provided semen and blood samples, measured the size of their testicles (testicular volume), and completed a questionnaire on health behaviour.

Body fat percentage was divided into four categories: low (less than 8% fat), normal (8-19%), high (20-24%) and very high (25% or more).

Compared to normal body fat percentage, men who had high body fat percentages had, on average, a 23% lower sperm count; men with very high body fat percentages had a 15% lower sperm count. They also had lower sex hormone-binding globulin (SHBG) and testosterone levels, and higher levels of luteinising, oestradiol and free androgen index, reflecting an alteration in the balance of male reproductive hormones. There was a tendency towards lower semen volume and concentration, although this was not statistically significant. There was no consistent pattern for sperm motility, form (morphology) and testicular volume.

A reduction in sperm count of around 15% in men with very high body fat percentage, compared to men with a normal body fat percentage, corresponds to a reduction in sperm count from an average of about 105 to 89 million per ejaculate. In men with a high body fat percentage, a 23% reduction in total sperm count corresponds to a reduction from 105 to 81 million. These reductions are above the World Health Organization's lower limit of 39 million, but the researchers say they may still be clinically relevant for men with borderline semen quality.

The researchers looked at how body fat percentage compared to BMI using standard deviation (SD) – a way of describing a typical amount of variation within a population.

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.

"For men, maintaining a healthy body composition, not just a healthy body weight for height, may be important for fertility. 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. More broadly, the results highlight the importance of preventing excess body fat through healthy lifestyle initiatives, which may benefit both reproductive health and overall health.

"Future studies should confirm our findings in other groups, particularly our findings in men with a normal BMI. 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."

Strengths of the study include the large number of men taken from the general population and with detailed measurements of body composition, reproductive health and lifestyle factors. No other study has compared body fat percentage with BMI and waist-to-hip ratio, and no study has investigated whether body fat percentage is associated with impaired semen quality even among men with a normal BMI. Limitations include the low (19%) participation rate in the FEPOS.

Now the researchers are collecting detailed information on reproductive health, including semen samples, in 1,250 Danish men and 1,250 Norwegian men as part of the large BIOSFER project ('Untangling the biological and social determinants of low fertility in modern societies') led by last author of the current study, Cecilia Høst Ramlau-Hansen, and collaborators from Norway and Germany. The researchers look forward to replicating this study in these new data.