BMI and body fat percentage: a clearer signal for male fertility and sperm count

A man’s BMI may tell clinicians much less than they assume about fertility.

In a new study of more than 1,000 men, a higher body fat percentage was associated with lower sperm counts and shifts in key reproductive hormones, including testosterone.

Notably, the same relationship was seen even among men whose BMI sat firmly in the normal range.

The work used participants from the Fetal Programming of Semen Quality group, which sits within the far larger Danish National Birth Cohort.

That wider cohort includes around 100,000 mother–child pairs recruited from 1996 to 2002.

BMI has always had a blind spot

Researchers have long understood that carrying excess weight or living with obesity-typically assessed using BMI-often coincides with poorer male reproductive health.

However, BMI only offers an indirect proxy for fatness. It cannot distinguish fat mass from muscle mass, so two men with identical BMI values may have very different body fat levels.

“Our study found that men with a high body fat percentage generally had lower sperm counts than men with a normal body fat percentage,” said lead author Nis Brix, an associate professor at Aarhus University.

“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.”

The analysis also showed that men with a normal BMI but comparatively high body fat had reduced sperm counts and a greater share of abnormally formed sperm.

Brix added that body fat percentage could help pinpoint men at risk of reproductive difficulties who would not be identified by BMI alone.

How the study was conducted

From March 2017 to December 2019, the researchers contacted 5,697 young men around their 18th birthday. A total of 1,058 men-about 19 percent-agreed to participate.

Body fat was assessed using a bioimpedance analyser, which passes a very small electrical current through the body and uses the resulting voltage measurements to estimate fat, water and muscle.

For each participant, the team also recorded BMI and waist-to-height ratio. Semen and blood samples were taken, testicular size was measured, and participants completed a lifestyle questionnaire.

Body fat percentage was grouped into four bands: low (under 8 percent), normal (8 to 19 percent), high (20 to 24 percent) and very high (25 percent and above).

Higher body fat, fewer sperm

On average, men in the high body fat group had sperm counts that were 23 percent lower than those with normal body fat. Men in the very high group averaged 15 percent lower.

Across both higher-fat groups, the researchers observed lower sex hormone-binding globulin and testosterone, together with higher luteinising hormone, oestradiol and free androgen index.

Taken together, these patterns indicate that the balance of male reproductive hormones was shifting.

There were also indications of reduced semen volume and sperm concentration, although those particular results did not quite reach statistical significance. Meanwhile, sperm motility, sperm shape and testicular size did not show a consistent trend in either direction.

In straightforward terms, the 15 percent reduction seen in men with very high body fat corresponds to a drop from roughly 105 million sperm per ejaculate to about 89 million.

The 23 percent reduction among men with high body fat equates to a fall from 105 million to around 81 million.

Both figures are still above the World Health Organization’s lower cutoff of 39 million. Even so, the researchers point out that a decrease of this magnitude could be important for men who are already close to the threshold for poor semen quality.

Body fat gave clearer answers

To compare BMI and body fat percentage directly, the team used standard deviation-a statistical measure describing how much values typically vary within a population.

“About 95 percent of people have values within two standard deviations [SD] of the average, so a change of one SD represents a meaningful and comparable difference,” said Brix.

Researchers used standard deviations (SDs) to compare body fat percentage and BMI on the same scale.

Within the group of men whose BMI was normal, a one-SD rise in body fat percentage was linked to a 13 percent lower sperm count. By contrast, the same one-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,” said Brix.

A better way to assess fertility

“For men, maintaining a healthy body composition, not just a healthy body weight for height, may be important for fertility,” Brix explained.

“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.”

The next step for fertility research

A key strength of the study was its large sample drawn from the general population, paired with unusually detailed measurements of body composition, reproductive health and lifestyle.

The authors note that no earlier research had directly set body fat percentage alongside both BMI and waist-to-height ratio.

They also state that previous studies had not tested whether body fat percentage can indicate poorer semen quality among men whose BMI remains in the normal range.

The main limitation was participation: only 19 percent of those invited ultimately took part.

The researchers are now extending this line of work through the larger BIOSFER project-short for “Untangling the biological and social determinants of low fertility in modern societies.”

That project is currently collecting in-depth reproductive health information, including semen samples, from 1,250 Danish men and 1,250 Norwegian men.

The team aims to determine whether the same patterns persist once these new data are available.