• Posted August 4, 2026

High Body Fat, Not BMI, Linked To Lower Sperm Counts

A man’s body mass index (BMI) is a poorer indication of his fertility than other, more precise measures of body fat, a new study says.

A higher body fat percentage is linked to lower sperm counts and changes in the balance of testosterone and other male hormones, researchers reported July 29 in the journal Human Reproduction.

This association held even if the men had normal BMIs but higher percentages of body fat, researchers said. BMI is an estimate of body fat based on height and weight.

“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,” lead researcher Dr. Nis Brix said in a news release.

“Importantly, body fat percentage was a better predictor of reduced sperm count than BMI or waist-to-height ratio,” Brix said. He is an associate professor of public health at Aarhus University in Denmark.

BMI has been losing favor as a measure of overweight and obesity, because it doesn’t distinguish between fat mass and muscle mass, researchers said in background notes.

For example, very fit athletes and bodybuilders who add lots of muscle to their bodies can have a BMI that would indicate obesity, researchers noted.

For the new study, researchers employed a very precise method for analyzing body fat — a bioimpedance analyzer, which uses a weak electrical current to estimate body fat, body water and muscle mass.

This device was used to track body fat of more than 1,000 Danish men participating in a study of male fertility. The men also had their BMI and waist-to-height ratio calculated.

Men who had high body fat (20% to 24%) had on average a 23% lower sperm count than men with normal body fat (8% to 19%), the study found. Men with very high body fat (25% or more) had a 15% lower sperm count.

These reductions could interfere with fertility among men with borderline semen quality, researchers said.

Further analysis showed that increases in body fat were associated with a larger drop in sperm count than increases in BMI.

“We found that a one-standard-deviation (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,” Brix said. “This suggests that body fat percentage is a stronger predictor of sperm count than BMI.”

Results also showed that higher levels of body fat were linked to imbalances in male reproductive hormones, as well as a tendency toward lower semen quality and concentration.

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

More information

The American Society for Reproductive Medicine has more on obesity and reproduction.

SOURCE: European Society of Human Reproduction and Embryology, news release, July 29, 2026