Men with higher body fat percentages have lower sperm counts than men with lower body fat levels, according to a study published July 29 in the journal Human Reproduction. The research also found that body fat percentage was a better predictor of male reproductive health than body mass index, a common but limited measurement tool.
Reported by Healthline, the study found that higher body fat percentages were associated not only with lower sperm counts but also with lower semen volume, lower sperm concentration, and poorly formed sperm. Changes in male reproductive hormones, including testosterone, were also observed.
One of the most significant findings was that men with a normal BMI but a relatively high body fat percentage still showed lower sperm counts and reduced sperm quality. That matters because BMI does not distinguish between fat mass and lean body mass such as muscle. As a result, two men with the same BMI can have very different body fat levels, making BMI an imprecise tool for assessing reproductive risk.
"[Our research] suggests that measuring body fat percentage may identify men at risk of impaired reproductive health who would not be identified based on BMI alone," said Nis Brix, MD, an associate professor in the Department of Public Health at Aarhus University in Denmark and first author of the study, in a statement.
The study analyzed data from 1,058 Danish men. The researchers are also involved in a broader project called BIOSFER, which examines biological and social factors behind low fertility. That project is collecting reproductive health data, including semen samples, from 1,250 Danish men and 1,250 Norwegian men.
Philip Werthman, MD, a urologist and director of the Center for Male Reproductive Medicine and Vasectomy Reversal in Los Angeles, said the issue of declining male fertility deserves serious attention. Werthman was not involved in the study.
"The decline in male fertility is real and significant," he told Healthline. "[It's] not a subtle shift. It's a major public health signal."
Previous research has linked obesity and weight-related issues to worse semen quality and reduced fertility in men. This new study adds to that body of evidence while pointing to body fat percentage as a more precise measure for identifying men who may be at risk. Researchers said men may be able to improve reproductive health by maintaining a healthy body weight, eating a balanced diet, and reducing stress.
