A new study suggests that a man’s body mass index (BMI) may provide doctors with far less insight into his fertility than commonly assumed. The research, involving more than 1,000 men and published in Human Reproduction, shows that body fat percentage is linked to lower sperm counts and changes in reproductive hormone levels, including testosterone.
This association held true even among men with a completely normal BMI. The study drew on data from the Fetal Programming of Semen Quality cohort, a subgroup of the much larger Danish National Birth Cohort, which includes roughly 100,000 mother‑child pairs recruited between 1996 and 2002.
Scientists have long known that excess weight or obesity — typically assessed via BMI — tends to be associated with poorer male reproductive health. The issue is that BMI only indirectly reflects body fat; it cannot distinguish fat from muscle. This means two men with the same BMI may have very different body fat levels.
“Our study showed that men with a high percentage of body fat tend to have fewer sperm than men with a normal percentage of body fat,” said lead author Nis Brix of Aarhus University. “They also had lower semen volume and lower sperm concentration. Importantly, body fat percentage was a better predictor of reduced sperm count than either BMI or waist‑to‑height ratio.”
Researchers also found that men with a normal BMI but relatively high body fat had fewer sperm and a higher proportion of abnormally shaped sperm cells.
Brix noted that measuring body fat percentage could help identify men at risk of reproductive issues who might otherwise be overlooked if doctors relied solely on BMI.
Between March 2017 and December 2019, the team contacted 5,697 young men around the age of 18; 1,058 of them (about 19 %) agreed to participate. Researchers measured body fat using a bioimpedance analyzer, which sends a weak electrical current through the body and measures voltage to estimate the amounts of fat, water, and muscle.
The team also recorded each participant’s BMI and waist‑to‑height ratio, collected semen and blood samples, measured testicle size, and asked all men to complete a lifestyle questionnaire. Body fat percentage was categorized into four groups: low (less than 8 %), normal (8–19 %), high (20–24 %), and very high (25 % or more).
Men with a high body fat percentage had, on average, a 23 % lower sperm count compared to those with a normal fat percentage. For men in the very high fat category, the reduction was 15 %. Both groups also showed decreased levels of sex hormone‑binding globulin and testosterone, along with increased levels of luteinizing hormone, estradiol, and the free androgen index — indicating a real shift in the balance of male reproductive hormones.
There was also some decline in semen volume and concentration, though these results did not reach statistical significance. Sperm motility, morphology, and testicle size did not show any clear trend. In concrete terms, a 15 % drop for men with very high body fat translates roughly from 105 million sperm per ejaculate down to about 89 million.
Although both figures remain above the World Health Organization’s lower threshold of 39 million, researchers note that such reductions can matter for men whose sperm quality is already borderline low.
To compare body fat percentage and BMI on the same scale, the researchers used standard deviations (SD), a statistical measure of how much values typically vary within a population.
“About 95 % of people fall within two standard deviations of the mean, so a change of one SD represents a meaningful and comparable difference,” Brix explained.
Using this approach, the team found that, among men with normal BMI, an increase in body fat by one standard deviation was associated with a 13 % reduction in sperm count. A similar increase in BMI was linked to only an 8 % reduction.
“This indicates that body fat percentage is a more reliable indicator of sperm count than BMI,” Brix said.
“For men, maintaining a healthy body composition — not just a healthy weight relative to height — may be an important factor for fertility,” Brix added. “For clinicians, assessing a patient’s body fat percentage may help identify men at risk of reduced fertility who might be missed when relying on BMI alone.”
“More broadly, the findings highlight the importance of preventing excess fat accumulation through a healthy lifestyle, which can benefit both reproductive and overall health,” he continued. “Future studies should confirm our results in other populations, especially among men with normal BMI. We also need to investigate why higher body fat percentages may reduce sperm counts. This will help us identify at‑risk men earlier and develop strategies to improve male fertility.”
The study featured a large sample drawn from the general population and collected unusually detailed data on body composition, reproductive health, and lifestyle. No prior research had directly compared body fat percentage with BMI and waist‑to‑height ratio in this way.
Moreover, no earlier study had examined whether body fat percentage predicts poorer semen quality in men whose BMI falls within the normal range. The main limitation was the participation rate: only 19 % of the invited men took part.
Researchers are now building on these findings through the larger BIOSFER project (Biological and Social Factors Determining Low Fertility in Modern Societies). This ongoing initiative is collecting detailed reproductive health data, including semen samples, from 1,250 Danish and 1,250 Norwegian men. The team hopes to verify whether these results hold up with the new dataset.
About the Author
