Hidden body fat may be lowering sperm counts – Earth.com
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07-31-2026
Body fat may be lowering sperm counts, even in men with a healthy weight
ByAndrei Ionescu
Earth.com staff writer
A man’s BMI might be telling doctors far less than they think about his fertility
New research involving more than 1,000 men found that body fat percentage was linked to lower sperm counts and changes in reproductive hormones such as testosterone
That connection held up even in men whose BMI appeared completely normal
The study drew on the Fetal Programming of Semen Quality group, a subset of the much larger Danish National Birth Cohort
The data included roughly 100,000 mother-child pairs recruited between 1996 and 2002.002
Scientists already knew that being above average weight or obese, usually judged by BMI, tends to go hand in hand with worse reproductive health in men.
The problem is that BMI only guesses at body fat indirectly. It cannot tell the difference between fat and muscle. That means two men with the same BMI could have very different amounts of body fat
“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 researchers also found that men with a normal BMI but a relatively high body fat percentage had lower sperm counts and more poorly formed sperm
Brix noted body fat percentage may identify men at risk of reproductive problems who would not be flagged by BMI alone
How the study was conducted
Between March 2017 and December 2019, researchers reached out to 5,697 young men right around their 18th birthday, and 1,058 of them, about 19 percent, agreed to take part
The team measured body fat with a bioimpedance analyzer. The device sends a faint electrical current through the body. It then measures the voltage to estimate fat, water, and muscle content
The researchers also recorded each participant’s BMI and waist-to-height ratio. They collected semen and blood samples, measured testicular size, and had everyone complete a lifestyle questionnaire
Body fat percentage got sorted into four categories: low, under 8 percent; normal, 8 to 19 percent; high, 20 to 24 percent; and very high, 25 percent and up
Higher body fat, fewer sperm
Men in the high body fat category had, on average, 23 percent lower sperm counts than men with normal body fat. Men in the very high category came in 15 percent lower.
Both groups also showed lower sex hormone-binding globulin and testosterone, alongside higher luteinising hormone, oestradiol, and free androgen index
These are all signs the balance of male reproductive hormones was genuinely shifting.
There was a hint of lower semen volume and concentration too, though that particular finding didn’t quite reach statistical significance. Sperm motility, shape, and testicular size didn’t show any clear pattern either way
Put in plain numbers, that 15 percent drop for men with very high body fat works out to a decline from roughly 105 million sperm per ejaculate down to about 89 million.
The 23 percent drop for men with high body fat means falling from 105 million to around 81 million.
Both numbers remain above the World Health Organization’s lower cutoff of 39 million. However, the researchers note that a drop of that size could matter for men already near the threshold for poor semen quality
Body fat gave clearer answers
The researchers used standard de It is a statistical measure of how much values typically vary across a population
“About 95 percent of people have values within two standard dea meaningful and comparable difference,” said Brix.
Researchers used standard de to compare body fat percentage and BMI on the same scale
Among men with a normal BMI, a one-SD increase in body fat percentage was linked to a 13 percent lower sperm count. The same 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
This study included a genuinely large sample from the general population. Researchers also collected unusually detailed data on body composition, reproductive health, and lifestyle habits
No previous study had compared body fat percentage with both BMI and waist-to-height ratio
None had examined whether body fat percentage predicts poorer semen quality in men whose BMI falls within the normal range
The main limitation was participation: only 19 percent of the men invited actually joined the study
Researchers are already building on these findings through the larger BIOSFER project, short for “Untangling the biological and social determinants of low fertility in modern societies.”
That project is currently gathering detailed reproductive health data, including semen samples, from 1,250 Danish men and 1,250 Norwegian men
The team hopes to see whether these findings hold up once the new data become available
The study is published in the journal Human Reproduction
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