July 30, 2026, 3:39 pm | Read time: 4 minutes
When assessing their weight, people often look at the Body Mass Index (BMI). However, the BMI has a significant drawback: it does not differentiate between fat and muscle mass. A new Danish study examined whether body fat percentage is related to male fertility characteristics. The results suggest that body fat percentage might be more indicative than BMI in this regard.
The More Fat, the Fewer Sperm
The most important finding of the study is that men with a higher body fat percentage tended to have fewer sperm and changes in key reproductive hormones. Levels of testosterone and sex hormone-binding globulin (SHBG), which transports testosterone in the blood, were lower.1
At the same time, concentrations of luteinizing hormone (LH), which regulates testosterone production, and the hormone estradiol were increased. Estradiol is partly derived from testosterone and can be produced in greater amounts with higher body fat percentage.
How the Study Was Conducted
The background of the study is that obesity is increasing worldwide, while studies simultaneously describe declining sperm counts and rising infertility rates. Since male factors can play a role in up to half of all involuntarily childless couples, the researchers wanted to examine whether body fat percentage is related to fertility.
For the cross-sectional study, they analyzed data from 1,058 young Danish men with an average age of just under 19 years. Body fat percentage was measured using bioelectrical impedance analysis (BIA). Participants were divided into four groups based on their results:
- under 8 percent body fat
- 8 to 19 percent body fat (reference group)
- 20 to 24 percent body fat
- at least 25 percent body fat
Additionally, participants provided semen and blood samples, self-measured their testicular volume, and answered questions about their lifestyle. The researchers then examined possible connections between body fat percentage, semen quality, testicular volume, and hormone levels.
Body Fat Percentage Predicts Sperm Count Better Than BMI
Compared to men with a body fat percentage of 8 to 19 percent, participants with 20 to 24 percent body fat had, on average, 23 percent fewer sperm.
When the scientists considered body fat percentage as a continuous value, a similar pattern emerged: with each 5.7 percent increase in body fat percentage, the total sperm count decreased by an average of eight percent. The researchers found no clear connections for sperm motility and shape or for testicular volume.
The connections between body fat percentage and sperm count were more pronounced than with BMI or waist-to-height ratio. They were also evident in some men who were considered normal weight according to BMI. This suggests that body fat percentage might better reflect actual fat mass and its potential impact on fertility than BMI.
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Strengths and Weaknesses of the Study
The strengths of the study include the large number of participants, the direct measurement of body fat percentage, and the consideration of many possible confounding factors. Additionally, the researchers compared body fat percentage with BMI and waist-to-height ratio. A limitation is that only 19 percent of the invited men participated. They self-measured testicular volume, so measurement errors cannot be ruled out. Furthermore, the measurement of body fat percentage can be influenced by fluid balance.
The decrease in sperm count with higher fat percentage was visible but statistically so small that it cannot be ruled out as a coincidence. As a cross-sectional study, the investigation also cannot prove cause-and-effect relationships.
Also interesting: What happens to men’s sperm as they age
Losing Weight Really Helps
A meta-analysis from Guangzhou University in 2024, which evaluated 50 studies with a total of 71,337 men, reached a similar conclusion to the current study from Aarhus University: Men with obesity had significantly fewer sperm, lower semen volume, and slightly poorer sperm motility compared to men with a BMI under 25.2
The intriguing question is whether this can be reversed. A Danish research team from the University of Copenhagen investigated this in a randomized study. Fifty-six men with obesity consumed only 800 calories daily for eight weeks and lost an average of 16.5 kilograms. Afterward, their sperm concentration increased by about 1.5 times, and their total sperm count increased by about 1.4 times.3
However, what happened afterward was crucial: The men maintained their weight either through exercise or with the help of a medication (GLP-1 receptor agonist). Both methods helped to secure sperm quality in the long term. For those who regained weight, the effect disappeared. Weight loss, however, had no impact on semen volume and sperm motility.