Summary
A new study finds that body fat percentage is a stronger predictor of male reproductive health than body mass index (BMI). Higher body fat percentages were linked with lower sperm counts, reduced semen volume and concentration, and changes in male reproductive hormones including lower sex hormone-binding globulin (SHBG) and testosterone.
The study
Researchers published the findings in Human Reproduction on July 29. They analyzed 1,058 Danish men who were 18 years and 9 months old. Body fat percentage was measured with a bioimpedance analyzer and divided into four groups: low (<8%), normal (8–19%), high (20–24%), and very high (≥25%). The team also recorded BMI, waist-to-height ratio, semen and blood samples, testicle size, and health behavior questionnaires.
Key findings
– Men in the high body fat category had, on average, a 23% lower sperm count compared with men in the normal body fat group. Men categorized as very high had a 15% lower sperm count than those with normal body fat. Higher fat categories were also associated with lower semen volume and sperm concentration.
– Higher body fat percentages corresponded with lower SHBG and testosterone and altered balances of reproductive hormones.
– Body fat percentage outperformed BMI and waist-to-height ratio as a predictor of reduced sperm count, and men with a normal BMI but relatively high body fat still showed poorer semen metrics.
Interpretation and implications
Lead author Nis Brix, an associate professor at Aarhus University, said measuring body fat percentage may identify men at risk of impaired fertility who would be missed if clinicians relied on BMI alone. The researchers are part of the larger BIOSFER project, which is collecting reproductive and social health data from 1,250 Danish and 1,250 Norwegian men to study biological and social determinants of low fertility.
Expert perspectives
Urologist Philip Werthman, not involved in the study, emphasized that declining male fertility is a significant public health concern. He noted that excess fat tissue can convert testosterone to estrogen and directly impair sperm production. Werthman recommends a semen analysis when fertility issues are suspected because it is a noninvasive, inexpensive test that provides information on sperm count, motility, and morphology.
Urologist S. Adam Ramin said the findings help explain why sperm counts drop with increasing body fat and cautioned that low sperm counts may signal broader health problems.
Context and prevalence
Infertility affects about 15% of couples in the United States. The male partner is solely responsible for roughly 20% of infertility cases and contributes to another 30–40%. While some recent research suggests sperm concentration has been stable in some populations, other long-term studies show declines in sperm counts worldwide. This study adds evidence that body composition, not just weight-for-height, matters for male reproductive health.
Risk factors for male infertility
Common contributors to low sperm count and other male fertility problems include:
– Obesity and excess body fat
– Testicular injury or surgery
– Sexually transmitted infections
– Exposure to pesticides, industrial chemicals, or radiation
– Excessive alcohol use, smoking, or recreational drugs
– Anabolic steroid use
Practical steps to support reproductive health
The study authors and clinical experts recommend general lifestyle measures that can benefit both fertility and overall health:
– Aim for a healthy body composition through diet and regular exercise
– Eat a balanced diet rich in nutrients and consider antioxidant support when advised by a clinician
– Minimize stress and prioritize consistent, quality sleep
– Quit smoking and limit alcohol intake
– Get a semen analysis if you suspect fertility problems
– Favor loose-fitting clothing to support testicular blood flow
– Avoid prolonged exposure of the testicles to high heat (for example, long hot baths or frequent sauna/jacuzzi use)
Bottom line
Body fat percentage appears to be a better marker than BMI for identifying young men at risk of reduced sperm count and hormonal changes. Assessing body composition and addressing excess fat through lifestyle changes may help improve reproductive outcomes and overall health. Men worried about fertility should consider clinical evaluation, including a semen analysis, and discuss targeted strategies with a healthcare provider.