On July 29, 2026, researchers published a study in the journal Human Reproduction finding that body fat percentage is a better predictor of male reproductive health than body mass index — including in men whose BMI reads as perfectly normal. The headline number: men with higher body fat percentage had roughly 23% lower sperm counts than men with normal body fat, alongside lower semen volume, lower sperm concentration, and altered reproductive hormones including testosterone.

Why It Matters

Male fertility testing is one of the fastest-commercializing corners of sexual health — at-home sperm kits from Legacy and Yo Sperm, sperm-health supplement lines, and fertility-adjacent TRT telehealth all sell against a vague narrative of declining counts. This study hands that market a more specific and more actionable metric than the one it's been using. If body fat percentage outperforms BMI, the obvious product implication is that body-composition measurement (smart scales, bioimpedance, DEXA) becomes a legitimate on-ramp to male fertility services rather than a wellness afterthought. It also complicates the GLP-1 story: weight-loss telehealth platforms increasingly cross-sell sexual health, and "lose weight, improve fertility" is a cleaner pitch if the endpoint is fat mass rather than the scale. Clinicians, meanwhile, get a reason to look past a normal BMI in men presenting with unexplained infertility.

The work drew on the Fetal Programming of Semen Quality group, a subset of the Danish National Birth Cohort, which recruited roughly 100,000 mother-child pairs between 1996 and 2002. Between March 2017 and December 2019 the team contacted 5,697 young men around their 18th birthday; 1,058 — about 19% — agreed to participate. Researchers measured body composition using a bioimpedance analyzer, recorded BMI and waist-to-height ratio, collected semen and blood samples, measured testicular size, and administered a lifestyle questionnaire.

The comparison is the interesting part. Among men with a normal BMI, a one-standard-deviation increase in body fat percentage was associated with a 13% lower sperm count, while 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," lead author Nis Brix, an associate professor at Aarhus University, said in a statement. Men with normal BMI but relatively high body fat also showed more poorly formed sperm. Brix's practical read: "For men, maintaining a healthy body composition, not just a healthy body weight for height, may be important for fertility."

The limitations are honest and worth stating. Participation was only 19% of those invited, which raises selection-bias questions, and the cohort is a single national population of 18-year-old Danish men. Brix has said future work should confirm the findings in other groups and probe the mechanism. His team is already collecting reproductive-health data, including semen samples, from 1,250 Danish and 1,250 Norwegian men through the larger BIOSFER project on the biological and social determinants of low fertility.

Outside experts framed it against a broader trend. Urologist Philip Werthman, director of the Center for Male Reproductive Medicine and Vasectomy Reversal in Los Angeles, who was not involved in the study, told Healthline that "the decline in male fertility is real and significant… not a subtle shift. It's a major public health signal."

Sources

Update — 2026-08-01

{Initial entry — story first created.}