Men with higher body fat percentages have significantly lower sperm counts and altered reproductive hormones, regardless of whether their overall weight appears normal on a standard scale. A new study published in the journal Human Reproduction found that body fat percentage is a better predictor of male reproductive health than body mass index (BMI) alone, revealing a gap in how doctors currently assess fertility risk.

Why Body Fat Matters More Than Your Weight?

Researchers from Aarhus University in Denmark studied 1,058 young men around 18 years old, measuring their body fat using a bioimpedance analyzer, a device that sends a weak electrical current through the body to estimate fat, water, and muscle mass. The findings were striking: compared to men with normal body fat levels, those with high body fat percentages had a 23% lower sperm count, while men with very high body fat had a 15% lower sperm count.

The key insight is that BMI, which divides weight by height, doesn't distinguish between fat and muscle. Two men with identical BMI readings can have vastly different body compositions. "Measuring body fat percentage may identify men at risk of impaired reproductive health who would not be identified based on BMI alone," explained Nis Brix, MD, the study's lead author and an associate professor in the Department of Public Health at Aarhus University.

"For men, maintaining a healthy body composition, not just a healthy body weight for height, may be important for fertility. 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," said Nis Brix, MD.

Nis Brix, MD, Associate Professor, Department of Public Health, Aarhus University

How Does Excess Body Fat Damage Sperm Production?

The biological mechanism behind this connection is well understood. Excess fat tissue acts like an endocrine organ, converting testosterone into estrogen. This hormonal shift directly impairs sperm production and reduces sex hormone-binding globulin (SHBG) and testosterone levels. Men in the higher body fat categories also showed altered balances of male reproductive hormones overall.

"Obesity alone plays a major role as excess fat tissue converts testosterone into estrogen and directly impairs sperm production. It is important we recognize that modern life is actively working against male reproductive health," explained Philip Werthman, MD, a urologist and men's health specialist at the Center for Male Reproductive Medicine and Vasectomy Reversal in Los Angeles.

Philip Werthman, MD, Urologist and Men's Health Specialist, Center for Male Reproductive Medicine and Vasectomy Reversal

The decline in male fertility is becoming a significant public health concern. Infertility affects approximately 15% of couples in the United States, with the male partner solely responsible for about 20% of cases and contributing to another 30% to 40%. While a 2025 study found that sperm concentration remains stable among men in the United States with no known fertility challenges, earlier research has shown concerning declines in sperm counts globally.

Steps to Improve Male Reproductive Health

The good news is that male fertility can be improved through lifestyle changes. Here are evidence-based strategies men can implement:

• Regular Exercise: Physical activity helps reduce body fat percentage and improve overall metabolic health, which directly supports reproductive function.
• Balanced Diet: Eating a well-balanced diet supports healthy body composition and provides nutrients necessary for sperm production.
• Stress Management: Minimizing stress through relaxation techniques, meditation, or counseling can help maintain hormonal balance.
• Antioxidant Supplements: Vitamin C and other antioxidants may help protect sperm quality, though supplementation should be discussed with a healthcare provider.
• Semen Analysis: Men concerned about fertility should obtain a semen analysis, which is inexpensive and non-invasive, to measure sperm count, motility (movement), and morphology (shape).

"You cannot optimize what you haven't measured," Werthman noted, emphasizing the importance of getting tested if there are any signs of fertility issues.

What Other Health Risks Signal Low Sperm Count?

Male infertility is often linked to broader health problems. A low sperm count may indicate other serious health conditions developing inside the body. Male infertility typically involves issues with sexual function and semen quality, which can include decreased sperm count, motility, or quality; decreased libido; erectile dysfunction; and low levels of hormones such as testosterone and prolactin.

Several risk factors increase the likelihood of low sperm count and male infertility beyond body fat percentage. These include injury or surgery to the testicles, sexually transmitted infections (STIs), exposure to pesticides or industrial chemicals, exposure to radiation, excessive drug use or alcohol consumption, and anabolic steroid use.

The takeaway for men is clear: body composition matters more than the number on the scale. By focusing on reducing body fat through exercise and diet rather than simply losing weight, men can protect their reproductive health and potentially improve their fertility prospects. For those with concerns, a simple semen analysis can provide critical insight into reproductive health and guide next steps with a healthcare provider.