Researchers say body fat percentage may be a better indicator of male reproductive health than BMI, warning that men with higher levels of body fat may have lower sperm counts and changes in reproductive hormones.

The research, published in Human Reproduction, found that measuring body fat percentage could provide a better indication of male reproductive health than body mass index (BMI).

Researchers found that men with higher body fat generally had lower sperm counts, lower semen volume and sperm concentration, as well as changes in hormones, including testosterone.

The findings were particularly notable because the association was also observed among men whose BMI was within the normal range but who had relatively high levels of body fat.

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The researchers studied 1,058 young Danish men who were approximately 18 years and nine months old. Their body fat percentage was measured and divided into four groups.

The researchers also measured BMI and waist-to-height ratio and collected blood and semen samples. Testicular size and information about participants’ health behaviours were also recorded.

Compared with men who had a normal body fat percentage, those in the high body-fat group had, on average, a 23 per cent lower sperm count. Men with very high body fat had a 15 per cent lower sperm count compared with those in the normal body-fat group.

Higher body fat was also associated with lower levels of testosterone and sex hormone-binding globulin (SHBG), as well as changes in the balance of male reproductive hormones.

The researchers concluded that body fat percentage was a better predictor of reduced sperm count than BMI or waist-to-height ratio.

BMI is commonly used to determine whether a person is underweight, within a healthy weight range, overweight or obese. However, it does not distinguish between body fat and lean tissue such as muscle.

This means two men can have the same BMI but very different levels of body fat.

The study found that even some men with a normal BMI but higher body fat had lower sperm counts and a greater proportion of poorly formed sperm.

Nis Brix, associate professor at Aarhus University in Denmark and first author of the study, said measuring body fat could help identify men at risk of impaired reproductive health who might otherwise be overlooked.

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

Philip Werthman, a urologist and men’s health specialist in Los Angeles who was not involved in the research, said obesity can contribute significantly to male infertility because excess fat tissue can affect testosterone and estrogen levels and interfere with sperm production.

S Adam Ramin, a urologist and urologic oncologist who was also not involved in the research, cautioned that low sperm count can sometimes indicate an underlying health problem.

“A low sperm count may be a signal of something more serious happening inside the body,” Ramin said.

Infertility affects many couples and can involve factors affecting either partner. Male infertility can be associated with problems involving sperm production, sperm movement and sperm shape, as well as hormonal or sexual health.

A low sperm count does not necessarily mean a man is infertile. Fertility depends on several factors, and sperm levels can vary over time. A semen analysis can help determine sperm count, movement and morphology and may be recommended when a couple has difficulty conceiving.

The new findings suggest that when it comes to male reproductive health, body composition rather than BMI alone could provide doctors with additional information when assessing men at risk of reduced fertility.