Being either too thin or too heavy may lower a man’s sperm count, in some cases enough to impair fertility, researchers report.

In a study of nearly 1,600 young Danish men, the investigators found that those with either a low or high body mass index (BMI) had differences in reproductive hormones, as well as lower sperm counts than normal-weight men.

In some cases, these weight-related effects would likely be enough to reduce a man’s fertility, study leader Dr. Tina Kold Jensen, of the University of Southern Denmark, told Reuters Health.

For example, men with a high or low BMI were more likely than normal-weight men to have a sperm count below 20 million per milliliter of semen, which is abnormally low.

The findings raise the possibility that the high prevalence of Obesity in the Western world may be contributing to fertility problems, Jensen and her colleagues report in the journal Fertility & Sterility.

If that’s the case, they note, it would mean that some cases of impaired fertility may be preventable.

However, Jensen said, it’s not yet known whether maintaining a normal weight can help a man’s chances of fathering children. To answer that question, she added, researchers could study whether overweight men who shed their excess pounds show changes in semen quality.

Body weight has already been shown to affect a woman’s ability to conceive. Obese women, for example, have a heightened risk of menstrual irregularities and infertility

To see whether BMI - a measure of weight in relation to height - may be related to male fertility, Jensen’s team studied young men who were undergoing physicals for potential military service.

They found that, on average, those with a low BMI - defined as below 20 - had a 36 percent lower sperm count than men with a normal BMI. Overweight men - those with a BMI over 25 - had a 24 percent lower sperm count compared with average-weight men.

In addition, the men’s testosterone levels decreased as BMI increased, as did certain other reproductive hormones. According to Jensen’s team, it may be such hormonal effects of a high BMI that explain the poorer semen quality they found in overweight men.

As for the underweight men, malnutrition or “subclinical” health problems may take some of the blame for their lesser semen quality, the researchers speculate. They add, though, that hormonal imbalances could have been at work in these men as well.

It’s also possible, according to Jensen, that the types of lifestyles that promote extremes in weight may explain the link between BMI and semen quality. Men in the study were asked about their smoking and drinking habits, but factors such as lack of exercise and poor diet may also be important, Jensen noted.

SOURCE: Fertility & Sterility, October 2004.