NEW YORK – Adding to evidence that obesity may affect a man's sperm quality, a new study finds that obese men tend to have less-mobile sperm than their thinner counterparts.

Studies have come to conflicting conclusions as to whether obesity impairs a man's fertility. But several recent ones have found that obese men tend to have poorer quality sperm than leaner men do -- including lower sperm counts and fewer progressively motile sperm, which refers to sperm that swim forward in a straight line rather than moving about aimlessly.

In the new study, published in the journal Fertility and Sterility, researchers from Argentina evaluated semen samples from 749 men who were each part of a couple seeking help for fertility problems.

They found that the 155 obese men tended to have fewer motile sperm and fewer rapidly moving sperm than their normal-weight and overweight counterparts.

Obese men also had generally lower levels of neutral alpha-glucosidase, or NAG -- an enzyme secreted into the fluid of the epididymis, which is a structure at the back of the testes where sperm mature and acquire their motility.

The concentration of NAG in the semen is considered a marker of how well the epididymis is functioning.

"To our knowledge, ours is the first study proposing a deleterious effect of obesity on epididymal function," lead researcher Dr. Ana Carolina Martini, of the National University of Cordoba in Argentina, told Reuters Health in an email.

However, the effects she and her colleagues found on sperm quality would not have a significant effect on a man's fertility, according to Martini.

"A man would not become sterile because of some weight increase," she said.

While the study linked obesity to lesser sperm motility, it found no effects on other measures of semen quality, including sperm count, testosterone levels and the percentage of normally shaped sperm.

Still, Martini said, it might be possible for an obese man to improve his sperm quality by shedding some weight. Research has shown that weight loss can reverse the imbalance in reproductive hormones that is linked to obesity, she noted.

The study had a number of limitations, Martini and her colleagues point out. One is that it focused on men who were part of couples with fertility problems; whether the findings extend to obese men in general is unknown.

Another is that the researchers gauged obesity using the men's body mass index, or BMI, a measure of weight in relation to height. The problem is that BMI does not directly reflect a person's level of body fat, and other studies have suggested that abdominal fat is more closely related to sex-hormone levels than BMI is.

More studies, the researchers conclude, are needed to better understand the relationships among obesity, sperm quality and fertility.

"Because the incidence of obesity is growing," they write, "it is expected that the number of obese men with reduced fertility will increase as well."

SOURCE: Fertility and Sterility, online January 7, 2010.