WEDNESDAY, July 9 (HealthDay News) -- Diabetes and being over- or underweight can have a negative effect on male fertility.

That's the conclusion of two reports to be presented July 9 at the annual conference of the European Society of Human Reproduction & Embryology, in Barcelona, Spain.

While semen samples from diabetics look normal under the microscope, a closer examination revealed DNA damage, Dr. Con Mallidis, of Queen's University in Belfast, Northern Ireland, said in a news release issued by the conference sponsors.

"Sperm RNA was significantly altered, and many of the changes we observed are in RNA transcripts involved in DNA repair," he said. "And comparison with a database of men of proven fertility confirmed our findings. Diabetics have a significant decrease in their ability to repair sperm DNA, and once this is damaged, it cannot be restored."

Sperm DNA quality is known to be tied to decreased embryo quality, low embryo implantation rates, higher miscarriage rates and some serious childhood diseases, including cancers.

"We found a class of compounds known as advanced glycation end products (AGEs) in the male reproductive tract. These are formed as the result of glycation (the addition of sugar)," Mallidis said, "and accumulate during normal aging. They are dependent on lifestyle, diet, smoking, etc., and in many diabetic complications are centrally implicated in DNA damage. We believe that they play a similar role in the male reproductive system."

The researchers plan to now determine how AGEs cause and contribute to DNA damage.

Obesity, which often plays a factor in diabetes, and being too thin, was also found to cause problems with sperm. In a separate study, scientists found that men with a higher body mass index (BMI, a ratio of weight to height) had less seminal fluid and more abnormal sperm.

The findings showed that men with an optimal BMI of 20 to 25 had higher levels of normal sperm than those who were either overweight or underweight. They also had higher semen volume.

The researchers did not look at DNA damage in the sperm, though.

"Our findings were quite independent of any other factors," scheduled presenter A. Ghiyath Shayeb, from the University of Aberdeen, in Scotland, said in the news release from the conference, "and seem to suggest that men who are trying for a baby with their partners should first try to achieve an ideal body weight."

"Adopting a healthy lifestyle, a balanced diet, and regular exercise will, in the vast majority of cases, lead to a normal BMI," he said. "We are pleased to be able to add improved semen quality to the long list of benefits that we know are the result of an optimal body weight."

More information

The American Urological Association has more about male infertility.