NEW YORK (Reuters Health) - For most men, exercise appears to have no relationship to either the quality or quantity of sperm, according to U.S. researchers.
NUGGET : But they found one exception: Men who biked for at least five hours per week had fewer and less active sperm than couch potatoes.
"It is likely that most forms of exercise have no effect on semen quality and that only certain subtypes of activity, and/or those performed at higher intensity levels, have an effect," study author Dr. Lauren Wise at Boston University told Reuters Health in an e-mail.
Previous research has suggested competitive athletes may have issues with their sperm. The new report, published in the journal Fertility and Sterility, looked at the relationship between sperm health and exercise in the average man.
The researchers focused on more than 2,200 men who provided semen samples to three fertility clinics, and answered questions about their general health and physical activity.
To eliminate the influence of additional factors that could affect sperm health independent of exercise, the authors used statistical tools to remove the influence of taking multivitamins, body weight, blood pressure, choice of underwear, and others.
Looking specifically at exercise, they found that men who exercised regularly -- even vigorously -- were no more likely to have problems with the quality or quantity of their sperm than men who never exercised.
However, when Wise and her team looked at specific types of exercise, they saw that men who said they spent at least five hours per week biking were twice as likely to have both a low sperm count, and relatively few sperm with healthy movement.
Among men who did not get regular exercise, 23 percent had low sperm counts -- but so did more than 31 percent of those who biked for at least five hours per week. And nearly 40 percent of frequent bikers had low numbers of sperm with healthy movement, versus only 27 percent of men who didn't exercise.
Previous research looking at competitive athletes has linked biking to genital or urinary problems and poor semen quality, Wise explained.
"However, we were uncertain whether we would find an association among a sample of men engaged in more moderate levels of physical activity," she said. It's possible that trauma or temperature increases in the scrotum may explain the relationship between biking and semen health, "but more research is needed to investigate these mechanisms further."
It's also too early to say that regular biking caused the sperm problems, Wise cautioned. "More studies are needed to replicate our findings before they can be considered causal."
For instance, it's possible the men included in the study may not be representative of the general population, since they were all attending a fertility clinic and therefore are more likely to have problems with their sperm, Wise said. Their physical activity may not differ from that of other men, however, so it is difficult to say how the results might change when looking at men who haven't been to a fertility clinic, she noted.
By Alison McCook