Men who watch 20 hours of TV a week may have only half as many sperm as men who watch very little TV, a small study suggests.
On the plus side, 15 hours a week of moderate to vigorous exercise may improve sperm count by as much as 73 percent, the researchers report.
"Guys, turn off the TV and put on the running shoes," said senior researcher Dr. Jorge Chavarro, an assistant professor of nutrition and epidemiology at the Harvard School of Public Health. "Adopting a less sedentary lifestyle may have a positive impact on sperm counts."
The report was published online Febraury 4 in the British Journal of Sports Medicine.
To measure TV's effect on sperm, 189 young men, aged 18 to 22, were asked about their TV watching and exercise habits. They were also asked about other habits, such as smoking and diet, and whether they had any reproductive health condition or suffered from stress.
Men who sat in front of the TV for 20 hours or more a week had sperm counts 44 percent lower than men who watched less TV, the researcher found.
Although sperm count was lower, the sperm were healthy in terms of motility, shape and sample volume, the study authors noted.
Men who exercised 15 or more hours a week also had healthy sperm and more of them. This, however, was only among men whose exercise routine was moderate to vigorous. Light exercise didn't affect sperm count, the researchers noted.
The reason why TV was associated with a lower sperm count is unclear, and it may be that TV is a signal for other factors.
"The associations of TV watching and physical activity with sperm counts were independent of each other," Chavarro said. "What we cannot rule out entirely is that our finding for TV watching is specific to TV or sedentariness in general."
It's also not clear whether the TV-loving men would have trouble fathering children, the researchers said.
"Men in this study had not attempted to have children before, so we do not know how their own fertility is influenced by TV watching or physical activity," Chavarro said.
"We know from other studies that sperm counts are related to fertility — higher sperm counts are related to higher fertility. Men with low sperm counts can still father children although they may have difficulties doing so," he said.
One expert said there are too many factors to consider to say TV is responsible for a lower sperm count. There's an association, but it isn't necessarily the cause, said Dr. Pravin Kumar Rao, an assistant professor of urology and director of reproductive medicine and surgery at Johns Hopkins University.
"The results are believable from a physiological standpoint, but it's difficult to conclude for sure that it's TV that lowers sperm count," he said.
Other factors could account for the finding, such as whether the men who were TV watchers had other undiagnosed medical conditions that might also affect sperm count, like diabetes or high blood pressure, Rao said.
"Exercise and a sedentary life style most likely have an effect on fertility, but I am not sure if we can quantify how much is due to one factor versus another," he said.
However, Rao does have a takeaway message for men.
"I tell my patients the testis prefers to have a very healthy and very stable environment to do its work the best," he said, which means staying active and eating healthfully. "Living a healthy lifestyle can help your sperm production," Rao said.