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."