Men who watch a lot of TV have half the sperm count of men who rarely watch TV, according to a report released Monday.
The study, performed by researchers at the Harvard School of Public Health, could help explain why male fertility has declined in western countries over the past several decades.
The study followed 189 seemingly healthy college-aged men. Those who watched more than 20 hours of TV per week had nearly half the sperm count of men who watched just a couple hours a week. Men who exercised at least 15 hours a week had sperm counts that were 73 percent higher than those who rarely exercised.
"These associations with sperm counts suggest that lifestyle changes such as increases in physical activity may positively influence sperm count and concentration in reproductive-aged men," the study's authors write.
In the past, sedentary behavior has been linked to higher scrotal temperatures, which can harm sperm production. Previous studies have found that men with low sperm counts more commonly worked office jobs or performed other sedentary work.
Alternatively, the study's authors suggest that higher rates of obesity could be to blame for the lower sperm counts found in men who watch a lot of television.
"It was difficult to disentangle the effect of obesity from that of inactivity. The modifying effect of TV watching on the association between physical activity was unexpected as this has not been documented in previous literature," they write. "It is possible that this might be a chance finding. Therefore, further research is needed to confirm this result."
Researchers say that men with lower sperm counts can still be fertile, though previous studies have suggested that sperm concentration can be used to predict fertility.
"It is not possible to conclude from these findings whether the observed differences in sperm counts translate into clinically relevant differences in fertility," they write. "Our findings suggest that a more physically active lifestyle may improve semen quality."