20 hours of television per week may halve men's sperm counts

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Watching television for too long may affect more than belt size: New research shows the amount of time sitting in front of the tube could also take a toll on a man's sperm count.

A new study published in the British Journal of Sports Medicine on Feb. 4 showed that men who watched more than 20 hours a week of TV had almost half the sperm count of men who didn't watch TV at all.

"There had been a few reports linking very high levels of physical activity to higher sperm quality, but these studies had been linked to professional athletes and professional bicyclists who reached levels of physical activity higher than anyone else," study author Dr. Jorge Chavarro, assistant professor of nutrition and epidemiology at Harvard School of Public Health, said to CBSNews.com. Chavarro and his team wanted to find out how levels of physical activity affected non-athletes.

They hypothesized that the typical person with higher-than-average levels of physical activity would have better sperm quality.

The authors used data from the Rochester Young Men's Study, which involved 189 men between the ages of 18 to 22 who were enrolled in 2009 and 2010. The study mostly looked at how environmental contaminants affected subjects, but also looked at a wide variety of factors including diet and television watching.

On average, the men had about eight hours of physical activity and watched 14 hours of TV a week.

Just as their hypothesis predicated, men who had the highest level of physical activity -- 15 hours a week or more -- had a 73 percent higher sperm count than those who had the lowest level of physical activity, which was less than five hours a week.

Interestingly, men who watched more than 20 hours of TV a week had a 44 percent lower sperm count than men who watched the least amount of TV. The lowest category included anyone who watched less than four hours a week, but the average viewing time was 0 hours for that whole group.

"I was surprised to see the strength of the association," Chavarro said. "We expected an inverse relationship, but we didn't know to what extent."

Although other studies have shown that sedentary behavior can affect sperm quality, those typically looked at higher scrotum temperatures or sitting in front of a computer screen all day -- not a relaxing activity like watching TV, Chavarro pointed out.

Chavarro said he believes that the reason why men who watch TV more had lower sperm counts is because of their oxidative stress. He explained that a wide variety of chemical reactions that the body goes through generate oxygen. These oxygen levels can damage cells such as sperm cells like spermatozoa, which develop into sperm.

"We know from other studies that physical activity is associated with lower oxidative stress and being sedentary is associated with high oxidative stress," he said.

Dr. Pravin Kumar Rao, an assistant professor of urology and director of reproductive medicine and surgery at Johns Hopkins University, who was not involved in the research, told HealthDay that the results are believable but it's difficult to conclude watching television was to blame. He adds conditions like diabetes or high blood pressure can also affect sperm count.

"Exercise and a sedentary lifestyle 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.

Chavarro said that you don't have to become a professional athlete to have a great sperm count, but getting off the couch and moving more can help.

"More exercise is better for sperm production," he concluded.

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