Young men who work out frequently have as much as 73 percent more sperm than those who don't, and the more television one watches, the lower the count goes, according to a study by Harvard University researchers.
College-aged men who exercised more than 14 hours a week had the highest sperm counts. Watching TV had the opposite, with sperm counts almost halved for those viewing 20 or more hours a week, according to the study published in the British Journal of Sports Medicine.
"The message is pretty clear," said Jorge Chavarro, an assistant professor of nutrition and epidemiology at the Harvard School of Public Health. "It makes sense to turn off the TV, and it makes sense to put on your running shoes or sports gear and get out there."
The findings may influence how people think about exercise and men's reproductive health. Past studies looking at sperm counts in athletes focused on highly trained cyclists and long-distance runners, and found that intense exercise by those athletes can reduce sperm. The Harvard researchers said their study participants included all types of athletes such as those who ran or who played soccer, basketball, baseball or football.
The researchers examined semen samples from 189 men who reported their exercise and TV viewing habits over three months. Exercise was counted as any physical activity that made the subjects "somewhat to very" winded or sweaty.
Sperm counts started to rise after about eight hours a week of exercise, said Chavarro, the study's senior author.
"More physical activity is better," he said. Those that exercised eight to 14 hours a week had sperm counts 27 percent higher than sedentary men, while working out more than 14 hours a week increased sperm count by nearly three quarters.
"That's still quite a bit of exercise, compared to what most people achieve," Chavarro said.
Those watching 20-plus hours of TV a week had sperm counts 44 percent lower than those who watched very little, the study found. That could be because the lack of activity was bad for health, or because being regularly stuck to the couch raised temperature in the scrotum, which can hurt semen production, said Chavarro.
The researchers didn't break down semen quantity by participation in different types of sports, though they may do so in future research.
The good news for TV watchers was the study found little correlation between amounts of exercise or TV viewing volume with sperm quality, meaning that their sperm weren't deformed and could move in the right way.