Healthy young men who watch more than 20 hours of television weekly have half the sperm count of those who abandon the couch and the TV remote control for the gym, medical researchers have found.
The findings of the survey, carried out on nearly 200 men aged between 18 and 22 in Rochester in New York state, are published in the British Journal of Sports Medicine today.
Men who were vigorously physically active for more than 15 hours every week – light exercise does little to improve counts – recorded a sperm count 73 per cent higher than the laziest.
Television viewing sends the figures into a spiral. Those who watch more than 20 or more hours of programmes a week had a sperm count that was 44 per cent lower than those who watched the least.
More than half of the men surveyed were within the normal range for weight for their height, and three out of four were non-smokers. Few had known reproductive health issues.
However, the Rochester sample does not offer evidence that exercise improves a sperm’s motility – its ability to move towards an egg.
A reduced sperm count “does not necessarily curb a man’s fertility or his chances of being able to father a child”, but the findings suggest exercise may improve semen quality, say researchers.
Describing the findings as plausible, Dr Allan Pacey of the University of Sheffield said sedentary jobs and tight underwear were known to cause “testicular heating”, decreasing sperm counts.
However, he said “it remains to be seen if coaxing a TV-watching couch potato into doing some regular exercise could actually improve” sperm count.
“Before all worried men hunt for their sports bag it’s important to note that other research suggests that doing too much exercise can be harmful to sperm production,” he said.
