The importance of nutrition has been so broad and universal when it comes to the human body that its relevance in the roles of every biological system are undeniable. Micronutrient intakes are 'strongly' associated with improved sperm DNA quality.
But now researchers say activity could be just as important.
Men who are mostly sedentary lose out to those who are moderately active, says the Harvard University study in the British Journal of Sports Medicine.
Experts say the effect could be due to lack of exercise, or to overheating of the testicles caused by prolonged sitting.
Men wanting to father a child are currently urged against wearing tight underpants to improve sperm quality, while drivers and cyclists may also be at risk.
In the latest study from the Harvard School of Public Health (HSPH), healthy young men who watched more than 20 hours of TV each week had a 44 per cent lower sperm count than those who watched almost no TV.
Men who exercised for 15 or more hours weekly at a moderate to vigorous rate had a 73 per cent higher sperm count than those who exercised less than five hours per week. Mild exercise did not affect sperm quality.
Study leader Dr Audrey Gaskins said ‘We know very little about how lifestyle may impact semen quality and male fertility in general so identifying two potentially modifiable factors that appear to have such a big impact on sperm counts is truly exciting,’
The researchers analysed the semen quality of 189 men between the ages of 18 to 22 participating in the Rochester Young Men’s Study during 2009-2010 at the University of Rochester.
The men were asked about their physical activity and how much time they spent watching TV, DVDs or videos over the previous three months, in addition to health issues that may affect their sperm quality, such as diet, stress levels, and smoking.
Over half the men were of normal weight, and levels of diagnosed infertility were low.
The amount of moderate to vigorous physical activity taken each week ranged from five to 14 hours, while weekly TV screen time varied from four to 20 hours.
Those who watched most TV, 20 hours or more, had a sperm count almost half that of men watching the least, and TV viewing seemed to cancel out the benefits of exercise.
Jorge Chavarro, senior author of the study and assistant professor of nutrition and epidemiology at HSPH, said: ‘The majority of the previous studies on physical activity and semen quality had focused on professional marathon runners and cyclists, who reach physical activity levels that most people in the world cannot match.
‘We were able to examine a range of physical activity that is more relevant to men in the general population.’
However, he added, while a reduced sperm count has been linked to lower fertility, it does not necessarily ruin a man’s chances of fathering a child.
Dr Allan Pacey, Senior Lecturer in Andrology, University of Sheffield, said ‘The results are intriguing and suggest that men who do regular exercise have higher sperm counts than men who watch a lot of television.
‘On the face of it, this seems like a well conducted study albeit with a relatively small number of participants.
‘Having said that, the authors appear to have used good methodology and have attempted to control for all of the obvious variables that might differ between the two groups of men (e.g. diet, smoking, Body Mass Index).
‘Their conclusions are plausible, and I would agree that there is evidence to suggest that moderate exercise could change men’s physiology sufficiently to improve testicular health.
‘Similarly, we already know that testicular heating through sedentary jobs or tight underwear can decrease sperm counts and so arguably the same effect might be seen in men who spend too many hours on the sofa watching television.
‘However, it remains to be seen if coaxing a TV watching couch potato into doing some regular exercise could actually improve his sperm count.
'Or whether there exists an unknown fundamental difference between men who like exercise and those who do not which might account for the findings.
‘This should be a relatively easy study to perform, but 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 and this study did not examine the type and intensity of exercise their participants were undertaking.
‘My advice would be everything in moderation - and that includes time in the gym as well as watching TV, or perhaps both at the same time!’