Men who exercised for 15 or more hours weekly at a "moderate to vigorous" rate had a 73-per cent higher sperm concentration than those who exercised less than five hours per week. Participants were divided into four groups based on their physical activity levels.
The findings come after decades of research in several Western countries into whether declines in sperm quality from men going to fertility clinics indicate widespread drops in the general population of healthy men or are just blips.
"In this population of healthy men, higher moderate-to-vigorous activity and less TV watching were significantly associated with higher total sperm count," said Jorge Charravo, assistant professor of nutrition and epidemiology at the Harvard School of Public Health.
"Our findings suggest that a more physically active lifestyle may improve semen quality."
To explore the questions involved, Harvard researchers tested sperm samples from 189 men aged 18 to 22 and asked them questions about their physical activity levels, diet and TV viewing.
Charravo and his co-authors published their results in Tuesday's online issue of the British Journal of Sports Medicine.
None of the participants had sperm counts that would be a cause for alarm, said Warren Foster, a professor in the department of obstetrics and gynecology at McMaster University in Hamilton, Ont., who was asked about the results.
"One of the things that we know is that when you engage in moderate to strenuous exercise on a regular basis, serum testosterone increases," Foster said in an interview with CBC News.
"Serum testosterone … is involved in sperm production. As a consequence of that, one would expect to see that the more fit you are that you would potentially see a modest increase in semen quality."
Scientists suspect that regular physical activity may prevent against oxidative stress damage and protect sperm from damage.
Everything in moderation
Higher scrotal temperatures have also been proposed as an explanation, but whether that is a consequence or cause of impaired sperm production is unclear, Charravo's team said.
The researchers acknowledged that study was limited by the relatively small number of volunteers who provided only a single sperm sample.
Dr. Allan Pacey, senior lecturer in andrology at the University of Sheffield, noted that other research suggests that too much physical activity may harm sperm production.
Charravo's study did not look at the type and intensity of physical activity.
"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!)," Pacey told the Australian Broadcasting Corporation.
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