Young men who want a high sperm count would do well to exercise vigorously and restrict their television watching, a new study suggests.
Researchers tested the sperm of 189 health US college men, aged from 18 to 22, in Rochester, New York state.
Participants who did at least 15 hours a week of moderate-to-vigorous activity had a 73 per cent higher sperm concentration than those doing no more than five hours.
Those who watched at least 20 hours of television a week had a 44 per cent lower sperm concentration than those who watched no television.
The study, published in the British Journal of Sports Medicine, said sperm quality appeared to have declined in past decades, but reasons for the decline were unresolved. Rising sedentary behaviour may be a contributing factor.
"In this population of healthy men, higher moderate-to-vigorous activity and less TV watching were significantly associated with higher total sperm count and sperm concentration."
The researchers could not say whether the differences in sperm count were big enough to have any effect on fertility. Other sperm characteristics, such as swimming speed, were similar for all groups.
Links between activity, television and sperm counts held up even when weight and dietary patterns were factored in, co-author Jorge Chavarro from the Harvard School of Public Health said.
The authors suggested metabolic changes related to inactivity could be a factor, as could heating of the scrotum due to sitting in front of a television for long periods.
Chavarro also suggested the findings might reassure active men who were worried about studies which suggested vigorous activity could decrease sperm counts.
Those earlier studies had focused on endurance athletes, such as marathon runners and professional cyclists, whose bodies might be under unusual stresses.
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