Watching too much television can lower a man’s sperm count by up to 44 percent, according to a recent study conducted by the Harvard School of Public Health.

Published Monday in the British Journal of Sports Medicine, the study also found that otherwise healthy young men who do not exercise have significantly lower sperm counts than men who do exercise.

Researchers at HSPH analyzed the semen quality of 189 college-aged men and recorded data about their average physical activity and TV viewing patterns.

“When we analyzed the data, we found that men who watched more than 20 hours per week had 44 percent lower sperm counts than those who watched almost no TV,” said Audrey J. Gaskins, lead author of the study and a doctoral student at HSPH. “Conversely, those who exercised the most at a moderate to vigorous rate had 73 percent higher sperm count than those who exercised less than five hours per week.”

Previous studies have found that male sperm counts have declined in western countries over the last 60 years, but none have yet identified such a direct link between sperm count and physical habits.

“So much of the research that has been done in this area has been related to environmental chemicals. What chemicals have been introduced into the environment in the last 60-year period that may be the reason behind the decreasing sperm counts?” said Jorge E. Chavarro, senior author of the study and an assistant professor of nutrition and epidemiology at HSPH. “But something that has not received as much attention is whether lifestyle factors such as diet, where people live, [and] how people move around might have also influenced these declines in sperm counts.”

Gaskins added that previous investigations did not target the average population.

“The majority of them have focused on professional athletes like marathon runners and elite cyclists. They generally reach activity levels that most people in the world can’t match,” she said. “So we set out to look at this question in a population of men that is somewhat more relevant to the population.”

The mechanism by which a more sedentary lifestyle diminishes sperm is not yet fully understood, researchers said. But Gaskins offered some potential explanations.

“Sedentary behavior has been linked to increased levels of oxidative stress, and oxidative stress can damage male germ cells...responsible for producing sperm,” she said. “There’s also the rationale that sedentary conditions have been linked to increased sperm temperature, and spermatogenesis is a temperature-dependent process.”

—Staff writer Rachel J. Sapire can be reached at rsapire@college.harvard.edu. Follow her on Twitter @racheljsap.