Watching too much television and exercising too little might be at least partly to blame for a possible decline in the sperm counts of American men, a Harvard School of Public Health study suggests.

In findings published Monday in the British Medical Journal, the researchers assessed sperm quality in 189 college students ages 18 to 22 and found that those who exercised vigorously and often — more than 14 hours per week — and who watched no television had the highest quality and quantity of sperm compared with those who exercised less and watched more than 20 hours a week of television.


The result was surprising because it contradicts previous research suggesting that male athletes are more likely to have low sperm counts. “Those previous findings were looking at elite marathon runners, triathletes, and cyclists, and I’ve always been skeptical that they apply to nonathletes,” said Dr. Jorge Chavarro, an assistant professor of nutrition and epidemiology at the School of Public Health.

While Chavarro emphasized that the study findings need to be replicated to verify the association, he said moderate exercise of any kind appears to cause no harm to a man’s fertility and is likely beneficial.

Both too much TV and a sedentary lifestyle independently raised a man’s likelihood of having fewer sperm, but men who exercised and watched a lot of TV didn’t fare as poorly from their excess TV viewing as those who didn’t exercise at all.

The research, however, has several limitations. The study was relatively small and wasn’t designed to prove whether altering exercise or TV viewing habits could improve a man’s sperm quality. Although the researchers accounted for smoking, obesity, poor diet, and high stress levels — all of which could lower sperm production — they couldn’t account for all of the potential differences between the two groups that might have influenced fertility.


“It’s an interesting study, and I’m always one to look for research showing that physical activity benefits fertility,” said Alice Domar, a fertility researcher and psychologist who heads the Domar Center at Boston IVF. “But it’s also possible that college students who aren’t as healthy and don’t produce ample sperm are also more likely to watch TV and avoid exercising.”

What’s more, the study could not conclude that men who watched the most TV and did the least exercise actually were less fertile, even though their sperm counts were significantly lower.

“We cannot answer that question for these particular men,” Chavarro said, “because they were college students who haven’t yet tried to have kids.”

Researchers have been debating for some time whether men in the United States and other westernized countries have become less fertile in recent decades. In a study published in the British Medical Journal Open last year, Danish researchers examined the semen of nearly 5,000 young men and found that only 23 percent of the study participants had optimal sperm quantity and quality, which was much lower than the percentage of men who were studied in the 1940s. The researchers predicted that such low sperm counts would probably delay pregnancy and that 15 percent of the study participants would need fertility treatment.

Dr. Peter Schlegel, chair of urology at Weill Cornell Medical College, wrote in an e-mail that men shouldn’t interpret the new research as the answer to infertility. “Studies such as this are of interest, but they don’t ask the most important question. If you turn off channel 5 and go for a run each night, can you change your fertility? Can you even increase your sperm concentration?”


Domar agreed and said she’s not going to advise men with low sperm counts to limit their TV time or start working out more intensively.

What Domar does advise -- based on previous research -- is that men with fertility problems avoid biking for extended periods, choose boxers instead of briefs, and remove warm laptops from their laps. Anything that raises the temperature of the testicles can destroy sperm produced and stored in the glands.

Deborah Kotz can be reached at dkotz@globe.com. Follow her on Twitter @debkotz2.