Young men who get more exercise and watch less TV have higher sperm counts than those with less healthy habits, a U.S. study suggests.

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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  • Canned food

    The common household chemical bisphenol A (BPA) can lower sperm counts and motility, according to<a href=""> a study</a> in Reproductive Toxicology. Food packaging is a major source of BPA, which can seep into the food. Most food cans are <a href="">coated with BPA resin</a>, and acidic canned foods are especially risky.

  • Sex toys

    Certain plastics, including vinyl used in some sex toys, release phthalates — plastic-softening chemicals that have been <a href="">tied to lower reduced sperm count and quality</a>. Glass, silicone and green sex toys are a safer bet.

  • The shower

    Phthalates are found in many shampoos, soaps, deodorants and shaving creams. Look for natural, phthlate-free products instead.

  • Marijuana

    It's not surprising that smoking cigarettes is bad for sperm, but smoking weed isn't much better. Heavy marijuana smokers have been found to have <a href="">significantly lower sperm counts and sperm with bad swimming strategies</a>.

  • Stress

    Stress can cut sperm count dramatically by <a href="">inhibiting the production of testosterone</a>.

  • Booze

    Women aren't the only ones who should slow down on drinking while trying to conceive. Alcohol can <a href="">inhibit sperm production</a> and lower sperm counts.

  • Receipts

    BPA is found on about 40 percent of cash register receipts, according to <a href="">a 2010 University of Missouri study</a>. The chemical can penetrate skin or be passed from hand to mouth to the digestive track. Skip the receipt or wash your hands after signing.

  • Soy

    Isoflavones found in soy products <a href="">mimic estrogen</a>, and a <a href="">2008 study</a> linked soy consumption to significantly lower sperm counts. The soy food industry <a href=",2933,303160,00.html">disputed the study's findings</a>.

  • Contaminated fish

    Polychlorinated biphenyls (PCBs) are a group of toxic chemicals that have been banned but have accumulated in certain fish — especially predator fish and bottom-feeders. They are associated with <a href="">decreased sperm counts and integrity</a>. Smaller, younger fish <a href=" fish are safer to eat?">are considered safer</a>.

  • Nonstick pans

    Chemicals called perfluoroalkyl acids are used in the manufacturing of nonstick products such as Teflon, Gore-Tex and wax paper and can significantly lower sperm counts, according to <a href="">a Danish study</a>.

  • Abstinence

    Of course not having sex at all won't help, but some couples abstain for days or weeks in hopes of saving sperm until the woman is ovulating. This doesn't work. Researchers have found that while it does take a day or two for sperm to replenish themselves after ejaculation, waiting any longer than that <a href="">can cause them to lose motility and change shape</a>.