Preliminary Study Suggests Frequent Cycling May Affect Male Fertility

Bicycling for five or more hours a week is associated with low sperm count and poor sperm motility among men, according to a study led by a researcher from the Boston University School of Public Health and BU’s Slone Epidemiology Center.

The study, published in the journal Fertility and Sterility, surveyed 2,200 men attending fertility clinics in the Boston area between 1993 and 2003. Working with a team of researchers, Lauren Wise, associate professor of epidemiology, gave the men, who were about to undergo an initial IVF cycle with their partners, a questionnaire that sought details about their exercise levels, type of exercise, general health and medical history. Each man also provided at least one semen sample.

The results of the prospective cohort study showed that, after adjusting for variables such as multivitamin use, blood pressure, weight, and type of underwear worn, exercise levels generally had no overall impact on sperm quality and quantity.

But when the researchers broke down the data into specific forms of exercise, they found that men who bicycled for at least five hours a week were more likely to have a low sperm count and fewer motile sperm than were men who did not exercise at all, or those doing other forms of exercise.

Less than a quarter of the non-exercisers had a low sperm count, compared to 31 percent of the frequent bike riders, the study found. Also, just over a quarter, or 27 percent, of the sedentary men had low total motile sperm, compared with 40 percent of the cyclists.

Previous studies of competitive cyclists have linked the sport to poor semen quality, as well as urinary and genital problems. No such links had been shown in non-competitive cyclists.

Wise speculated that semen may be affected by temperature increases in the scrotum or trauma while cycling. But she said it was premature to be sure of the cause, and that it was not yet known whether cycling itself actually affected sperm quality.

Since the men were attending fertility clinics, it also is not known whether the findings would hold up among the general population; further research would be needed to make any firm conclusions.

Lauren Wise
Lauren Wise

“A link between reduced sperm concentration and bicycling — the one class of exercise to show an effect — has some biologic plausibility,” the authors wrote. “Bicycling has been linked to genitourinary problems, including nerve entrapment syndromes (50%–91% of cyclists), erectile dysfunction (13%–24%), and other less common symptoms…

“Our findings warrant confirmation in larger studies of moderate bicyclers and men from the general population.”

Besides Wise, authors on the study included: Daniel W. Cramer, Mark D. Hornstein, Rachel K. Ashby, and Stacey A. Missmer, all of the Department of Obstetrics, Gynecology, and Reproductive Biology, Brigham and Women’s Hospital and Harvard Medical School.

The full study is available here.

Submitted by Lisa Chedekel

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