Does cycling damage men’s sperm?

Yes, suggests the findings of a study presented Monday at the annual meeting of the European Society of Human Reproduction and Embryology in Amsterdam. But you have to be doing some pretty heavy-duty cycling — close to 200 miles per week on your bicycle — for your sperm quality to significantly drop.

According to a press release (I couldn’t get the study’s abstract), researchers at the University of Cordoba in Spain examined sperm samples from 15 triathletes (athletes who enter competitions in which they must cycle, swim and run long distances). They had an average age of 33, and they had trained, on average, nine times a week for eight years.

All 15 triathletes had fewer normal-looking sperm in terms of shape and size than most fertile men, the study’s authors reported. But those men who cycled more than 186 miles a week had the lowest percentage of normal-looking sperm: 4 percent or less. Only cycling — not swimming or running — appeared to have this effect.

That level of quality sperm is low enough to cause fertility problems, and such problems may be permanent, said the study’s authors. In fact, they suggested that professional cyclists might want to consider freezing their sperm before they begin training.

Not a good predictor
Antoine Makhlouf, MD, an assistant professor in the department of urologic surgery at the University of Minnesota and an expert in male fertility problems, isn’t convinced.

"There may be a relationship between excess cycling and the impairment of fertility, but if there is one, it’s not going to be strong," he says.

And it’s certainly not a problem for the casual cyclist, he adds.

Prolonged cycling, particularly on a poorly designed bicycle saddle, can damage nerves and blood vessels in the groin area and lead to erectile dysfunction (impotence), Makhlouf says. But that’s a different issue from infertility.

One big problem with the Spanish study’s interpretation of its findings, says Makhlouf, is that the value of sperm morphology (size and shape) as a predictor of male fertility isn’t that strong. Scientists haven’t even agreed on what the cutoff value should be for normal morphology.

"Some use 4 percent as the cutoff, some use 14 percent, and some even use 55 percent," Makhlouf says. “I’ve seen men with a very low morphology who have conceived.”

Better measurements of male fertility, he says, are sperm count and sperm motility (the percentage of sperm swimming).

Cooling down
What is it about cycling training for triathlons that might produce poorer sperm quality? The authors of the Spanish study pointed to several possible causes, including friction of the testes against the bike saddle and oxidative stress imposed on the body’s cells (including sperm) by the huge amount of energy expended during intense cycling. They also cited heat generated by the triathletes’ tight cycling outfits.

Prolonged exposure to heat from any source — tight clothing, a sauna, or even long drives on heated car seats — can affect sperm and contribute to fertility problems, agrees Makhlouf. For sperm to form properly, it needs to be slightly cooler than body temperature. "But problems caused by heat exposure are clearly reversible," Makhlouf adds.

Laptop computers have also been blamed for sperm problems, but new research suggests that it’s more likely to be the heat from sitting rather than from the computer's battery that's at fault, says Makhlouf. And he wasn’t very impressed with data from last year’s study that linked radiofrequency electromagnetic waves from cell phones to infertility.

Much more common causes of male infertility, says Makhlouf, are infections, varicoceles (dilated veins in the scrotum), chemotherapy and genetic conditions. Minnesota, Makhlouf points out, has a higher-than-average number of cases of men born without a vas deferens (the tube that carries sperm out of the testes), a condition that tends to be more prevalent in families of Scandanavian heritage.

Still, says Makhlouf, the bottom line is that "we often don’t know why men’s sperm levels are low and why they can’t conceive."

As for all you wannabe dads out there: There’s no need to put a brake on riding your bike this holiday weekend.