Male cyclists who spend too long in the saddle risk damaging their sperm, research has suggested.

The gruelling training regimes followed by elite cyclists and triathletes, who spend many hours on the road and cover hundreds of miles a week, may lower the quality of their sperm to the point at which they suffer fertility problems, a study of triathletes in Spain has found.

The constant pressure and friction of the saddle against the groin, raised temperatures from tight-fitting Lycra and intensive exercise, as well as the stress of pushing the body to its limit, may all contribute to sperm damage, scientists at the University of Córdoba said.

The effects can be so marked that Diana Vaamonde, who led the research, said professionals might want to consider freezing their sperm at a young age to protect their fertility.

Recreational cyclists, however, are unlikely to be doing their chances of fatherhood much harm: the negative effects kick in only with a weekly mileage of about 300km (190 miles) at high intensity.

Sperm experts agreed that normal cycling activity is unlikely to be hazardous. Allan Pacey, senior lecturer in andrology at the University of Sheffield, said: “It is important to stress that even if the association between cycling and poor sperm morphology is correct, men training for triathlons are spending much more time in the saddle than the average social cyclist or someone who might cycle to and from work.

“There is no evidence that men who ride a bike are less fertile than other males. Indeed, if you look back in our history, only 40 years ago cycling was much more common and there is no evidence from that time that men were less fertile. In fact, quite the contrary: the postwar baby boom proves that.”

In the study, which was presented yesterday at the European Society of Human Reproduction and Embryology conference in Amsterdam, Professor Vaamonde’s team investigated 15 triathletes in Spain, all of whom competed at national or international level. The scientists examined their sperm quality after three days of sexual abstinence, and compared the results against details of their training regimes.

While the intensity of the athletes’ running and swimming training had no impact, there was a clear correlation between cycling mileage and sperm quality.

Professor Vaamonde said the findings were likely to be applicable to anybody who trains on a bike at such a high intensity, not just to triathletes. “We can conclude that a high cycling volume, especially when it’s above 300km a week, can be detrimental to sperm morphology,” she said. “Something that could be done is to have sperm frozen. That would be a safe way to preserve fertility, but when they start training they probably don’t realise the damage that can be done to their sperm.”

Three factors are probably involved, she said. Pressure of the saddle against the groin can inflict “microtrauma” on the testes. Tight cycling clothing and intense physical activity raise the body temperature, which is bad for sperm. The stress of gruelling exercise can also cause oxidative damage to sperm.

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