Professional cyclists should consider freezing their sperm before embarking on their careers, say researchers.
They found sperm quality drops dramatically with rigorous training.
The Spanish study of top triathletes found those who cover more than 186 miles (300km) a week on their bikes have less than 4% normal looking sperm.
At such levels, men would have "significant fertility problems", the European Society of Human Reproduction and Embryology heard.
However, a UK expert said the average man cycling to work would be unlikely to suffer fertility problems because of their time in the saddle.
Study leader Professor Diana Vaamonde, from University of Cordoba Medical School in Spain said other studies had shown very high levels of exercise affected fertility in both men and women.Read the full article here.