NEW YORK: Men who exercise to exhaustion experience changes in their hormone levels and sperm counts, a new study shows.

While these changes aren't permanent, Diana Vaamonde of the University of Cordoba in Spain and colleagues note, the findings suggest that exercise could impair fertility in men with poor sperm counts and low hormone levels.

Women who exercise intensely frequently show obvious changes in hormonal patterns, Vaamonde and colleagues point out in the International Journal of Sports Medicine. For example, they may stop menstruating. But the effects of exercise on men's fertility are not so clear.

The researchers set out to investigate how recreational exercise might influence male reproductive factors by assigning 16 healthy young men to either pedal to exhaustion on a special exercise bike four times a week for two weeks, or to avoid intense physical activity.

The exercisers showed a drop in their sperm concentration, ejaculate volume, and the number of sperm per ejaculate, as well as other changes in sperm quality.