Scientists from Spain have found that men who engage in exhaustive endurance exercise experience temporary changes in hormone levels and sperm counts that might affect their fertility (Vaamonde D et al. Int J Sports Med. 2006;27:680-689).
To study how intense physical activity might affect male reproductive factors, the researchers randomly assigned 16 healthy young men to short-term exhaustive endurance exercise (pedaling an exercise bicycle to the point of exhaustion 4 times per week for 2 weeks) or avoidance of vigorous activity. The men who vigorously exercised experienced a decrease in sperm concentration and volume of ejaculate and showed changes in sperm motility and the percentage of normal sperm, relative to their pretreatment values. Levels of follicle stimulating hormone and luteinizing hormone were decreased (but remained within normal range) among the exercisers and testosterone levels increased.
Stephenson J. Extreme Exercise and Fertility. JAMA. 2006;296(19):2307. doi:10.1001/jama.296.19.2307-d