Extreme Amounts of Exercise

Males who do extreme amounts of exercise, such as professional soccer players and runners who cover more than 40 or 50 miles a week, have less LHRH, LH, and testosterone in their circulation, smaller testes, less functional sperm. They also have higher levels of glucocorticoids in their bloodstreams, even in the absence of stress. (A similar decline in reproductive function is found in men who are addicted to opiate drugs.)

― Robert M. Sapolsky, Why Zebras Don't Get Ulcers:
The Acclaimed Guide to Stress, Stress-Related Diseases, and Coping