How much running is too much?

Running can create a lot of health benefits, but too much running has its problems.  For men, there is the drop in testosterone and disruption of reproductive functions that can occur with high mileage.  So how much mileage is "too much" in running?

One older study that looked at this compared high mileage runners (~60 mpw) to moderate mileage runners (~30 miles per week) to sedentary controls.  The study showed that the high mileage runners had significant sperm count and motility problems as compared to the other two groups.

This study showed that runners training for a marathon had reduced sperm motility and morphology.  This study says that beyond 80 minutes of running, the body switches to a "more catabolic environment". 

This study supports the idea of a "volume threshold effect" for endurance running.  I am working on getting the full-text of this study to see what their recommendations.  In the meantime, it seems that a maximum of 30 miles per week, and a maximum of 80 minutes on an individual run, may be good ceilings to keep in mind.