Why do toenails fall off?

Both my big toenails are about to fall off but I never experience any pain running. How do I determine if my shoes don't fit?

• My current trail runners have about 400 miles on them but, with the exception of diminished traction, feel just as comfortable as new, if not more so. I always read people write that nail problems are a shoe fit problem but mine seem so comfortable. One nail was sore after my first marathon and is about to fall off (nearly two months later) and the second one was just black for months. Only recently did I start figiting with it and conclude it will probably fall off, too.

• Answer:

  Toenails are for sissies! I only have 7 at any given time. Sometimes it's the shoe, but my left toenail bruised at the beginning of June, when I started training for my last marathon. It was not due to my shoes. I've lost that nail at least 2 other times, so maybe they do not grow back as strong? You might want to check out this article from Runner's World: http://www.runnersworld.com/tag/black-toenails