I have heard so much about lost toenails, black toes or black toenails and how common they are among marathoners, and especially common for ultrarunners. I have been running for a few years now, and completed 4 full marathons plus several halves and long training runs without any issues, imagine my surprise to have my nail, just fall off! I must admit... I am feeling pretty hardcore. I have earned this lost toenail and am going to embrace it!
When I told my mom I lost my first toenail, her response "Welcome to the toenail-less club!" Somehow, that made me feel so good! I finally, officially, have "runners feet". I will take pride in that!
Here’s what LiveStrong says to do about it (source: http://www.livestrong.com/article/331008-how-to-prevent-losing-toenails-from-exercising/)
Trim your toenails so the edge is straight across and just above the nail bed. Cutting your nails too short or with curved sides will increase the risk of infection or ingrown toenails. Perform regular toenail maintenance to keep them at the proper length.
Wear socks made from wick material to keep the feet dry during exercise and reduce the chance of blister formation around the toenail. Not all socks labeled as athletic have wicking properties. Look for socks made from merino wool or synthetic fabrics such as polyester or acrylic that have been specially designed to keep moisture away from your feet.
Verify your running or athletic shoes have enough room in the toe area. A shoe that is one-half to one size larger than your street shoes will provide room for the foot to expand when it strikes the ground during exercise. Have your foot measured and fitted at an athletic shoe store if you continue to have problems with toenail damage. These experts can ensure that your shoes are long enough and wide enough to protect your toenails.
Lace your shoes tighter than normal if your foot slides excessively back and forth in your shoe. Your foot should not slide enough for the toes to touch the front of the shoe.