Nipples bleeding when running is a common running problem among male runners. The bleeding happens when your running T-shirt rubs against your tender nipples.

This is how the bleeding happens

When you run your t-shirt moves up and down, that movement rubs on your nipples. The friction on your nipples makes them tender. Open wounds form and blood starts to flow out. When the salt from your sweat rubs on the open wounds you start feeling pain. Experts call this Nipple Chafing!

How to Avoid Bleeding Nipples

Apply a layer of vaseline over your Nipples, some marathons even offer Vaseline enroute. Put a bandage over the nipples. This will prevent direct contact.

Another funny solution that I found at a Runners world article on this topic was this one: