https://www.runnersworld.com/training/a ... your-body/Why do the nipples of some male runners bleed during a marathon, but those of females don’t?
Sweat is a mix of water, salt, and a handful of other minerals. When the water evaporates, you’re left with abrasive salt on your nipples, which are front-and-center in a high-sweat zone.
“After a few hours, a shirt rubbing against that salt feels like sandpaper,” says Bright, adding that beginner male runners are most susceptible because men typically sweat more than women, and novices take longer to complete a race. The abrasion causes chafing, which causes bleeding, which causes red stripes down the front of a white shirt, especially near the end of marathons. Women aren’t immune. Even nursing moms can be afflicted. “The skin around your nipples isn’t capable of thickening and getting stronger,” says Bright, medical director for the Columbus Marathon. The few women he has seen with bloody nipples were wearing no bra, a poorly fitting bra, or a cotton one. Stay hydrated. “When you stop sweating, all you have left on your skin is salt,” says Bright. “The liquid takes the edge off the salt.” Equipment fixes for men: Protect yours with circular Band-Aids or NipGuards. Women? A moisture-wicking, properly fitted sports bra.