Men's nipples can bleed when running primarily due to a condition known as "runner's nipple" or "jogger's nipple." This happens because of repeated friction between the nipples and clothing during running, which causes chafing and irritation of the skin. The constant rubbing over long distances can make the nipples raw, cracked, and sometimes bleed. Excess sweat can worsen the irritation by increasing moisture and friction, and cooler weather can make nipples more prone to chafing because they tend to become erect, increasing the likelihood of rubbing. Clothing material plays a big role; rough fabrics like cotton that get heavy with sweat tend to aggravate the skin more than smooth, moisture-wicking fabrics. Preventive measures include using lubricants such as petroleum jelly, wearing well-fitting and soft fabric shirts, applying adhesive bandages or tape over the nipples, and keeping the area clean and dry after running to avoid infection. Thus, bleeding is essentially caused by friction-induced skin damage on the nipples from running with clothing that repeatedly rubs the area during the motion.
