Why do marathon runners, regardless of gender, wear nipple covers?
Chest patches were originally a specialized item used by women to block areola, in order to achieve the effect of invisible bra. From a visual perspective, they are considered decorative items.
But marathon runners wearing silicone nipple covers are not for the sake of looking good, but for another clever use:
The main function of nipple covers is to protect the nipples
Marathoners will constantly rub their tops and nipples in contact during hours of running.
Adults run about 0.7 meters per step, and it takes about 1500 steps to run a kilometer. Roughly speaking, completing the entire marathon requires about 63000 steps, and with each step, the athlete's chest rubs against their clothes.

Moreover, long-distance running sportswear is made of fiber products, and it is inevitable for contestants to get their clothes soaked in sweat during the running process, which can cause greater damage from friction.
That is to say, athletes participating in the whole horse race, within four or five hours, rub their nipples and fiber made sportswear back and forth more than 63000 times, and this result can be imagined.
If a contestant does not wear a nipple cover, their nipples may have ruptured due to friction before they have completed the entire race, and even blood may soak through their clothes!
So, the main function of nipple covers in marathon sports is to protect athletes' nipples from friction, which is one of the equipment to protect athletes.