Football players spit so much because exercise increases the amount of protein in saliva. When you exercise, the amount of protein secreted into the saliva increases. A protein mucus named MUC5B makes your saliva thicker when you're exercising which makes it more difficult to swallow so we tend to spit more. It may occur during exercise because we breathe through our mouths more. MUC5B could activate to stop our mouths from drying out, therefore.