Asked by: John Farrugia, Qormi, Malta
Several studies have shown that exercise increases the amount of protein secreted into the saliva, especially a kind of mucus called MUC5B. This mucus makes the saliva thicker, which makes it harder to swallow, so we spit it out. It isn’t clear why we produce more MUC5B when exercising, but it may be because we breathe through our mouth more, so the mucus stops the mouth from drying out.
Some sports, such as basketball and tennis, penalise players for spitting, but football and rugby don’t, so the players are free to expectorate!
Read more:
Subscribe to BBC Focus magazine for fascinating new Q&As every month and follow @sciencefocusQA on Twitter for your daily dose of fun facts.
