Everyday saliva is generally easy to swallow back down, but running often causes us to produce thicker mucus.

According to Adeem Azhar, clinical pharmacist and co-founder of Core Prescribing Solutions, that’s because when we do intense exercise, we breathe more heavily. That then increases the rate of evaporation from our mouths – causing dry mouth or thicker saliva.

“Hydration levels can also affect the production and consistency of saliva; when you’re well-hydrated, your body produces an ample amount of saliva, which tends to be thin and clear. When you’re dehydrated, however, saliva production can decrease… because the body is trying to conserve water for vital processes.”

Saliva is regulated by the sympathetic and parasympathetic nervous systems, and exercise stimulates the sympathetic nervous system, which causes an increase of salivary flow rate and proteins that make saliva thicker.

Keep needing to spit when you run outdoors? It’s totally normal – here’s why it happens