While running my mouth gets thick saliva that i have to spit. i can still breathe fine. why is this?

Stay Hydrated: This is a common problem for any endurance athlete. Your body is constantly sweating to regulate body temperature and the fluid in your mouth is evaporating too. Your saliva is part mucus and part water, so your saliva is left feeling thick and stringy. My advice would be to stay hydrated and maintain your electrolytes before, after and even during the run if possible.

Can be dryness.: This is a nonspecific finding that may occur during running or other activity. It may be because of too little fluids, or it may be due to a chemical called Atropine which produces, among other things, dry mouth. This is used for many things, like bradycardia for resuscitation, because it blocks the vagus nerve. It is also used to dilate pupils in eye exams and to treat insecticide poisoning.

Nervous system: It's due to the sympathetic/parasympathetic nervous system. Sympathetic is for fight or flight. Parasympathetic is rest and digest. When you run you are using the sympathetic nervous system. Parasympathetic shuts down so that all your energy goes to sympathetic. Saliva is part of digestion. Parasympathetic (rest/digest) turns off, so less saliva giving you a think saliva feeling in your mouth.

Water: This is common during exercise..The best thing you can do is stay hydrated... Drink more water usually this happens when you have not been sufficiently hydrated.