The likely culprit: dehydration. “When you exercise, your fluid—water fluid and also your blood—is shunted from your mouth, your stomach, your GI tract to your muscles,” says Dr. Bob Adams, chair of USA Track and Field’s sports medicine and science committee. “Some people get kind of dry in the mouth, other people get a kind of thickness.”
Saliva is 98 percent water; the other two percent is made up of various substances including mucus and electrolytes. If you’re dehydrated, your saliva can become a thicker, more concentrated mixture of those substances. That white ring around your lips, Adams says, is likely a combo of mucus and electrolytes, including salt.
How to keep from looking like a rabid dog? Obviously, stay hydrated. Drink when you’re thirsty, and bring extra fluid on longer runs. Also, try chewing gum. “When you chew gum, digestive enzymes are released in your mouth, making it more moist in there,” and thinning out the mucus, Adams says.
The bottom line: Dehydration is probably causing your runner’s mouth, so stay on top of your fluids. Chewing gum can also help, but if you’re perfectly hydrated and continue to have bothersome mucus buildup, see your doctor to determine if something other than dehydration is the culprit.