Nasal breathing is responsible for bringing oxygen into and out of the lungs at a slower rate, resulting in increased oxygen absorption once inside the body. Most of the nitric oxide produced in the body comes from the paranasal sinuses, and this nitric oxide allows oxygen to be absorbed at a rate eighteen percent higher than it is when inhaled through the mouth.

Nitric oxide is associated with optimal cardiovascular function and is an important signaling molecule in many bodily processes, including metabolic and hormonal health. Without nasal breathing and nitric oxide, oxygen is inhaled through the mouth resulting in irritation to oral tissues, decreased oxygen absorption, and shallow breaths. These problems lead to bigger problems over time, like chronic fatigue, brain fog, forgetfulness and metabolic syndrome, and are associated with many diseases that are characterized by these symptoms, including Alzheimer's and heart disease.

Easy to observe symptoms that may signal someone is a mouth breather include: dry lips, dry mouth, snoring and open mouth while sleeping, numerous airway illnesses, including sinus and ear infections and colds.

The Tarahumara Native Americans, claimed by some to be the greatest marathoners in the world, run up to 60 miles a day, minimally shod, sometimes barefoot, in the Cooper Canyon of northern Mexico. They breathe almost exclusively through their noses, and maintain unstressed, peaceful faces. And Apache “spirit runners” have been trained from childhood by running in the desert while holding a mouth full of water. They learned to breathe deeply while holding a mouth full of water. They learned to breathe deeply and rhythmically through their noses, avoiding the panting that would dry their throats in the dry desert air.