The answer may shock athletes who’ve heard in yoga classes that “the nose is for breathing while the mouth is for eating.” Or athletes who have taken to heart the teachings of John Douillard, the chiropractor and author of the 2001 book extolling nasal over oral breathing, Body, Mind, and Sport, which Jurek refers to in his bestseller Eat and Run. But here it is:
“Breathe freely!” says Dr. James Shaffrath, a lecturer of exercise biology, neurobiology, physiology and behavior at the University of California, Davis. “I have never seen a study—and I look for them—in which any adopted pattern of breathing did anything to performance, oxygen consumption, efficiency, or fatigue.” Forget forcing yourself to breathe nasally, or to breathe in through your nose for two counts, then out through your mouth; breathing naturally, whether it’s nasally or orally, Shaffrath says, is what’s important.
“Almost everybody breathes nasally at rest,” he continues. “As you start to do light exercise, you’ll continue to breathe through the nose entirely until you’re at about two to three times your resting breathing rate.” At that point, you’ll start breathing through your mouth. And once you exceed four to six times your resting breathing rate—taking in 20 to 35 liters of air per minute—“everyone in the world is a mouth breather,” Shaffrath says. “You can’t push 21 liters of air through your nose comfortably.”
Often nasal advocates claim that we’re not breathing deeply enough—that we must "belly breathe" in order to use our lungs to their full capacity. However, the current scientific evidence for those claims is lacking. “Humans deepen each breath when they exercise—it’s what we do to increase our oxygen uptake,” Shaffrath says. “Nose vs. mouth? Doesn’t matter.”
The only time consciously breathing through your nose may be beneficial, Shaffrath says, is when the air temperature and humidity are extremely low, like in Minnesota in the winter, or on top of Denali. At that point, the your nose’s ability to humidify and warm the air before it enters the lungs may help you breathe more easily. In most climates where people are training, Shaffrath says, the air temperature and humidity shouldn’t make a difference in how you breathe.
And if you’re asthmatic, a 2008 study suggests that you, in particular, may want to make a general effort to breathe nasally, as mouth breathing may exacerbate asthma. In the study, eight resting volunteers with mild asthma breathed solely through their noses for one hour on one day, then solely through their mouths for one hour on another day. Lung function declined when they breathed only orally, and three subjects coughed and wheezed at the end of the mouth-breathing hour.
THE BOTTOM LINE: Just breathe. It doesn’t matter whether you’re breathing out of your nose or your mouth while you exercise. When you’re working hard, you’ll have to breathe through your mouth to take in enough oxygen. But if concentrating on your breathing helps keep you calm or mentally focused while training and competing, go ahead. Just don’t force yourself to breathe out of your nose if it isn’t comfortable—it won’t improve your performance.