Why Nasal Breathing During Training Changes Everything

The paranasal sinuses produce nitric oxide continuously. When you breathe through your nose, you draw that nitric oxide into your lungs, where it acts as a vasodilator — opening blood vessels in the pulmonary system and increasing oxygen exchange efficiency. Mouth breathing bypasses this entirely. The difference in nitric oxide delivery is approximately sixfold, which is a substantial physiological effect that most athletes are leaving on the table.

Patrick McKeown, who has worked extensively with elite athletes on breathing mechanics, has shown that nasal breathing during submaximal training improves CO2 tolerance over time. Why does CO2 tolerance matter? Because the urge to breathe is primarily driven by rising carbon dioxide levels, not falling oxygen. Athletes who can tolerate higher CO2 without panic — without that gasping, mouth-open breathing pattern — can maintain composure and make better decisions under fatigue. This is trainable. By deliberately nasal breathing during Zone 2 and moderate-intensity work, you're progressively raising your CO2 tolerance threshold.

There's also a filtration and humidification component. Nasal passages warm, filter, and humidify incoming air, which reduces airway inflammation and exercise-induced bronchoconstriction. For athletes training in cold or dry environments, this is particularly relevant. The data on nasal breathing and recovery between efforts are also compelling — athletes who nose-breathe during recovery intervals between high-intensity bouts return to baseline heart rate faster.