Nasal breathing during Zone 2 to train respiratory efficiency

Training with nose-only breathing during easy sessions builds diaphragmatic strength and CO2 tolerance that transfers to aerobic performance.

Why it works

Nasal breathing during low-to-moderate intensity exercise forces the use of the diaphragm as the primary respiratory muscle and increases CO2 tolerance (the main driver of the urge to breathe harder). Higher CO2 tolerance allows the body to extract more oxygen from each breath before triggering a stronger breathing reflex — a marginal but real efficiency gain. Nasal breathing also warms and humidifies air and releases nitric oxide, which dilates the airways.

How to do it

1. Attempt all Zone 2 training with mouth closed, breathing only through the nose.
2. If you need to open your mouth, reduce your pace until you can maintain nasal breathing comfortably.
3. This will feel frustrating initially — your effective Zone 2 pace will be much slower at first.
4. After 4–8 weeks of consistent nasal training, your nasal-breathing pace at the same heart rate typically increases.

Evidence

Nasal breathing during exercise is mechanistically supported (diaphragm loading, nitric oxide release, CO2 tolerance) and used in clinical breath training. Direct RCT evidence for nasal training improving VO2 max specifically is limited. (mechanistic)

The nasal breathing for performance claim (popularized by Patrick McKeown and James Nestor) is largely mechanistic and practitioner-reported; high-quality RCTs on VO2 max outcomes are sparse.

Common mistake

Trying nasal breathing during high-intensity sessions rather than Zone 2 — at maximal effort, the respiratory demand genuinely cannot be met by nasal breathing alone, and forcing it impairs performance without benefit.