Most athletes are told either to breathe through the nose or to breathe however feels natural. Both pieces of advice are half right, because the best route for your breath depends on how hard you are working. This is actually also true at rest.

At easy effort your body needs relatively little air, and the nose can deliver it. As intensity climbs, ventilation rises steeply, and at some point the nose becomes a bottleneck. Athletes who force nasal breathing all the way to the finish line end up fighting their own airway, while those who breathe through the mouth from the first warm-up minute give up benefits that only the nose provides. As so often in life, finding balance is where the magic lies.

Easy effort: why the nose wins

Below your first ventilatory threshold, roughly the pace at which you can still hold a conversation, nasal breathing has real advantages. The nose filters, warms and humidifies incoming air, and it releases nitric oxide, a gas that widens blood vessels and helps match airflow to blood flow in the lungs. Its narrow passages also add resistance, which slows each breath and lets CO2 rise slightly, and because haemoglobin releases oxygen more readily where CO2 is higher (the Bohr effect), working muscle gets its oxygen more easily, which is the mechanism Patrick McKeown builds the Oxygen Advantage method around.

Early data from elite athletes points the same way. A 2026 preprint on SportRxiv, not yet peer-reviewed, tested 13 elite male cyclists and triathletes at their first ventilatory threshold. Nasal breathing lowered estimated ventilation by about 11.8 L/min and improved oxygen extraction by roughly 13.8%, without changing metabolic demand. In plain terms, the same work cost the breathing muscles less, though as a preprint it is a strong signal rather than settled fact.

Near maximum effort: let the mouth in

Near maximum effort the picture changes, and the reason is simple physics. Air moving through the nose meets far more resistance than air moving through the mouth, and at high workloads the ventilation your muscles ask for outruns what that nose can supply comfortably. Breathing turns laboured, the shoulders tense, and the breath climbs into the upper chest instead of using the diaphragm. Opening the mouth alongside the nose lowers the resistance and lets the diaphragm keep working at full stroke.

A 2025 randomised cross-over study in Frontiers in Physiology tested this with 12 well-trained male cyclists and triathletes, comparing oral-only, oronasal and decongested oronasal breathing during incremental tests. Minute ventilation and VO2max did not differ significantly between conditions, and time to exhaustion was 2.8% longer with oronasal breathing and 4.2% longer with a decongested nose, although neither gap reached statistical significance. Blood lactate after the test was significantly lower with oral-only breathing, and the authors cautioned that exclusive mouth breathing may alter metabolic responses without improving performance. The takeaway is not “always use the mouth” but “let the nose and mouth share the work when the effort demands it”.

The three-zone approach

At Atemstark we teach three zones instead of one rule. In the base zone, where you can speak in full sentences, breathing stays nasal. At the threshold, where speech shrinks to short phrases, the mouth joins the nose in a controlled hand-over and the breath stays rhythmic rather than ragged, while at maximum effort breathing is free and unforced. We track where each athlete’s breathing starts to break down alongside their BOLT score, because CO2 tolerance influences how early the urge to gulp air arrives, a theme we cover further on our breathing for sport page.

The hand-over is a skill, not a switch. As the workload rises, keep the exhale long and relaxed while the mouth opens slightly, because a long exhale keeps the diaphragm working through its full range and stops the breath climbing into the upper chest. Practising this in threshold sessions teaches you to recognise the moment the nose stops being enough, before your breathing turns uncontrolled.

This matters for endurance work in particular. Athletes who build CO2 tolerance through nasal breathing often find the urge to gulp air arrives later, which is part of what we describe in our piece on CO2 tolerance and threshold power in cyclists. If running is your sport, our article on nose or mouth while running applies the same zone logic to pace.

Try this on your next session

Here is one thing to do today. On your next easy run, ride or session, keep your mouth closed and breathe through your nose at a pace where you could hold a conversation. If you have to open your mouth to keep up (for inhales or exhales), you have found your first threshold, so note the pace or heart rate at that point. Spend most of the session just below it, and repeat the test weekly to see whether that point moves.

None of this replaces coaching for a specific race, but it gives you a clear rule to test. To see how your CO2 tolerance shapes the point where your breathing gives way, take the BOLT test.

FAQ

Should I breathe through my nose or my mouth when I exercise?

It depends on intensity. Stay nasal at easy effort, let the mouth join the nose near your threshold, and breathe freely at maximum effort.

Is nasal breathing during training a performance hack?

Not on its own. At easy pace it lowers the cost of breathing, but at maximum effort the data shows no significant gain either way.

How do I find my first ventilatory threshold without a lab?

Use the talk test. The pace where you can no longer breathe comfortably through the nose is a practical stand-in.

Does this mean I should stop breathing through my mouth entirely?

No. Mouth breathing at high effort is normal, while constant mouth breathing at rest or easy pace forfeits the nitric oxide and slower breaths the nose provides.