You breathe roughly 20,000 times a day. And if you are like most cyclists, you are doing it wrong for about 19,500 of those breaths.
Here's the thing nobody tells you about breathing on the bike: the limitation isn't how much air you can get in. It's how efficiently your body uses the air it already has. And that efficiency question starts in a place most riders completely ignore — your nose.
I spent years gasping through my mouth on every ride, treating my lungs like a pair of bellows that just needed more volume. Then I started reading the work of Patrick McKeown and James Nestor, tested a few things on long base rides around Girona, and realised I had been solving the wrong problem. The issue was never oxygen supply. It was carbon dioxide tolerance.
Let me break this down properly.
What Your Nose Actually Does That Your Mouth Cannot
Your nasal passages are not just a smaller, less convenient version of your mouth. They are a processing plant for air.
When you breathe through your nose, three things happen that mouth breathing skips entirely. First, the air is warmed and humidified before it reaches your lungs — this matters more than you think, especially on cold morning rides where dry, cold air hitting the bronchial tubes can trigger constriction. Second, the nose filters out particles and pathogens. Third, and most importantly for performance, the paranasal sinuses produce nitric oxide.
Nitric oxide is a vasodilator. It widens blood vessels and improves the transfer of oxygen from alveoli into the bloodstream. A 1995 study published in the journal Thorax by Lundberg and colleagues measured nitric oxide output during nasal breathing at roughly 15 times higher than during oral breathing. You are literally making a performance-relevant molecule every time you breathe through your nose, and bypassing production entirely every time you open your mouth.
This isn't marginal. The downstream effect is better oxygen delivery to working muscles at the same cardiac output. You do the same work with less cardiovascular strain. For endurance athletes — and that is exactly what we are on four- and five-hour rides — that matters enormously over cumulative hours.
The CO2 Tolerance Problem Most Cyclists Don't Know They Have
Here is where it gets interesting, and where most of the actual benefit sits.
Most cyclists switch from nose to mouth breathing not because they are running out of oxygen. Blood oxygen saturation stays above 95 per cent in nearly all conditions short of altitude or severe respiratory disease. You switch because rising carbon dioxide levels in the blood trigger a sensation called air hunger — that panicky feeling that you need more air, right now.
But CO2 is not a waste product in the way most people think. It's a signalling molecule. It's involved in the Bohr effect, which governs how readily haemoglobin releases oxygen to tissues. Higher CO2 tolerance actually means your muscles get better oxygen offloading at the cellular level. When you train yourself to tolerate a slightly higher partial pressure of CO2 without triggering the air hunger response, you breathe less but more effectively.
Patrick McKeown talks about this extensively in The Oxygen Advantage. The body's sensitivity to CO2 is adjustable. It's a thermostat, not a fixed threshold. If you spend most of your day — and most of your easy rides — mouth breathing, you keep that thermostat set low. Any slight rise in CO2 triggers the reflex to breathe more, breathe harder, breathe through the mouth. You ventilate excessively and blow off CO2 that was actually useful.
Nasal breathing forces you to tolerate higher CO2 because the airflow is naturally restricted. Over weeks of consistent practice, your chemoreceptors recalibrate. The air hunger threshold moves. And suddenly you find that what used to feel like suffocation at 65 per cent of max heart rate now feels perfectly manageable.
The BOLT Score: Measuring Where You Start
Before you change anything on the bike, measure where you are. McKeown developed the BOLT score — Body Oxygen Level Test — as a practical assessment of CO2 tolerance, and it takes about 30 seconds.
Sit quietly. Breathe normally for a minute or two. Take a normal breath in through your nose. A normal breath out through your nose. Then pinch your nose closed and start a timer.
The number you want is the time until the first distinct urge to breathe. Not how long you can white-knuckle it. Not a breath-hold competition. The moment your diaphragm contracts or you feel your throat wanting to swallow — stop the timer. That is your BOLT score.
Under 20 seconds: your CO2 tolerance is poor. You are almost certainly a chronic over-breather, and nasal breathing on the bike will feel noticeably difficult at first. This is where most of the population sits.
20 to 30 seconds: average. You have room to improve and will see noticeable changes within a few weeks of nasal breathing practice.
30 to 40 seconds: good. You likely already have reasonable breathing efficiency and will find nasal zone 2 rides manageable fairly quickly.
Above 40 seconds: excellent. McKeown considers this the target for trained athletes. At this level, nasal breathing at moderate intensities should feel natural.
Test yourself before you start training this, and retest every two weeks. The number will move if you are consistent.
Nasal Breathing as a Zone 2 Verification Tool
Here is the most practical application for cyclists, and the reason I now use nasal breathing on nearly every endurance ride.
Zone 2 training is the backbone of endurance development. We all know this. But one of the persistent problems with zone 2 is knowing whether you are actually in it. Heart rate drifts. Power zones are only as accurate as your last FTP test. Perceived effort is subjective and gets distorted by caffeine, sleep quality, and how irritated you are with the headwind.
Nasal breathing gives you a binary check. If you can sustain nasal-only breathing at your target intensity, you are in zone 2 or below. If you cannot — if you keep needing to gulp air through your mouth — you are either riding too hard or your CO2 tolerance is too low. Both of those are useful pieces of information.
This does not replace a heart rate monitor or a power meter. But it adds a physiological feedback loop that those tools cannot provide. It tells you something about your metabolic state, not just your mechanical output.
The 6-Week Nasal Breathing Protocol for Cyclists
Right. Enough theory. Here is how to actually train this on the bike.
Week 1-2: Introduction blocks. Start every easy ride with 10 minutes of nose-only breathing. Keep your mouth closed. If you feel the air hunger rising, slow down — drop a gear, reduce your cadence, let your pace fall. The instinct will be to open your mouth. Resist it. After 10 minutes, return to normal breathing for the rest of the ride. Do this on every ride that is zone 2 or below. Skip it on interval days.
Week 3-4: Extending duration. Push the nasal breathing block to 20-30 minutes. You will notice it getting easier — this is the CO2 tolerance adapting. If 30 minutes feels comfortable, try alternating: 20 minutes nasal, 5 minutes mouth, 20 minutes nasal. You are building the capacity to sustain it, not testing your willpower.
Week 5-6: Full ride nasal breathing. Aim to complete entire zone 2 rides — 60 to 90 minutes initially — breathing only through your nose. If you are doing a three-hour endurance ride, try the first 90 minutes nasal and allow mouth breathing after that. Over time, extend until the full ride is nasal.
Ongoing: Intensity creep. Once fully nasal zone 2 rides are comfortable, start testing nasal breathing at slightly higher intensities. Tempo efforts. Sweet spot. See where the threshold sits. You are not trying to race with your mouth closed — you are trying to push that switchover point higher.
A few practical notes. Your nose will run. Carry a small pack of tissues or learn the farmer's blow. Your pace will initially drop by 5-15 per cent during nasal breathing blocks — this is expected and temporary. If you have a deviated septum or chronic nasal congestion, address those first; McKeown's work covers nasal decongestion techniques that can help.
When Mouth Breathing Is the Right Answer
I want to be clear about this because the nasal breathing community can get dogmatic, and dogma is useless on a bike.
Above roughly 70-75 per cent of your maximum heart rate, mouth breathing is not just acceptable — it is necessary. Your ventilatory demand at threshold and above simply exceeds what nasal passages can deliver. Trying to force nasal breathing during a 20-minute FTP test or a VO2max interval is like trying to fill a swimming pool through a garden hose. You will limit your performance and you will not get any additional CO2 tolerance benefit because you are already well above the adaptive zone.
The rule is simple: train nasal, race oral. Use nasal breathing to build your aerobic efficiency and CO2 tolerance at lower intensities. When the effort demands it, open your mouth and breathe as hard as you need to.
There is a grey zone between zone 2 and threshold where you might be able to sustain nasal breathing on good days but not on bad ones. That is fine. The grey zone shifts as your tolerance improves. Do not force it. Let it come.
The Off-Bike Work That Amplifies Everything
Here is where James Nestor's reporting in Breath connects the dots. His work made a compelling case that how you breathe during the other 22 hours of the day affects what happens during the two hours you are on the bike.
Chronic mouth breathing during rest lowers your baseline CO2 tolerance. If you walk around all day with your mouth open, breathe through your mouth while working at your desk, and sleep with your mouth open, you are resetting that thermostat to its most sensitive position every single day. Then you get on the bike and wonder why nasal breathing feels impossible.
Three off-bike habits that accelerate on-bike progress:
Daytime awareness. Set a reminder on your phone — every 30 minutes for the first week, then hourly. When it goes off, check: is your mouth closed? Is your tongue resting on the roof of your mouth? If not, correct it. This sounds absurdly simple. It is. And it works because the body defaults to whatever pattern you practise most frequently.
Mouth taping at night. This sounds extreme until you try it. A small strip of surgical tape — 3M Micropore works well — placed vertically over the lips before sleep. It does not seal your mouth shut; you can open your lips and dislodge it easily. What it does is encourage nasal breathing during sleep, which improves sleep quality, reduces snoring, and prevents eight hours of CO2 tolerance regression every night. Nestor tried this during his Stanford experiment and reported noticeably better sleep metrics within days.
Breathing exercises off the bike. McKeown's Oxygen Advantage method includes several drills. The simplest and most relevant for cyclists: walk for 10-15 minutes breathing only through your nose, then add breath holds. Take a normal breath in, normal breath out, pinch your nose, and walk 10-15 paces before releasing. Repeat. This trains CO2 tolerance without any cycling-specific fatigue. Do it on rest days.
The Research Context: What We Know and What We Don't
I want to give you an honest picture of where the evidence sits.
The nitric oxide research is solid. Lundberg's work and subsequent studies have consistently shown that nasal breathing produces significantly more nitric oxide than oral breathing, and the vasodilatory effects are well-established in physiology.
The CO2 tolerance adaptation is well-supported by respiratory physiology principles and by clinical evidence from McKeown's work with thousands of athletes and patients. The Bohr effect is textbook physiology — higher CO2 partial pressure does facilitate oxygen offloading from haemoglobin.
What we have less of is large-scale, controlled studies specifically examining nasal breathing interventions in trained cyclists over meaningful time periods. Most of the cycling-specific evidence is observational, anecdotal, or extrapolated from general respiratory research. A 2018 study in the International Journal of Kinesiology and Sports Science by Dallam and colleagues did compare nasal and oral breathing during steady-state exercise and found similar oxygen consumption with lower breathing rate during nasal breathing — supporting the efficiency argument — but we need more work like this.
My position: the mechanistic evidence is strong enough, the risk is essentially zero, and the practical feedback you get from using nasal breathing as a zone 2 tool is immediately useful regardless of whether every physiological claim pans out. Try it for six weeks. Your BOLT score will tell you whether anything changed.
Putting It All Together: A Practical Summary
Start with the BOLT score. Know your baseline.
Introduce 10-minute nasal breathing blocks on easy rides. Extend them over four to six weeks until you can complete full zone 2 rides nasally.
Fix your off-bike breathing: mouth closed during the day, tape at night, walking breath holds on rest days.
Do not force nasal breathing above 70-75 per cent of max heart rate. Train nasal, race oral.
Retest your BOLT score every two weeks. Track progress.
Use nasal breathing as a zone 2 intensity check — if you cannot sustain it, you are either going too hard or you have work to do on your CO2 tolerance.
This is not about restricting airflow for the sake of it. It is about training a specific physiological adaptation — CO2 tolerance — that makes you a more efficient breather at the intensities where you spend the vast majority of your riding time. And it costs you nothing except a few weeks of mildly uncomfortable endurance rides.
The riders who do this consistently tell me the same thing: they breathe less, they ride easier, and their zone 2 pace quietly drifts upward without any increase in perceived effort. That is the kind of adaptation worth chasing.
If you want to go deeper on this and the other marginal gains that actually move the needle for masters cyclists, the Roadman Cycling community on Skool is where we break this stuff down in detail. Come say hello.