A few weeks ago, I opened up my training plan for the week and saw this.

What is this nasal breathing stuff?

Why am I worried about my breathing?

What impact does this have on my goals for OCR dominance?

Despite my questioning, I trust my coach implicitly so I jumped on the rower and did the workout.

It seemed quite easy.

I completed the 5k without much strain and keeping my HR around 100 BPM exclusively nasal breathing.

Fast forward a few weeks and the workout changed.

Now I needed to do the same 5K unloaded on the rower while nasal breathing, but at an 8/10 RPE.

Again, I thought this should be easy. I jumped on the rower and half way through my heart rate started to climb into the 130s and 140s. I found myself taking huge breaths in and out through my nose. By the end I was breathing harder than I ever had through my nose. I did it, but man was it hard!

I continued to perform this workout week after week and over the course of several weeks my time to complete the 5K was going down and down.

In my first attempt at this workout I completed the 5K in 21:57 with an average heart rate of 127 and a peak heart rate of 135. My current PR for the workout is 20:37 with an average heart of 138 and a peak heart of 147…..breathing exclusively through my nose!

It was clear to me that this nasal breathing thing had some value to it, but what exactly was happening? What was nasal breathing actually doing? Why did it seem that I could exercise at a higher heart rate even though I was not taking in more air? Was there anything happening “under the covers” that I couldn’t see reflected in my numbers?

Let’s Take A Dive Down The Nerd Chute

Breathing is how we take in oxygen, duh!

You might think that is the sole purpose for us breathing, but there is something more important happening than getting oxygen into the body….we are exhaling carbon dioxide!

To better understand why exhaling carbon dioxide is important lets take a dive down the nerd chute and see what is happening with oxygen and carbon dioxide in the muscle.

To make your muscles work (i.e. contract when you need it to), your muscles consume oxygen and substrates like glucose (carbohydrates) and fats to produce the energy necessary. The byproduct of this energy production is carbon dioxide.

The amount of carbon dioxide in the blood needs to be kept in balance. In order to keep the right balance we breathe to remove carbon dioxide from the body.

That’s right, it is actually the buildup of carbon dioxide in the body that causes us to take a breath!

Too Much Of A Good Thing

It might seem like the more carbon dioxide we can remove from the body the better.

Quite the contrary actually.

Besides causing us to take a breath and in the process take in oxygen, carbon dioxide also plays another important role in the process of delivering oxygen to the working muscle.

Oxygen is transported around the body via your blood. The level of carbon dioxide in the muscle is what causes the removal of carbon dioxide from the muscle in exchange for oxygen. If the level of carbon dioxide in the muscle is too low, the exchange of carbon dioxide for oxygen will never occur.

That means if you remove too much carbon dioxide from the body, oxygen gets trapped in the bloodstream and will not be delivered to the muscles that need it.

Better Carbon Dioxide Tolerance = Better Fat Burning

The amount of carbon dioxide you can allow to build up in your body before having to take a breath is called carbon dioxide tolerance. As we discussed above, if your carbon dioxide tolerance is low you are going to breath more frequently and exhale more carbon dioxide. The lower the carbon dioxide in the blood the less oxygen gets delivered to the muscles that need it.

When the muscle has enough oxygen present it is easier for the muscle to rely primarily on fat to fuel its energy needs (up to a point, exercise intensity is also going to dictate whether the muscle will burn fat or carbs). In the case where there is not enough oxygen present, the muscle will not be able to oxidize fatty acids. In this scenario, the muscle will turn to glucose to fuel its energy needs since turning glucose into energy does not rely on the presence of oxygen.

When someone has a low carbon dioxide tolerance and is offloading a lot of carbon dioxide they will rely more heavily on glucose to fuel their energy needs regardless of the exercise intensity. In other words they are burning carbs even when sitting on the couch!

I know that was a lot of technical stuff, so here is a summary.

• Carbon dioxide causes us to breath
• If your ability to tolerate carbon dioxide is low, you will breathe more frequently removing more and more carbon dioxide
• If there is not enough carbon dioxide in the body, oxygen cannot be delivered to the muscles that need it
• Without enough oxygen your body will turn to glucose (carbohydrate) to provide the energy it needs
• Therefore if you have a low carbon dioxide tolerance you will forever be dependent on glucose to provide a majority of the fuel you need and hence decrease your metabolic flexibility

Want to burn more fat instead of carbohydrates? Then you better make sure you have adequate carbon dioxide tolerance!

Measuring Your Carbon Dioxide Tolerance

The folks from Shift Adapt have came up with a simple test to measure your carbon dioxide tolerance. The test protocol is very simple

• Get a stopwatch
• All breaths are through your nose only
• Take 3 normal nasal breaths
• Take 1 more full nasal inhale and fill your lungs all the way
• Start to nasal exhale, start your timer
• Exhale through your nose as slowly as you possibly can, for as long as you can
• DON’T hold your breath or swallow. If that happens, stop your timer
• When you have no air left to exhale, stop your timer

What do the results mean?

• >80 seconds –> Elite. Reflects an advanced pulmonary adaptation, excellent motor control, and low arousal.
• 60-80 seconds –> Advanced. Reflects a healthy pulmonary system, good motor control, and relatively low arousal.
• 40-60 seconds –> Intermediate. This range generally improves quickly with a focus on CO2 tolerance training.
• 20-40 seconds –> Average. Moderate to high arousal state. Breathing mechanics need improvement.
• <20 seconds –> Poor. Very high arousal and stress sensitivity.

If you have below average carbon dioxide tolerance you probably are relying too much on carbs and have impaired metabolic flexibility.

How do you build up a tolerance to carbon dioxide?

You train it of course!

Building Up Carbon Dioxide Tolerance

Your ability to tolerate more carbon dioxide is a stress just like exercise is a stressor.

And just like most other stressors on the body, exposing the body to it slowly over time allows the body to adapt.

It is very similar to when you start a new exercise program. You don’t go balls to the wall from the start (or maybe you do but you probably have experienced the consequences 😉), you start with a low volume and add more and more as you adapt.

The two most popular ways to build up carbon dioxide tolerance is breath holds and nasal breathing.

Nasal Breathing

Nasal breathing is exactly as it sounds, you breathe through your nose.

You might be saying to yourself “I’m golden, I already do that!”.

Think again.

Many studies in children have shown that greater than 50% of children breathe through their mouth. There have not been many studies done in adults, but we can imagine that the habits one develops as a child carry through to adulthood^[1]. I would imagine the statistics are similar in adults.

In addition people shift their breathing patterns during different activities.

You may breath through your nose when watching Netflix, but what about when you move?

Are you nasal breathing when walking, gardening, playing with your kids?

What about sleep?

What about when you are hiking, going for an easy bike ride, or doing body weight exercises?

If you can’t do easy aerobic exercise without breathing through your nose exclusively, that is a sure sign you need to improve your carbon dioxide tolerance.

Try it.

If you find yourself mouth breathing the best way to improve your carbon dioxide tolerance is to work on it.

If you can’t go for a walk without mouth breathing start there. Start off nasal breathing and make a conscious effort to extend the amount of time you can walk without breathing through your nose.

If walking is not an issue, but something like running, rowing, or biking is, even if the intensity is low, then take the same approach, start off nasal breathing for as long as you can. Then try and extend that, either in time or intensity. I especially like to correlate nasal breathing with my heart rate when exercising. If you can do the exercise at a higher heart rate but still nasal breath, you know you are improving my carbon dioxide tolerance.

Breath Holds

Another way to improve your carbon dioxide tolerance is to allow it to build up and not release it.

This can be done simply by holding your breath.

The folks from Shift Adapt have simple breath hold exercises you can do to improve your carbon dioxide tolerance. If you go to this page and enter your score from the breathing test above, it will give you personalized recommendation for performing these exercises.

Carbon Dioxide Tolerance From An Ancestral Perspective

While we can’t hook up ancient man to a metabolic cart and measure the amount of fat and carbs they were burning, it is a fairly good bet that they were metabolically flexible and had a very good carbon dioxide tolerance. Their anatomy was actually quite conducive to nasal breathing when compared to our own ^[2].

I myself never really paid much attention to the way I was breathing, particularly during exercise. I just breathed in a way that felt natural, and it felt natural to breathe mostly through my mouth.

Once I started to understand the connection between breathing and how that plays a role in our metabolic function, I was blown away. I always thought that heart rate and substrate availability was what dictated the fuel used. In other words, if you kept your heart rate low and did not consume an excess amount of carbohydrates you would burn mostly fat during exercise. Then after reading more about breathing patterns it became obvious that if your breathing pattern was not able to deliver enough oxygen to the muscle you would be burning carbohydrate regardless of your heart rate!

I now dedicate at least one training session a week to working on my breathing pattern during exercise by nasal breathing only at the highest intensity I can. By doing this I am training my body to be able to tolerate more carbon dioxide, which results in my blood delivering more oxygen to my muscle, and therefore allowing me to burn more fat at a higher heart rate.

The result?

Improved metabolic flexibility!

Identifying environmental mismatches and correcting those mismatches in order to improve athletic performance and quality of life is the goal of my health coaching practice. Each week I deliver information on exactly how to do this via my newsletter for free! Sign up now to get actionable information you can easily incorporate into your life and training to start making noticeable changes in the way you perform athletically and in everyday life.

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1. (2017, January 4). Prevalence of Mouth Breathing – YouTube. Retrieved September 17, 2020, from https://www.youtube.com/watch?v=-WdgW-IoMVU ↑

2. “James Nestor – These are some skulls from the Morton ….” https://www.facebook.com/108447400800358/posts/these-are-some-skulls-from-the-morton-collection-the-largest-collection-of-pre-i/136974771280954/. Accessed 19 Sep. 2020. ↑