CO2 Tolerance Explained: How To Train and Test It - Hanu HRV
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CO2 Tolerance Explained: How To Train and Test It

It doesn’t matter if you are an elite athlete or a high-performing business professional. Managing stress levels and the state of your nervous system is vital for health, wellbeing, and performance.

Your CO2 tolerance can significantly improve your body’s resiliency to physical and mental stress. There are physical benefits to increasing CO2 tolerance and a close correlation between anxiety and our ability to manage stress. In addition, having increased CO2 tolerance allows us to control our rate and depth of breathing better. 

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What is CO2 Tolerance? 

CO2 tolerance is your body’s ability to tolerate a certain amount of CO2 buildup before it tells you it’s time to expel some air from the body.

CO2, or carbon dioxide, is a byproduct of respiration, but contrary to commonly held beliefs, it’s certainly not a waste product. When you breathe in oxygen, your body expels carbon dioxide. Your cells create energy using oxygen, creating CO2 as an off-gas. As CO2 builds up in the body, a signal tells the body to breathe. As our friend Patrick McKeow always says, the primary stimulus to breathe is not to inhale oxygen; it’s to expel CO2.

However, buildups of CO2 in the body are not a bad thing, per se. CO2 is necessary for your body to coordinate the needed breathing rate at any time. When CO2 builds up in the body, your brain signals that it’s time to expel it and that you need to breathe out. So, it’s CO2 that signals your body that you need to breathe, not oxygen!

Your tolerance to CO2 is how much you can accumulate before your body becomes overly stressed and you need to take a breath. 

Why CO2 Tolerance Matters

The theory behind CO2 tolerance is that by increasing the amount of CO2 your body can handle, you train your body to address this temporary imbalance before returning to its baseline. Improving your CO2 tolerance strengthens your body’s ability to adapt to stressors.

While the buildup of CO2 in the body causes “negative” side effects in the short term, its long-term benefits can significantly improve your body’s ability to thrive under stress. When you hyperventilate or engage in endurance training, the buildup of CO2 begins to cause your blood to become acidic. This process triggers the need to exhale and get rid of the CO2. When you lift weights, you create tiny tears in your muscles. Your muscles repair these tears, strengthening your muscles as a result. Training your CO2 tolerance works in a similar mechanism. 

Developing a high CO2 tolerance can also help your aerobic metabolism get stronger. According to the Bohr Effect, the lower your blood pH is and the higher your blood CO2 levels are, the easier it is for your body to “absorb” oxygen. In other words, if you want to transport oxygen more effectively into cells, you need CO2 as the primary carrier. The best mechanism for increasing oxygen delivery is to use CO2 more effectively. 

A poor CO2 tolerance signifies poor breathing control and less ability to absorb more oxygen under stress. Suppose you want to improve the health of your body and its ability to respond to stressors. In that case, you should strive to have strong breathing muscles and excellent breathing control so that the lungs and respiratory system can handle stress efficiently. 

As your CO2 tolerance develops, your body will be able to tolerate higher levels of CO2 before it begins to switch into emergency mode, where it triggers hyperventilation and an intense need to exhale and steady the breath rate. For example, if you are an athlete, an improved CO2 tolerance will enable you to push yourself further and have improved endurance. 

There’s a very close correlation between CO2 tolerance and stress/anxiety. The better control you have over your breathing, the less generalized anxiety you feel. You’ll also have a better ability to center yourself when things get stressful. 

Whether you’re an athlete or an office worker, utilizing your nervous system to manage anxiety and arousal is imperative for health and performance. Low anxiety equals low cortisol. Low cortisol equals less fat, more muscle, and better physical and mental performance.

Breathing is key to developing increased tolerance to CO2. 

The CO2 Tolerance Test

The CO2 tolerance test is a powerful method you can use to understand better how your body responds to stress. All it takes is a timer and a few minutes of your time to assess your CO2 tolerance. 

CO2 tolerance training is a method that rescue divers pioneered to train themselves to be able to hold their breath for more extended periods. This method has been pioneered by these divers and furthered by performance training expert Andrew Huberman at Stanford University. 

Your body constantly addresses how much energy is available to use for performance and how much is for rest and recovery. Using the CO2 tolerance test, you can see how well your body performs and how adaptable it is. Your ability to adapt to new situations without getting overly stressed is a well-researched fact that demonstrates how healthy and happy you are.

The CO2 tolerance test consists of the following: 

  1. Get your stopwatch ready.
  2. Start by taking three breaths, all in and out through your nose.
  3. Take one more breath and fill your lungs all the way up
  4. As you begin to exhale this breath, start your stopwatch.
  5. Let this breath out as slowly as possible, trying to extend it as long as possible.
  6. If you hold your breath or swallow, start over. 
  7. When you are out of air, stop the timer. 

Depending on your results, you can glimpse the state of your body’s CO2 tolerance levels. 

  • < 20 seconds means your Co2 tolerance needs some work
  • 20-40 seconds is the average time that most people achieve
  • 40-60 seconds is an above-average score that represents good CO2 tolerance
  • > 60 seconds means a healthy pulmonary system and good stress + breath control  

BOLT Score

The Body Oxygen Level Test 

The body oxygen level test, or BOLT score, is a health assessment used by Patrick McKeown to determine relative breathing volume during rest and breathlessness during physical exercise. This test is another powerful method to increase your body’s tolerance to CO2. Through this breathing exercise, you can improve the amount of breath you can hold, demonstrating an increased CO2 tolerance. 

You can see your BOLT score by simply breathing in and out through your nose and then pinching your nose closed to prevent air from entering your lungs. Then, time how long it takes until you feel the desire to breathe. This isn’t a test of how long you can hold your breath, but more to determine how long until your body reacts to the CO2 buildup. Your BOLT score consists of how long you last, with 40 seconds being a good marker for a healthy CO2 tolerance. 

Our Hanu devices have a built-in BOLT tracking system that you can use to build up and test your CO2 tolerance using this beneficial technique. 

CO2 and HRV

The autonomic nervous system (ANS) is the part of the nervous system that controls the body’s automatic internal functions, including heart rate, digestion, and secretions of many glands. The ANS also controls respiration, which increases with stress and slows with rest. Our ANS comprises the sympathetic and parasympathetic responses, representing the fight-or-flight response and the rest-digest response. It is well known that mental and emotional states directly affect activity in the ANS.

Heart rate variability (HRV) represents your body’s ability to modulate between these two responses. A more variable trend of HRV scores tells us that our body is balancing between stress and rest very effectively. As you can infer, your HRV is similar to CO2 in that they tell us how well our body can respond to stress and bring itself back to baseline. CO2 tolerance also plays a role in breathing, a proven modulator of HRV levels

Breathwork is a profound method for improving your body’s stress response. The brain centers responsible for maintaining breathing rate and reactivity are some of the oldest systems in the body. Your breath is the first responder to stress. It’s a vital metric along with HRV to gain valuable insights into your physical and mental wellbeing. 

Tips for Improving CO2 Tolerance

Building up a tolerance to CO2 can help your body to resist stress more effectively and enable you to be more focused and alert as CO2 builds up in the body. Here are a few tips to improve your CO2 tolerance:

Nasal Breathing 

Next time you workout or try out some active rest, focus on breathing only through your nose.

Nasal breathing creates better use of our diaphragm and expends less CO2 when you breathe out. In addition, breathing through the nose only stimulates the release of nitric oxide (NO). Nitric Oxide is a vasodilator that enables increased blood flow and, as a result, improved healing and ability to perform. 

You can also try nasal breathing at all times and not just during your workouts.

The Physiological Sigh

The Physiological Sigh 

The physiological sigh is a proven technique that can activate the body’s natural stress reduction system, known as the parasympathetic nervous system. This technique can help you pull a natural “lever” to de-stress and relax your body. 

This basic breath technique is different than mindfulness breathwork – it is more targeted at directly triggering the mechanisms behind relaxation. To perform this technique, you must inhale twice and then let out an extended exhale. This breathing technique will send a signal to your parasympathetic nervous system that tells it to slow the heart down and trigger a restful response. 

When you exhale, your diaphragm moves down, creating more space for the heart. The double inhale also allows for greater gas exchange in the lungs, triggering the natural system in the body that works to de-stress your body. The double inhale is one of the only ways to calm down in real-time physiologically.

Endurance Training

Many people love to lift weights but dread the cardio workouts. Cardio workouts are an essential aspect of any complete workout routine. Cardio workouts help to improve your aerobic fitness and have a similar effect on improving your body’s tolerance to CO2. 

One recent study showed that ​​CO2 was beneficial for performance and muscle development during endurance exercise. Improved CO2 levels may enhance recovery from fatigue and support anabolic metabolism in skeletal muscles. In other words, the increased presence of CO2 resulted in a more pronounced recovery mechanism, demonstrating that both arms of the stress response lever are activated. 

Improve Your Nervous System Health Through CO2 Tolerance Training 

Contrary to what most people think, we do have control over our nervous system. We can choose to engage in actions that train us to be more resilient and able to handle stress in a more balanced, healthy manner. 

CO2 tolerance is yet another technique that allows us to purposefully train our bodies to go deeper into the stress response without becoming overwhelmed. As you train your CO2 tolerance, you’ll be able to go deeper into your training and notice an improved ability to handle stress without becoming overwhelmed. Keep track of your HRV using a Hanu wearable while strengthening your CO2 tolerance to see how your HRV scores improve.

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