I've read a few things about CO2 tolerance, the Bohr effect etc - this link has an overview of it but there are many similar sources including some scientific papers (although not necessarily focused on training athletes). https://remedypt.com/co2-tolerance-for-runners/

Is this something competitive cyclists should concern themselves with? Indeed, does this feature in any pro cyclists' training and coaching programme currently?

  • Is this just the opposite of ones max blood oxygen level?
    – Criggie
    Dec 28, 2021 at 23:29
  • 4
    I’ve never heard of this, or seen it referred to in exercise physiology papers. It does seem to be a legit concern for divers, but the linked web page sounds like snake oil.
    – Adam Rice
    Dec 29, 2021 at 0:16
  • @AdamRice I've heard of this in context of the legendary Czech distance runner Emil Zátopek [1922-2000]. Even then. His experiements in hypoventilation training were both about low O2 and high CO2.
    – Kaz
    Dec 29, 2021 at 22:43

2 Answers 2


At this time, this sounds like unvalidated pseudo-science.

I wouldn't take the article that you cite at face value. The website is primarily an advertisement for the author's private practice. The sources cited in the article aren't related to exercise, aren't primary research, or are neither. The author isn't a leader in the field, a respected researcher, or anything of the sort. She's a physical therapist whose primary client is the athletic department of a very small high school.

While there are, indeed, scholarly articles about CO2 tolerance, I was not able to find any that applied to athletes in general, much less cyclists in particular. Connecting the scholarly articles that I did find (which revolved around medical conditions) to athletics or cycling requires leaps of reason that aren't supported by the existing research.

A quick Googling of "CO2 tolerance atheletes" and "CO2 tolerance cycling" turned up articles similar to the one you cite, but nothing that I would consider authoritative. In fact, the article you cited was at the top of the first list and near the top of the second. The most relevant article I saw was from Bike Radar. However, that article didn't cite any external sources and contained enough links to specific products that I suspect its purpose is to generate advertising revenue rather than to inform readers. All of this indicates to me that this isn't currently a widely accepted training concern within the cycling community.

With hundreds of professional cyclists in the world, I wouldn't rule out the possibility that one or two has been swayed by one of these articles and has started to apply these ideas to his or her training. But that hypothetical professional cyclist would appear to be the exception, not the rule.

  • 1
    Thanks for looking for other papers, I did have some but lost the references. Happy that this is something I can ignore. :)
    – Wilskt
    Dec 29, 2021 at 8:57

Unless you specialize in cycling events where you hold your breath the entire race, I am beyond skeptical of some of the claims in that article.

Much of the claims about CO2 in breath control are true, but then the claims about the benefits of CO2 go off into the weeds. Yes, CO2 in the bloodstream is what triggers the sensation of needing to breath, rather than depletion of available oxygen. But that doesn't mean that being able to better tolerate higher CO2 levels will make you able to comfortably operate at lower blood oxygen levels, which could be problematic if you are pushing yourself too hard.

One of the most specious claims is "CO2 makes your muscles more efficient" and is due to a misunderstanding of the Bohr effect. The Bohr effect is basically that oxygen carrying capacity of hemoglobin reduces when the acidity increases, due to CO2, which causes the hemoglobin to 'deliver' the oxygen, as the article sort of claims. However, where they go wrong is understanding what is driving the relationship. The source of that acidity is the metabolic byproducts of the recipient muscle cells. It won't, as the article implies, be more capable of 'driving it into the cells'. It facilitates the release of O2, and the muscle cells that need it will have no shortage of the required CO2 to trigger the release.

An increase in tolerance will mean a higher blood CO2 levels, meaning more acidic blood, meaning less Oxygen carrying capacity overall, meaning less delivered to the body.

A situation where high tolerance, or other similar effects can be problematic is in something called shallow water blackout. What happens there is a diver often hyperventilates, which reduces their blood CO2 level to almost nothing. Normally, there is always some baseline level in a persons blood. They then hold their breath, and dive. Because feeling the need to breathe is controlled by CO2 levels in the blood, the diver can be comfortable and not feel the need to breathe, despite their blood O2 levels dropping. What then happens is their blood oxygen can get so low, that without warning they can just anticlimactically pass out without ever knowing how oxygen starved they were, and and in many cases drown as a result.

If you were able to mind over matter yourself enough to power through the need to breathe, you may be operating your body at oxygen levels below healthy, and can cause yourself a variety of problems.

Now, with all that said, if you are a track sprinter, you'll might have some benefit, as sprinting is an anaerobic effort, and a higher tolerance means you might be able to dig just a little bit deeper, push those watts for an extra half second which might mean the difference between victory and defeat. Frankly, a full gas sprint is not a whole lot different than an effort while holding your breath as long as possible. However, the article is basically alluding to breath control and pacing, steady state stuff, which is not where any of that comes into play.

  • Thanks for the additional context, some interesting stuff there.
    – Wilskt
    Dec 29, 2021 at 8:58
  • Especially the action of oxygen transfer to muscles - I started looking at this when someone mentioned preserving 'precious CO2' by calming the breathing, I too thought that there would never be a shortage of CO2 in the bloodstream!
    – Wilskt
    Dec 29, 2021 at 9:23

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