Coaches of cyclists, triathletes, and other endurance athletes are constantly trying new and improved methods to gain greater insight into their athletes’s training. These include assessments like continuous blood glucose monitoring and real-time blood lactate testing.
Recently, carbon monoxide rebreathing has come into the spotlight. And it brought some controversy along with it.
But what is carbon monoxide rebreathing, how is it used, and how does it benefit athletes? Let’s take a deeper dive into the facts and see if it deserves a bad rap.
The Long History and Recent History of Carbon Monoxide Rebreathing
During the second week of the 2024 Tour de France, Escape Collective released a story on carbon monoxide rebreathing in elite cycling. Writer Ronan Mc Laughlin revealed that at least three teams competing in the most prestigious Grand Tour were using it to test their athletes’ hemoglobin levels. Other outlets picked up the thread, leading to riders being grilled about the method in their press conferences.
Tadej Pogačar, then leading the race, initially said he didn’t know anything about it. Later clarifying that he misunderstood the question, he said, “It’s just a pretty simple test to see how you respond to altitude training. It’s not like we’re breathing exhaust pipes every day from cars.”
His archrival and eventual Tour winner Jonas Vingegaard confirmed that his team used carbon monoxide rebreathing as a diagnostic tool but said, “There’s nothing suspicious about it.”
However, the media went into overdrive, noting that this is a harmful gas and suggesting it could be abused to gain a performance advantage.
Carbon Monoxide Rebreathing Is Nothing New
With all the hype suddenly swirling around carbon monoxide rebreathing, you’d assume it’s a relatively recent phenomenon. But in fact, it has over 100 years of history.
Oxford University scientists John Haldane and Lorrain Smith first developed a way of using it to determine blood volume in a Journal of Physiology article published in 1900. Haldane later teamed up with his colleague, Claude Douglas, to study how altitude impacted hemoglobin levels during trials conducted in Tenerife and on Pikes Peak in Colorado.
Then, in the 1940s, Swedish scientists more accurately calculated blood volumes using carbon monoxide rebreathing and linked this to exercise capacity and cardiovascular function.
Two Austrian researchers then dialed in a simpler method in the 1990s, which was later modified by Nicole Prommer and Walter Schmidt. In 2005, Prommer and Schmidt found that breath-based results could be obtained just two minutes after testing. This led to more teams adopting the approach.
How Carbon Monoxide Rebreathing Works
To learn more about how carbon monoxide rebreathing is performed and applied, we reached out to Dr. David de Klerk. This South African doctor served on the medical team at UAE Emirates for three years and Alpecin-Deceuninck for the past year. He also contributed medical insight, coaching, biomechanics, and human performance expertise alongside colleague John Wakefield at Science to Sport.
He explained that carbon monoxide rebreathing is a way to measure blood volume and hemoglobin mass (Hbmass) in athletes (a topic he’s writing a thesis about).
Carbon monoxide rebreathing is sometimes performed with an automated machine, such as the Detalo model used by some pro cycling teams. You can also do it through a more manual process that uses a closed-circuit carbon monoxide rebreather system.
First, an athlete typically does a combination of breath and blood tests to measure their baseline carbon monoxide (CO) levels before the actual test is performed.
The test itself involves the athlete inhaling a small amount of carbon monoxide diluted with oxygen. This mixture is inhaled and exhaled through a closed circuit for two minutes. The carbon monoxide the athlete inhales then binds to the hemoglobin in red blood cells to form carboxyhemoglobin.
After two minutes, the amount of carboxyhemoglobin that remains in the breath and blood is re-measured and compared to the baseline levels measured at the start of the test. The results are then used to calculate the athlete’s total hemoglobin mass.
The Importance of Hemoglobin Mass
As hemoglobin is the main method of transporting oxygen around the body, a higher total hemoglobin mass is often correlated with a higher V̇O2 max.
Dr. de Klerk noted that V̇O2 max “also depends on other factors such as a well-functioning cardiovascular system, lots of blood vessels within the muscle, and abundant mitochondria to extract oxygen from the blood.”
That being said, in a healthy individual, hemoglobin mass is the most important determinant of their oxygen-carrying capacity. This makes it incredibly useful in evaluating how an athlete responds to training alone or combined with a stimulus like altitude or heat.
Assessing Altitude Adaptation
Dr. de Klerk shared that teams used to rely on a full blood count (a common blood test) to assess athletes’ adaptation to altitude training. These tests provided hemoglobin concentration and hematocrit levels, and they’re still widely used in conjunction with and (less accurately) in place of the carbon monoxide rebreathing method.
“But the problem is that if, for example, your blood volume goes up and your hemoglobin levels stay the same, your hematocrit comes down. This makes hemoglobin concentration and hematocrit poor predictors of whether an individual’s altitude camp was successful or not.”
Red blood cell count is a slightly better indicator, but it still underestimates the impact of altitude on blood volume. This is why carbon monoxide rebreathing is so useful.
“If you could take all the hemoglobin in your body and put it on the scale and weigh it, that would be the best way of doing it, and that is essentially what carbon monoxide rebreathing does,” he said. “The result is not affected by what your total blood volume is because you’re measuring the actual mass of the hemoglobin; it’s not relative to anything else.”
“If you could take all the hemoglobin in your body and put it on the scale and weigh it, that would be the best way of doing it, and that is essentially what carbon monoxide rebreathing does”
Dr. David de Klerk
Once the increase in hemoglobin mass–which Dr. de Klerk said is often estimated to be around 1.1 percent for every 100 hours spent at altitude but is highly individualized–has been calculated, performance staff can see who did or didn’t respond well to altitude training.
They then work backward and look at other biomarkers to find out why an individual may respond well to altitude training. As such, carbon monoxide rebreathing is a good place to start.
“It’s well known that if you don’t have enough iron, you can’t make more red blood cells, so we test for that,” Dr. de Klerk said. “We evaluate inflammation markers and indicators of illness too, as these inhibit the response to altitude. Injury is another factor that prevents adaptation. It’s also important to look at carbohydrate availability and if an individual is fueling correctly, along with the balance between training and rest.”
Uses Beyond Altitude Training
Assessing the impact of altitude training at the end of a camp is the main use case for carbon monoxide rebreathing in endurance sports. However, Dr. de Klerk suggested that extending the testing cycle could provide a broader view of the impact of high altitude.
He also noted that carbon monoxide rebreathing isn’t just for professional athletes. It’s also a useful diagnostic test “for competitive amateurs who have access to altitude, care about their results, and want to make changes.”
For athletes who respond well to altitude training, Dr. de Klerk said the results from their carbon monoxide rebreathing test and other physiological assessments could, for example, be compared to their power profile data in TrainingPeaks. This helps correlate whether a change in hemoglobin mass and other blood markers might impact values such as five- and 10-minute power output.
Dr. de Klerk believes that carbon monoxide rebreathing could also be applied to evaluating other training scenarios that don’t involve altitude exposure. “It would be useful to see how hemoglobin mass changes during the different training phases, for example,” he said.
He added that assessing hemoglobin mass with carbon monoxide rebreathing also has other areas of benefit, such as assessing the changes that occur in response to heat training.
“The use of heat training to raise hemoglobin mass has recently benefited from a lot of academic research,” Dr. de Klerk said. “Recent publications have shown that heat training can also induce changes in hemoglobin mass, similar to those that occur in response to altitude training.
“The same method of using carbon monoxide rebreathing to determine hemoglobin mass was applied in these studies, highlighting its benefit in the assessment of other training interventions.”
Responsible Testing Vs. Performance-Enhancing Inhalation
If carbon monoxide rebreathing is just a way to evaluate training response, what’s with all the controversy?
One of the main misconceptions about carbon monoxide rebreathing is that athletes inhale it for performance gains. This is a far riskier practice. It requires the athlete to frequently inhale the tracer to simulate hypoxia and prompt an increase in hemoglobin and other performance metrics.
While there is no evidence that cycling teams are doing this, it could be encouraged by the results of two studies.
The first was published in Frontiers in Physiology in 2019. It concluded that 12 soccer players who inhaled carbon monoxide before running on a treadmill five times a week for a month increased their hemoglobin mass and V̇O2 max by more than those who just did the endurance training.
A second study released two years later asked a group of amateur cyclists to inhale carbon monoxide five times a day. After three weeks, they increased their hemoglobin mass by 4.8 percent, with a corresponding rise in V̇O2 max.
“It’s disappointing to see some research suggesting that repeated carbon monoxide inhalation is effective at creating artificial hypoxia, which could theoretically have the same benefits as altitude training,” Dr. de Klerk said.
“In the context of abusing this method, it does potentially pose a risk to athletes, which would qualify as one of the three key criteria WADA uses to outlaw a practice or the use of a particular substance within competitive sport.”
“In the context of abusing this method, it does potentially pose a risk to athletes, which would qualify as one of the three key criteria WADA uses to outlaw a practice or the use of a particular substance within competitive sport.”
Dr. David De Klerk
It’s doubtful that teams in cycling, triathlon, or any other sports are using carbon monoxide inhalation in this way. But as long as the opportunity to gain an edge exists, there is the potential to exploit it to gain an unfair advantage.
Preventing Abuse and Creating Ethical Boundaries
This is partly because there are no rules from governing bodies and anti-doping agencies about carbon monoxide inhalation. But practitioners can go a long way toward preventing misuse and safeguarding the athletes in their care.
“Separation of the medical and performance teams helps protect athletes from potential harm,” Dr. de Klerk said. “If medical staff are needed to approve, conduct, and supervise all testing and the performance team focuses on interpreting and applying the results, it’s more likely that teams will function ethically and only use carbon monoxide rebreathing in a responsible manner.”
Currently, responsible practice means purposeful and infrequent testing to evaluate how athletes’ hemoglobin mass responds to training and preparation.
When the intention is simply to get measurements, there is little difference to evaluating blood glucose, lactate, or any other marker.
“In terms of scientific accuracy, carbon monoxide rebreathing is the most precise way of assessing hemoglobin mass and an effective method for determining if an altitude camp was successful,” Dr. de Klerk said.
“I wouldn’t want this legitimate diagnostic practice to be cast aside because of a fear that it was being misused. We need to draw a clear distinction between safe testing that offers no performance advantage and far more frequent inhalation that aims to create some kind of artificial improvement.”
Resources
Detalo Health. What is the CO Rebreathing Method? Retrieved from https://detalo-health.com/co_rebreathing_method/
Haldane, J., & Smith, J. (1900, August 29). The mass and oxygen capacity of the blood in man. Retrieved from https://pubmed.ncbi.nlm.nih.gov/16992538/
McLaughlin, R. (2024, July 7). Exclusive: Tour riders are inhaling carbon monoxide to optimise altitude training. Retrieved from https://escapecollective.com/exclusive-tour-riders-are-inhaling-carbon-monoxide-in-super-altitude-recipe/
Rønnestad, B., et al. (2021, January). Five weeks of heat training increases haemoglobin mass in elite cyclists. Retrieved from https://pubmed.ncbi.nlm.nih.gov/32436633/
Saunders, P., et al. (2013, December). Relationship between changes in haemoglobin mass and maximal oxygen uptake after hypoxic exposure. Retrieved from https://pubmed.ncbi.nlm.nih.gov/24282203/
Schmidt, W., & Prommer, N. (2005, October 13). The optimised CO-rebreathing method: a new tool to determine total haemoglobin mass routinely. Retrieved from https://link.springer.com/article/10.1007/s00421-005-0050-3
Schmidt, W., et al. (2020, September). Chronic Exposure to Low-Dose Carbon Monoxide Alters Hemoglobin Mass and V˙O2max. Retrieved from https://journals.lww.com/acsm-msse/fulltext/2020/09000/chronic_exposure_to_low_dose_carbon_monoxide.4.asp
Garvican-Lewis, L., et al. (2015, June). Altitude Exposure at 1800 m Increases Haemoglobin Mass in Distance Runners. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4424472/
Wang, J., et al. (2019, June 6). A New Method to Improve Running Economy and Maximal Aerobic Power in Athletes: Endurance Training With Periodic Carbon Monoxide Inhalation. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6562501/
