Heat adaptation – the process of repeated heat exposures to improve performance in the heat – is well established as the best overall way to optimize performance in hot conditions, as shown by our own Dr. Cheung in his research. Benefits can be seen as short as 1-2 weeks.
Most athletes are familiar with the obvious benefit: improved performance and lower perceived effort when racing or training in the heat. Many of these improvements are driven by increases in blood plasma volume, which allows the cardiovascular system to work more efficiently.
Heat training has become (pun intended) a hot topic in endurance sport over the past few years. Here, the goal isn’t just to improve performance in hot conditions, but to elicit changes that can help performance even in temperate conditions. The main way this has been proposed to work is through an increase in hemoglobin – the molecule in our red blood cells that carry oxygen. This takes much more time than found with heat adaptation.
If proven true, this could mean that athletes who are unable to spend three weeks atop a volcano training at altitude, including amateurs like most Pez readers, could still gain some of the same benefits.
This idea gained attention following work by Bent Rønnestad and colleagues. In a 5 week intervention, endurance-trained athletes completed five one-hour heat sessions per week and experienced meaningful increases in both plasma volume and total hemoglobin mass. The body of evidence on this topic is still relatively small, the relationship between heat training and increases in hemoglobin mass is still emerging, and the additional training load, fatigue, laundry, and logistical headache of regular heat sessions are all worth considering before undertaking this intervention. However…
After seeing all the excitement surrounding this study, I kept wondering: Can the average athlete measure heat adaptation or heat training changes?
Performance changes are relatively easy to monitor. You could track heart rate relative to power or pace during a standardized ride and watch how that relationship changes over time. The same goes for RPE. But what about the physiological adaptations happening beneath the surface? Can you measure changes in plasma volume? Is there any practical way to monitor hemoglobin adaptations?
Measuring Hemoglobin Isn’t That Simple
The first roadblock I ran into while investigating this was that the gold-standard measurement used in research isn’t something most of us can access.
Rønnestad and colleagues measured total hemoglobin mass using carbon monoxide rebreathing. During this test, an athlete breathes a very small, carefully controlled amount of carbon monoxide mixed with oxygen for a few minutes. Because carbon monoxide binds almost exclusively to hemoglobin, researchers can calculate the total amount of hemoglobin in the body based on how much gas is absorbed.
This is not a standard test at your doctor’s office, and it requires specialized equipment and trained personnel found only in a few sport science labs, making it unrealistic for most of us.
The way many of us have seen hemoglobin measurements reported is through a complete blood count (CBC), often performed as part of routine blood work. The hemoglobin value reported on a CBC measures hemoglobin concentration, not total hemoglobin mass. As plasma volume expands during heat adaptation, that concentration often decreases because the blood becomes diluted, this is known as hemodilution. This can actually make your red blood cells and hemoglobin on a CBC test drop, even though the cells haven’t decreased; they’re simply in a larger volume of plasma.
This means that measuring hemoglobin concentration alone is not particularly helpful for determining whether total hemoglobin mass has changed. However, we can estimate changes in plasma volume, which is one of the better-established adaptations to heat training.
The Dill & Costill Equation
The Dill & Costill equation estimates changes in plasma volume using just two values from a standard blood test:
- Hemoglobin concentration (Hb)
- Hematocrit (Hct). This is the ratio of solid (mostly red blood cells) to liquid (plasma) in the blood (remember the late ‘90s and early ‘00s when Hct was used as a crude proxy for blood doping?)
By comparing values before and after a heat intervention, the equation estimates the percentage change in plasma volume. This is not a direct measurement of absolute plasma volume, just an indirect estimate of before/after changes, but it provides a practical estimate based on changes in blood concentration.
For most of us, this is probably the easiest field method we have available. This is also how I decided to measure my own progress when I started heat training.
My Heat Training Experiment
I completed a five-week heat training block consisting of:
- 20 total sessions
- Approximately 16 hours of total heat exposure
- Core temperature targeted at ~101°F using an in-ear thermometer
- Room-temperature water allowed during sessions
- Exercise performed at roughly 50% VO₂max power
Most sessions combined active and passive heat exposure.
Active sessions consisted of riding indoors while wearing a paint suit and winter clothing and using a small space heater for 25–40 minutes. I generally started around 180 watts before gradually increasing to 200–220 watts. Once my heart rate reached roughly “tempo” intensity or my ear temperature approached 101°F, I reduced power and focused on maintaining temperature rather than workload.
Passive sessions involved spending 15–20 minutes in a portable sauna after riding.
One important caveat: my protocol was less demanding than Rønnestad’s. Their participants accumulated approximately 28 hours of heat exposure while exercising at closer to 60% VO₂max. My intervention was similar in design but lower in both total heat dose and exercise intensity.
My Results vs. the Research
In Rønnestad’s study:
- Plasma volume increased 7.6% in the heat group versus 5.3% in the control group.
My results:
- ~4.7% increase in estimated plasma volume
Final Thoughts
The question I wanted to answer wasn’t whether heat training works. The question was whether I could detect meaningful physiological adaptations using tools that are actually available to everyday athletes, or simply put “how much does heat training work for me.”
By estimating plasma volume changes from routine CBC, I hoped to see whether my response resembled the adaptations observed in research, and whether those changes might hint at the potential for longer-term increases in hemoglobin mass.
While I ultimately can’t measure total hemoglobin mass, I felt like stumbling across the Dill & Costill equation as a way to estimate plasma volume changes felt like a win. It’s also worth noting that plasma volume increased in the control group in Rønnestad’s study, but without the change in Hb mass. That suggests that there’s more to heat adaptation than plasma volume expansion alone.
For future experiments, I’ll definitely include standardized performance tests alongside RPE tracking. I think this experiment offered some useful insight and a practical way to move beyond simply feeling fitter and start measuring whether heat training was actually changing my physiology.
FAQ SECTION
Does heat training improve cycling performance?
Yes. Heat adaptation increases plasma volume, reduces cardiovascular strain, lowers perceived effort and improves performance in hot conditions.
How long does heat adaptation take?
Most athletes begin seeing measurable improvements after 7–14 days, with larger physiological adaptations developing over several weeks.
Can you measure heat adaptation?
While direct measurement requires specialized laboratory testing, routine blood work combined with the Dill & Costill equation can estimate changes in plasma volume.
What is plasma volume?
Plasma volume is the liquid portion of blood. Heat training typically increases plasma volume, improving cardiovascular efficiency and helping athletes tolerate heat more effectively.
Can a CBC measure total hemoglobin mass?
No. A complete blood count measures hemoglobin concentration, not total hemoglobin mass, which is why plasma volume changes can make hemoglobin values appear lower despite improved adaptation.
References
Tyler, C. J., Reeve, T., Sieh, N., & Cheung, S. S. Effect of heat adaptation on physiology, perception, and exercise performance in the heat: an updated meta-analysis., Journal of Science in Sport & Exercise, 6(3), 195-217. https://0.1007/s42978-023-00263-8
Rønnestad, B. R., Hamarsland, H., Hansen, J., Holen, E., Montero, D., Whist, J. E., & Lundby, C. (2021). Five weeks of heat training increases haemoglobin mass in elite cyclists. Experimental physiology, 106(1), 316–327. https://doi.org/10.1113/EP088544
Dill, D. B., & Costill, D. L. (1974). Calculation of Percentage in Changes in Volumes of Blood, Plasma, and Red Cells in Dehydration. Journal of Applied Physiology, 37, 247-248. https://doi.org/10.1152/jappl.1974.37.2.24 7
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