Heat Training for Improved Oxygen Uptake

Summary: In 2019, an NRK article was circulated suggesting that heat training might offer similar benefits to altitude training. The main question is whether easy cycling in a warm room, several times a week, can actually improve oxygen transport in a way detectable in tests. In this text, we explore the hypothesis, the setup, and the results from the study, which was later published more comprehensively. The conclusion is that heat training appears promising as a practical tool, but the effects should be interpreted cautiously, and more research is needed before equating the method with classic altitude camps.

The Study

In 2019, an exciting article from NRK began to spread on various endurance sports websites and forums, "Unique Research: Training in Heat Gives You Better Endurance".

With the renowned sports researcher Bernt Rønnestad at the helm, research on the effects of heat training found results similar to those from altitude training. We then wrote this article. However, the complete study hadn't been released when we recorded that episode, so we couldn't dive deep into the study. Now we can present a more comprehensive picture of the results and whether you should incorporate this into your training.

5 times/week for 4 weeks, then 4 times in the final week

The study's hypothesis is that light training at 38–40 degrees for 1 hour a day (5 times/week, see above) over 5 weeks can yield similar results to altitude training by raising the hemoglobin level (improved oxygen transport). If you want to read more about the pros and cons of altitude training, you can read it here.

Why Hemoglobin is Interesting

For a well-functioning and high-performing system for oxygen uptake, a high hemoglobin value (Hb value) is required. Hemoglobin is the protein that carries oxygen molecules to our working muscles. An increased Hb value means more red blood cells, leading to improved maximal oxygen uptake.

The Hb value is somewhat individual, but males have a reference range of 138–180 g/L, while women should be around 121–151 g/L. During training at high altitudes, the Hb value rises as a response to lower partial pressure of oxygen, and this is fundamentally positive. However, long-term high-altitude stays have shown decreases in total plasma volume, which is not beneficial from a performance perspective. Athletes try various ways to counteract this—and here, heat training becomes more interesting.

Doping: 1–1.3 liters of blood provided a 4–9% increase in oxygen uptake. 150 g HB versus 42 g.

During heat training, it has been observed that plasma volume can increase up to 20 percent after just a few days, then stabilize around a 10 percent increase. Researchers hypothesized that heat training over 5 weeks could lead to increased plasma volume, leading to increased EPO production and thus a raised Hb value. Erythropoietin (EPO) is the hormone that signals the body to create new hemoglobin and new blood cells. The body produces EPO in response to conditions like oxygen deprivation in the blood and kidneys. This has been used both as a medicine and as doping in synthetically produced preparations.

"The mechanisms for plasma volume reduction during altitude acclimatization remain unresolved. Dehydration from drinking insufficient fluid to offset urinary, respiratory, and transcutaneous fluid losses, which all increase with hypoxia, may contribute (75). However, preventing dehydration does not prevent the plasma volume reduction in lowlanders at altitude (146). Recently, the magnitude of plasma volume reduction in acclimatizing lowlanders was shown to be closely related to the amount of circulating protein concomitantly lost from the plasma (146). This suggests an oncotically mediated hemoconcentration, but how the proteins are leaving the vascular space is unclear" – https://pubmed.ncbi.nlm.nih.gov/10694114/

Summary: Training at high altitudes increases the amount of hemoglobin carrying oxygen, but it decreases the total blood volume (plasma). If you raise Hb from 100 to 150 g per liter of blood while simultaneously lowering plasma volume, the total effective value may be less than if the plasma volume hadn't decreased.

Training in heat increases plasma volume, which signals the need for more hemoglobin (the body wants to maintain a specific Hb concentration per liter of blood). Therefore, the hormone EPO, which promotes hemoglobin production, increases. In this case, the increased plasma volume can trigger an increase in hemoglobin production. At high altitudes, it's the lack of oxygen that triggers it. Both methods can ultimately have the same effect, and this is the hypothesis the researchers aimed to test.

Method and Test Setup

To test their hypothesis, the researchers had 23 elite cyclists (VO2max = 76.2 ± 7.6 mL·min‑1·kg‑1) perform light training in about 38 degrees Celsius for 50 minutes a day over 5 weeks.

Participants were divided into two equivalent groups based on VO2max. The heat group (Heat; n = 11, age = 19 ± 2 years, height = 178 ± 8 cm, weight = 68.6 ± 6.9 kg) and a control group (CON; n = 12, age = 19 ± 3 years, height = 179 ± 5 cm, weight = 70.8 ± 5.6 kg). Participants underwent several tests before and after the 5-week period, including incremental cycling tests examining cycling economy, power, oxygen uptake, and blood lactate. VO2max was also tested, and a maximum test was conducted starting with 30 minutes of cycling at 2 mmol lactate (threshold for conversation pace) followed by a 15-minute performance test where participants could adjust the resistance to achieve maximum average power. The entire testing period took place during the cyclists' base training.

The actual heat training was conducted on a trainer in a room kept at 37.5–38.5 degrees Celsius, with humidity at 64–66 percent. Sessions were held in the afternoon and lasted 50 minutes. Intensity was 45 percent of OBLA (4 mmol·L‑1). Resistance was adjusted for the upcoming session if perceived exertion wasn't between 11 and 15 on the BORG scale. The control group followed the same setup without the heat element.

During the test period, neither training volume nor intensity differed between Heat and Control:

• Zone 1, < 55% of FTP: 6.52 ± 1.75 vs. 7.90 ± 2.52 hours
• Zone 2, 55–75% of FTP: 1.22 ± 1.05 vs. 1.47 ± 1.07 hours
• Zone 3, 76–105% of FTP: 1.70 ± 0.18 vs. 1.88 ± 1.55 hours
• HIT (Zone 4, 106–120% of FTP): 0.23 ± 0.27 vs. 0.10 ± 0.10 hours

The number of strength sessions didn't differ either (0.7 ± 0.9 vs. 0.8 ± 0.8 sessions). However, there were significant variations even within the groups.

Hb volume and blood volume were measured on four occasions: two before the test period and two afterward. Post-test period measurements were conducted 1–2 days and 3–4 days later, respectively.

Results

The most significant finding after the 5-week test period was that Hb volume increased by 4.6% in the heat group, while it remained unchanged in the control group. Red blood cells and total blood volume remained unchanged for both groups throughout the test period.

From the physiological tests, it was observed that both groups increased their VO2max: the heat group by 4.6 ± 5.6% and the control group by 3.2 ± 3.9% (p = 0.002).

Wmax showed no change in either group. However, the heat group increased their performance at 4 mmol·L‑1 [La‑] (p = 0.035), whereas the control group showed no increase (9.1 ± 12.4% vs. ‑0.4 ± 5.1%).

Both groups increased their average power output during the 15-minute time trial test, with a greater increase seen in the heat group, though not statistically significant (Heat group 6.9 ± 8.4% vs. Control group 3.4 ± 5.1%). In other performance parameters, the results were similar between the groups.

Discussion

It has long been studied that Hb volume increases with high altitude training, but seeing similar results from heat training is relatively new. Besides this study, similar results were observed in a Danish study from 2015, where Danish cyclists, who were not heat-acclimatized, trained outdoors in 36 degrees in Qatar and stayed in air-conditioned hotels during other times. After just 2 weeks, it was observed that plasma volume increased by 13.5% and Hb volume by 60 g. In the Norwegian study, the increase was 42 g for comparison.

A 42 g increase corresponds to 4.6%, which aligns with what has been seen in similar studies conducted at high altitude (train low – live high), but with the difference there that it involved untrained individuals. This makes the result even more interesting since this study was conducted on well-trained cyclists — and it's significantly harder to achieve performance gains in well-trained athletes compared to amateurs.

Regarding the results of the physiological tests, the increase in power output at 4 mmol·L‑1 is likely due to the increased Hb volume, which enhances the availability of oxygen to the working muscles. Previous studies have also shown that regular exposure to heat leads to improved mitochondrial function in skeletal muscles. The increased Hb volume likewise serves as a reasonable explanation for the improvement observed in the 15-minute time trial test. Average power output increased for both groups, but significantly more for the heat group, though it wasn't statistically significant. A 7% increase versus a 3% increase is nonetheless a noticeable difference in already well-trained elite cyclists.

Summary

This is the first study to demonstrate increased Hb volume from heat training in an indoor lab environment. This has led many coaches and athletes to begin testing and implementing it in their training. However, the researchers are cautious to note that more research is required to verify the effects, and if one plans to use this as preparation before important competitions, it should be tested in advance.

There will certainly be more research in this area, and it would be interesting to compare heat training against high altitude training in a specific study to evaluate the pros and cons of each option.

Read the full article here.