Heat Training

Sound familiar? You’re scrolling through your feed and every week there’s a new "Magic Bullet." Yesterday it was polarized training and ketones; today we’re all riding in Zone 2, measuring our blood glucose in real-time, and spooning down bicarbonate.

As exhausting as these hypes can be, what’s fascinating is that there is almost always some truth to them. None of these trends emerge in a vacuum; there is almost always a valid physiological core worth looking at. The art lies in understanding this core and contextualizing it correctly.

This is exactly where I see my role in coaching: building scientific expertise on a topic, sorting through the knowledge, and communicating it to the athlete in a way that allows us to implement the useful building blocks while discarding the nonsense.

A current topic that is ubiquitous right now: Heat Training.

The promises sound tempting: Simply wrap up warm, spin easily on the trainer for an hour, sweat, and reach a new level of performance.

Let's look at what science says, which physiological mechanisms this trend is based on, and how sensible it is to integrate heat training into your own routine.

The Physiological Turbo: Why Heat Works

Heat training has long been established in professional sports ahead of hot races like the Ironman Hawaii or the Cape Epic. However, due to climate change and the globalization of sport, it is affecting more and more major events in our latitudes, such as the Olympic Games in Paris.

In those cases, the focus is purely on getting used to the heat, with the goal of being able to deliver maximum performance even at high temperatures. However, research is showing increasing evidence that heat training can also lead to performance improvements in cool conditions. In English, a distinction is made between two terms:

Acclimation (Heat Tolerance): This is about habituation—being able to deliver roughly the same performance under hot conditions as under cool ones. For this adaptation, the density of the stimulus can be lower. It takes about 12–15 heat sessions to achieve sufficient adaptation. This can be well integrated into a plan with, for example, 3 sessions per week.
Acclimatization (Structural Adaptation): However, if we want the long-term, structural increase in hemoglobin mass to be faster even in the cold, a significantly higher stimulus density is required. Studies usually cite 5 x 50 minutes per week over a period of 3–5 weeks.

The underlying physiological mechanism is fascinating and two-staged:

The Short-Term Plasma Boost: One of the body's most important reactions to heat stress is the expansion of blood plasma. More plasma means more blood volume. The blood becomes "more fluid," leading to better thermoregulation (sweating) and a higher stroke volume of the heart.
The Long-Term Hemoglobin Boost: A higher plasma volume mathematically leads to a lower hematocrit (the solid portion of the blood). Since our body always strives for homeostasis (balance), the kidneys register this "diluted" state and, given sufficient stimulus density, stimulate the production of erythrocytes (red blood cells) via EPO.

The Result: We have more red blood cells available for oxygen transport. Therefore, heat training is often referred to as "Poor Man’s Altitude Training," as the physiological adaptations in the blood are similar to those of altitude training.

"Heat acclimation improves aerobic exercise performance in temperate-cool conditions and provide the scientific basis for employing heat acclimation to augment physical training programs." — Lorenzo et al. (2010)

In this groundbreaking 2010 study, cyclists improved their VO2max by 5% and their time trial performance by 6% after 10 days of heat training—even at a cool 13°C (55°F). While Lorenzo (2010) laid the foundation, the most exciting current data—as is often the case—comes from Norway, but also from Switzerland, particularly from the research group around Bent Rønnestad at the Inland Norway University of Applied Sciences and Carsten Lundby. They are intensively investigating how to integrate the effects of heat training into everyday training.

A frequently cited study is Rønnestad et al. (2020/2021) on the topic of "Heat Suit Training":

The Protocol: Elite cyclists completed 5 sessions per week over 5 weeks. Each session consisted of 50 minutes of training in a "heat suit," wearing winter clothing or rain jackets at low intensity, while a control group did the same training in normal clothing.
The Result: The heat group showed a significant increase in hemoglobin mass of 4% compared to the control group. In addition, physiological lactate thresholds and VO2max improved more significantly.

Further studies in recent years—Cubel et al. (2024), Lundby et al. (2023), Oberholzer et al. (2019), and Mikkelsen et al. (2019), Rønnestad et al. (2022b & c)—confirm the results that heat training leads to increased hemoglobin mass.

The Hobby Athlete's Dilemma: The 10-Hour Calculation

The scientific findings regarding the increase in plasma volume and hemoglobin mass are promising. But now, let's leave the lab and enter your living room or your boiler room.

We need to look at the training reality of an average amateur athlete who trains between 6 and 12 hours per week. This is where the biggest challenge of heat training lies: Transferability to our everyday lives.

An average amateur triathlete trains around 8 to 12 hours per week. A pure cyclist or runner sometimes less. If an athlete wants the benefit of a long-term structural adaptation of hemoglobin mass and applies the protocol mentioned above from the studies—i.e., 5 x 50 minutes per week—they have to invest nearly 4 hours.

That is 30 to 50% of the total weekly training time flowing into heat training. If they train less, the percentage is even higher. So, let's assume that half of the training time during a heat block is spent in the low-intensity zone. At the same time, the training is physiologically and psychologically very demanding. Due to the high overall load, it becomes more difficult to set important intensive stimuli at the threshold or above. Training during a heat block will therefore be lower in intensity, and the opportunity to integrate high intensities is limited.

Is It Worth It?

The results of the mentioned studies show increases in hemoglobin mass of 2–4%, which corresponds to about 20–40 grams for an adult. According to Schmidt & Prommer (2010), one can expect an increase in VO2max of approx. 4 ml/min for every 1 g of additional hemoglobin mass. A 2-4% increase would therefore correspond to an increase of 60–120 ml/min.

For elite athletes operating at their physiological limit, an improvement of this magnitude is definitely relevant and can decide between victory and defeat.

How much is 60-120ml/min for an amateur athlete?

From my own experience, I can say that a five-week, focused training block—for example with two high-intensity sessions per week—often achieves similar, if not greater, improvements in VO2max. This, of course, depends on the athlete's training status and history. However, there is one difference regardless of the athlete: Through higher intensity, you train muscular resilience at high power outputs or running speeds and are thus more specifically prepared for competition demands than after five weeks of easy riding in the heat.

Another factor is the total load of a heat block: 50 minutes in the heat or 30 minutes following an interval session is not an "easy add-on" to training. Physiologically, the stress on the autonomic nervous system is enormous. Usually, you see this through a rising heart rate at constant or decreasing power (Cardiac Drift). Additionally, carbohydrate consumption increases significantly under heat conditions (Febbraio, 2001). The body is heavily challenged. And whenever the body is challenged to the max, the mental component is also a relevant factor. Heat training implies maximum psychological and physical stress, which requires corresponding recovery and the previously mentioned reduction in load elsewhere.

Weighing Risks and Opportunities

To integrate heat training sensibly into a training plan, you shouldn't blindly follow a trend. You must read up on it well beforehand and do an honest cost-benefit analysis. Three factors are decisive here:

The Physiological Potential: How big is the performance benefit for you really? have you been training at a very high level for years and are bumping against your physiological limits with established training methods? Or might you profit more from a five-week, targeted VO2max training block?
The Mental Toll: What does the block do to you mentally? Do not underestimate the mental component. Are you really ready to "wrap yourself in plastic" five times a week for an hour on the trainer, sweat profusely, and suffer in your own juice? Heat training is monotonous and unpleasant.

Do the Basics Right: Consistency Beats Extremes

My philosophy as a coach is simple: Do 80% of things 100% right and you will be able to increase your performance enormously. The biggest lever in amateur endurance sports is not finding the last percent through heat training, but maintaining the continuity of the load over very long periods. Logging hours week after week, staying healthy, being able to complete the intensive key sessions, and keeping the joy in the sport—that is what leads to performance development.

Extreme protocols that come from science or professional sports are often the "icing on the cake" and open up new possibilities for performance enhancement in those specific areas. But to stay with the metaphor: You can't put icing on a cake that hasn't been baked yet.

Conclusion

Heat training works. The study situation is clear, and the physiological adaptations are measurable. If you want to prepare for a competition in the heat and need to deliver your maximum performance there, heat training is an indispensable tool.

For the normal athlete preparing for a highlight event under hot conditions, [a specific block before the] competition is sufficient. This is enough for solid heat habituation (Acclimation).

However, torturing yourself in the boiler room for weeks in the winter to produce artificial puddles of sweat is rarely worth it. In this phase, a classic VO2max block or consistent base training usually makes much more sense. The price for the physiological "blood boost"—measured in time, stress, and mental energy—is often too high for someone with a 40-hour job and family and bears no relation to the benefit.

Before you lock yourself in the boiler room, ask yourself the following questions:

Are you doing the basics right?
Are you sleeping enough?
Are you paying attention to your nutrition?
Have you already been training for years at a high level, adapted to your physiology and your competition demands?

If you can clearly answer these questions with YES, then heat training with the goal of increasing your hemoglobin mass is certainly a tool you can try to squeeze out that last percent of performance.

If not, then leave the heater off. Focus on the basics first—training, nutrition, and sleep—and use heat training specifically in the last weeks before a hot race for acclimatization.

References

Febbraio, M. A. (2001). Alterations in energy metabolism during exercise and heat stress. Sports Medicine, 31(1), 47–59.
Lorenzo, S., Halliwill, J. R., Sawka, M. N., & Minson, C. T. (2010). Heat acclimation improves exercise performance. Journal of Applied Physiology, 109(4), 1140–1147.
Lundby, C., & Robach, P. (2015). Performance enhancement: What are the physiological limits? Physiology, 30(4), 282–292.
Lundby, C. et al. (2025). Mechanisms of haemoglobin mass expansion following heat stress. Journal of Physiology. (Published online ahead of print).
Oberholzer, L. et al. (2019). Hematological and performance adaptations to heat acclimation in endurance-trained males. Frontiers in Physiology, 10, 1379.
Rønnestad, B. R. et al. (2020/2021). Heat suit training increases hemoglobin mass in elite cross-country skiers. Medicine & Science in Sports & Exercise.
Schmidt, W., & Prommer, N. (2010). Impact of alterations in total hemoglobin mass on VO2max. Exercise and Sport Sciences Reviews, 38(2), 68–75.
Verdel, N. et al. (2021). Reliability and validity of the CORE sensor to monitor core body temperature during cycling. Sensors, 21(17), 5932.