Comparing Heat and Altitude Training – Similarities and Differences in Adaptations
Editor’s note: CORE often receives the questions, 'Can heat training prepare me to race at altitude?‘ and 'Can heat training ease my transition to altitude training?‘ We asked Lukas Scherl, a sports scientist who has studied elite athletes at altitude, to answer these questions.
Benefits of altitude and heat training
It has been known for decades that altitude training can provide benefits for competing in endurance sports, not only at altitude, but also at sea level.. One of the main physiological adaptations is an increase in total hemoglobin mass (Hbmass) and therefore an increase in maximum oxygen uptake (VO2max). However, going to altitude for several weeks just to train comes with a significant logistical and financial effort, which makes its use almost exclusive to elite athletes.
In the last few years, there has been a growing body of research showing that heat training can also provide a variety of beneficial adaptations, not only when competing in the heat but also in moderate climate conditions. These include an increase in plasma and total blood volume, an increased sweat rate, and – like seen in altitude training – also an increase in hemoglobin mass. At the same time, it is much cheaper and relatively easy to perform wherever you want.
Can heat training replace altitude training?
This growing understanding of heat training yields the question, 'Could heat training potentially be an alternative to altitude training, or even replace it for some athletes?‘ To answer this, we need to compare the physiological mechanisms and adaptations, as well as their respective implications for endurance performance.
When going to altitude, the body senses the reduced oxygen availability and responds with an increase in EPO expression, which then leads to the production of red blood cells and hemoglobin (Hb), the oxygen transport molecules in our blood. The literature suggests that a 3–4-week altitude training camp leads to an average increase in Hbmass of 3-7 %, or a mean increase of 1.1 % per 100 hours spent at altitude. Depending on the baseline Hbmass of each athlete, this corresponds to a total increase of approximately 20-70 g.
Because Hbmass directly influences the maximum oxygen uptake, 1 g of additional Hb means an increase in absolute VO2max of 4 ml. Furthermore, altitude training has been shown to improve ventilatory capacity, capillary density, total number and efficiency of mitochondria, as well as the buffer capacity in the muscle. Several studies also showed improved performance at sea level after altitude training, but the literature here shows conflicting results, and physiological changes don`t always directly result in better performance.
Becoming a hot topic both in elite endurance sports practice and training science over the last years, heat training has been shown to result in several physiological adaptations. Similar to (but maybe slightly less) than in altitude training, research shows that 3–5 weeks of regular heat training results in an average increase in Hbmass of 2-4.5 % Furthermore, it is associated with increased plasma, total blood volume, sweat rate, and heat tolerance; better thermoregulation and skin blood circulation; as well as a decrease in core temperature and heart rate during exercise in both hot and moderate climate conditions. Additionally, several studies showed improvements in VO2max, power output at lactate threshold and time trial performance after heat training.
As you can see, certain mechanisms, especially some of the hematological changes, overlap in both training mechanisms. That is why some may call heat training "the poor man`s altitude,“ as it is much easier to perform and comes with a fraction of the costs and logistical challenges of altitude training. But neither of them can serve as a replacement for the other, as both induce their own unique adaptions and are performed for several reasons. When we look at the world`s best endurance athletes, we see both altitude and heat training performed regularly and not one of them exclusively.
Heat training to prepare for competition at altitude
A question often asked is: "Can I do heat training in order to prepare and perform better in competitions at altitude?“ A study by White et al. (2016) found no difference in VO2max at altitude after heat training, but slightly better performance in a 16 km time trial of 28s on average (p = 0.07). Another small benefit of heat training for performance under hypoxia could be the elevated plasma volume and therefore decreased blood viscosity, as well as improved cardiac efficiency, both having positive effects on endurance performance.
Heat training to prepare for training at altitude
A systematic review by Willmott and colleagues (2024) looked at possible cross adaptation from heat stress to hypoxia. Their results suggest that heat training can have several effects which could help preparing for training and competing in hypoxic conditions: higher oxygen saturation and lower heart rate during rest and submaximal exercise, as well as small effects on ventilation, peak power and time trial performance during maximal intensity.
This is in line with experiences from coaches and athletes, who report that they use heat training in the weeks leading up to altitude training camps in order to shorten the acclimation period when traveling to altitude. In the first days of altitude training, athletes typically don`t feel great and their ability to train is limited. Therefore, making the transition as smooth as possible could lead to a higher overall quality of the altitude training camp.
To potentially optimize the quality of training and performance under hypoxia, a protocol consisting of both heat and altitude acclimations could look as follows:
- When still at home, perform 3-5 heat sessions per week for several weeks (>= 5 weeks are recommended) in order to increase total blood and plasma volume as well as hemoglobin mass. This can lead to the cross-adaption effects described above and make the transition from sea level to altitude smoother.
- Altitude acclimation is still highly recommended, because some physiological mechanisms are mainly triggered through hypoxia, e.g. changes in ventilation, buffer capacity, capillary and mitochondrial density.
- After acclimation to hypoxia, athletes are now optimally prepared to compete and perform at altitude.
Using heat training to maintain gains made at altitude
Another valuable use case of heat training, shown in a recent study by Rønnestad and colleagues (2024), is the ability to maintain the hematological adaptions of altitude training after returning to sea level. The authors found that 3 weekly heat sessions maintained the Hbmass increase 3,5 weeks after returning to sea level, whereas the control group lost 75% of their adaptions up to that point. Additionally, the heat training group increased plasma (+11.6 %) and total blood volume (+5.8 %), similar to the findings of other studies.
Summary and conclusions
In summary, heat and altitude training can have similar effects, especially regarding total hemoglobin mass. Each of them has been shown to have positive effects on endurance performance, both in their respective environments (altitude & heat) as well as at sea level and under moderate conditions. The big advantage of heat training is that it is accessible and easy to implement for athletes, no matter where they live, as long as body core temperature is correctly controlled.
Overall, the evidence on benefits of heat training on performance at altitude is limited. That being said, specificity is still key, which means that for competitions at altitude, acclimation and, if possible, training at altitude is still highly recommended. The same goes for races in the heat: if you don´t adapt and train in the heat, you most likely won`t be able to perform to the best of your abilities.
Lukas Scherl is a sports scientist (M.Sc. University of Bayreuth, Germany & Norwegian School of Sports Sciences, Oslo, Norway) and endurance sports coach. In the lead up to the Paris Olympics 2024 he was part of a study together with the German Swimming Association, where they scientifically accompanied a group of world class middle and long distance swimmers for altitude training camps and investigated a multitude of physiological and external parameters to identify factors influencing the hematological response to altitude training. Lukas also worked at the Institute of Applied Training Science in Leipzig, Germany, in the months before and during the 2024 Olympics and is now leading a small performance diagnostics lab in Bayreuth, Germany.
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