Dossier acclimatization. Altitude or Heat?

In this estival period the appeal of the mountains is strong among endurance athletes. Like top-level athletes like Paula Radcliffe or Fred Van Lierde at Font Romeu, Helle Frederiksen at Boulder or riders from the Kenyan highlands, amateur athletes also want to benefit from the effects of altitude training camps, known as “hypoxic” (see more).

However, these camps are not always synonymous with benefits. A recent post by Vincent LUIS following the WTS in Leeds well illustrates the importance of placing this type of stage in relation to the objective:

“Not in the loop from start to finish… Probably the result of a poorly managed altitude descent.”

On the other hand, the summer period is also accompanied by the call of the beach. In a recent article we discussed the principles of heat acclimatization to help endurance athletes perform well in high temperatures. Because the ambient heat also increases the physiological stress during training to reinforce the athlete’s progression.

That being said, you now identify the purpose of this article : compare the effects of altitude training to the effects of heat training !

In people’s minds, altitude training is by far the most popular method of acclimatization. It was used again by the English and American teams during the last Rio 2016 Olympic Games.

Heat acclimatisation is the “last little one”. It is beginning to make itself known while laboratory studies have been interested in it for about twenty years.

On this observation, the question then arises: is it a battle lost in advance? To answer them, we will review 4 points. For each of them, the approach will be to cross scientific data and logistical reality of the athletes to facilitate your practical application.



This point is essential: it is it which primarily conditions the interest of the coach / athlete. And yes, an easily implemented method is an adopted method.

1.a. In a natural environment.

If you opt for hypoxia, you will have to climb to altitude where the body’s ability to capture O2 is more difficult (16% O2 at 2000m altitude compared to 21% in the plain). From there 2 protocols are recommendable:

– The “Live-High Train-Low” (living at altitude and training in the plain): this strategy seems to be the most effective when one cannot often do a training camp in altitude. This protocol requires many trips, so we see the difficulty for an athlete to descend from his mountain 1 to 2x/day to train on the plain.

– The “Live-High Train-High” (living in altitude and training in altitude): it is the most effective method if the athlete has the possibility to do regular training camps in altitude (at least 3 camps/year) AND if he also manages his training load correctly (see more).

Points +: hypoxia guaranteed by altitude, acclimatisation possible while sleeping, change of environment of the athlete.

Points –: site to site travel, fatigue accumulation, housing and transportation costs.

If you opt for heat, the implementation will be easier. Yes, no need to leave for a training camp, the summer period in France often allows high temperatures. So you can combine family holidays by the sea with your acclimatisation.

However, unlike hypoxia, heat cannot be used during sleep to acclimatize because high temperatures are unfavourable to sleep. To compensate for this lack of exposure, all the rest phases you can spend outside when it is hot will be beneficial.

Points –: hazards of the weather, need to voluntarily expose to heat, travel (possible).

because the altitude guarantees hypoxia while the summer period does not guarantee heat.

1.b. In artificial environments

To avoid moving in the mountains and simulate altitude, hypoxic tents have emerged. O2 percentage is adjustable up to a simulated altitude of >3000m (<14.5% O2) but the CO2 control, less reliable, can cause headaches. If this happens, acclimatization should be interrupted until the athlete recovers clear thinking.

Many top athletes are accustomed to training and/or sleeping in this type of tent because their effectiveness has been shown to be comparable to the effects of altitude.

Note: we hear about “hypoxic masks” as an alternative to altitude. But make no mistake about it! Rather than recreating mountain conditions, these masks strengthen your ventilatory muscles – which is interesting but a little less relevant.

Points +: possibility to acclimatize in plain, similar efficiency to the altitude, acclimation possible while sleeping, controllability of hypoxia in relation to the feeling.

Points –: purchase price (from 350€), CO2 headaches, sleeping alone in a hypoxic tent, material disturbance.

Heat-acclimatisation can also be artificially simulated. This is ideal when a race takes place overseas but you are in the cold season. It’s also perfect in summer when the outside temperature suddenly fails but you need to continue acclimatizing.

A first “active” possibility consists of training in a room heated to about 35°C (living room, bathroom, bedroom, garage, sauna, 2x2m mini-tent, storage room). We have successfully tested this option with many of our athletes although accommodation constraints can temporarily complicate family life.

Other innovative “passive” protocols are also effective provided the thermal load is sufficient: hot baths (40°C), saunas (80-100°C). Particular mention for the bathroom, because it allows to have heat + humidity but also training + bath!

Points +: easy access to heated rooms, guaranteed heat, controllable heat, reduced costs, short-term exposures (40-80′).

Point –: material disturbance.

because heat sessions are less socially and financially annoying.


Whatever the acclimatization, many adaptations occur in the body. We address them here and see what performance improvements can then be expected.

2.a. Physio

Altitude-related changes are fairly well known and are generally confined to improving blood transport of O2 via the stimulation of EPO (erythropoietin, the red blood cell production hormone). This is important for endurance performance because after 4 weeks at 2500m altitude, the EPO level and hemoglobin mass can increase by ~50%. For a VO2max gain between 3% and 10%.

Also, we think less about it but hypoxia increases the “buffering” power of bicarbonates: they reduce the acidity of muscles during intense efforts (relaunches, uphill). In addition, the ability to ventilate improves and promotes brain oxygenation and CO2 evacuation. Finally, the lower density of the air at altitude favours the work of the technical ranges to hope for a positive transfer back to the plain.

In terms of adaptation speed, some changes are rapid (EPO secretion) while others require >7 days before they can be observed (increased hemoglobin mass). These differences have an impact on the competition schedule at the end of the camp (see point 3.). Physiological changes also have an effect on the athlete’s condition: headaches / sleep disorders may accompany the ascent to altitude, require descending to the plain and thus interrupt acclimatization.

Points +: hematological (O2 transport) and metabolic (muscle pH) adaptations.

Points –: speed of adaptation of the body, training load, ability to recover well.

The mechanisms related to heat-acclimatization are less known. In fact, we are told that exercise is not recommended when it is hot… Obviously, compete under 35°C without being prepared… Hence an acclimatization.

By repeatedly raising the body temperature, the plasma volume reacts in 2-3 days and can finally increase by 10-15%. This increases the body’s hydration level and reduces exercise HR. Also, the better dilation of blood vessels is very early. It allows warm blood to be brought effectively to the skin to dissipate body heat.

Sweating adjustments are observed after 7-10 days and are important: you can lose 20 to 50% more sweat per session, sweat earlier at the beginning of the exercise (after 3-4′) and lose less mineral salts (your sweat is then less salty and your t-shirts less yellow).

Finally, at the cellular level, the stress quickly becomes less pronounced (HSP72 stimulation). The consequence is not as visible as with sweating but the athlete feels it: his enzymatic activity is more efficient to produce energy and his recovery speed is accelerated between training sessions.

Points +: speed of adaptation of the body, cardiovascular and thermoregulatory adaptations.

Points –: training load.

because the heat induces more adaptations more quickly.

2.b. Performance

Training camps always represent an effort because they are accompanied by new adaptations, require an investment in time, organization… It is therefore normal that the athlete expects an effect on his performance. Since a new environment generates adaptations specific to that environment, endurance performance after a training camp is almost always improved when it takes place in the same environment as acclimatization.

However, the question that interests us is different: how does the athlete’s performance respond after the training camp when the event takes place on plain at neutral temperature (<200m and <20°C)?

In practice, about 1 athlete in 3 completes his hypoxic training camp without marked physiological adaptations. As a result, the average improvement in sea level performance is close to ~1%.

In cause the “dose” of hypoxia needed to perform better must be sufficient. Sufficient without being exaggerated: an altitude >3000m would guarantee physiological adaptations, but sleep disorders (apnea, waking up) and lack of recovery are recurrent there and counterbalance the physiological benefits. On this basis, here are the current recommendations:

– Follow a “Live-High Train-Low” protocol with >12h per day between 2200m and 2500m (15.3%-16% O2 in air) for 3-4 weeks. This duration may be difficult for the triathlete to achieve, but it is consensual. Indeed, the hemoglobin mass only starts to increase after 8-10 days. The advantage here is that training on the plains enables high intensity training to be maintained. This protocol can increase performance gains to ~4%.

– Follow a “Live-High Train-High” protocol: 3×3 weeks with 2 weeks interval. The interest of this protocol lies in the “physiological memory” of the organism: at each new stage the re-acclimation time is reduced and allows the athlete to approach his standard intensities. Ultimately, this protocol can induce performance gains of ~5-6%.

Although recommended, these protocols do not guarantee better performance. Indeed, there are “bad responders” at altitude. For example, the performance in the plain at the end of a training course in Font-Romeu (1850m) shows an improvement in performance of 5% for some athletes and a decrease in others.

Point +: the amplitude of performance ( provided a sufficient “dose” of hypoxia).

Points –: the reliability of the result, the management of the recovery.

After a heat-acclimatisation, performance in a temperate environment is not deteriorated as soon as the training load has been controlled: either performance improves (in 2 cases out of 3 and up to ~5%), or it stagnates (the athlete then feels better during exercise). If the question of the “dose” of heat is therefore obvious here too, it is not accompanied by bad surprises. We propose you 2 examples of acclimatization:

– Short-term (over 4 days): 2 heat exposures per day with one 70′ training session at low intensity and 35°C in the morning, and one hot bath at 40°C for 40′ at the end of the day. This double exposure boosts the main adjustments (plasma volume and vasodilation) but does not allow acclimatization to be completed (sweating rate). It will be relevant 8 days before the competition.

– Long-term (21 days): 3-4 heat exposures per week for 3-4 weeks (see more). This frequency allows not only complete acclimatization but also sufficient rest periods as well as high intensity training phases in cool conditions. The performance gain can then exceed 7%, especially in athletes who suffer a lot from heat.

In summary, the proportionality of the gain will depend on the athlete’s profile (good responder vs. neutral responder) and proportional to the acclimatization duration.

Points +: the absence of bad responder, the possibility to “boost” adaptations.

Point –: the management of the state of form.

because performance improvement is more likely after heat.


To tip the balance between altitude and heat, an important question for the athlete is also that of the time to keep between the training camp and the competition. This period should normally promote recovery without letting the organism lose all its benefits. The timing of the acclimatization before the competition is therefore important to guarantee its effectiveness because a well arranged training camp will only be of interest if it is correctly located in the programming.

Before looking at the differences between altitude and heat, let s look at their common points:

– The action of “dose-response”, which means that the more acclimatization is significant, the more its effects persist. Persistence may vary from 4-5 days to >3 weeks depending on the initial dose. When acclimatization is properly managed, persistence of >3 weeks is observed for both altitude and heat.

– The phenomenon of “re-acclimation” supposes that 2-3 exposures between D+4 and D+10 after the end of the stage make it possible to reactivate the benefits of acclimatization. Thus, the post-acclimation phase is optimal for a “low-cost” maintenance of the benefits of the stage.

At the end of a hypoxic training camp, the empirical data show that the performance should not be programmed between D+5 and D+10 (certainly because of the backlash related to the change of air) nor more than 4 weeks after the training camp (physiological maladjustment). Thus, it is recommended to perform within 2-3 days after the stage or after 2-3 weeks. Indeed, even if some benefits will then be blurred, the red blood cell mass will remain higher.

Points +: the possible re-acclimation, the duration of certain adaptations.

Point –: the contraindication to perform between D+5 and D+10.

The cardiovascular changes being the first to appear, they are the first to disappear 3-4 days after the stage. However, sweat adaptations and the improved cellular tolerance dissipate with more time: they allow you to take advantage of the benefits of the acclimatization still 3 weeks after the camp.

In terms of timing, there is no contraindication: the athlete can perform at D+2, D+7 or D+15 keeping in mind that the sooner the better.

Points +: re-acclimation possible, duration of some adaptations.

because the altitude can leave sequelae on the performance between D+5 to D+10 after the stage.


In the perspective of a transfer of benefits between 2 types of training (see more), recent studies have crossed acclimatization at altitude and acclimatization to heat. Here is what we keep in mind for the moment:

An absence of effect:
Sleeping in a hypoxic tent does not bring additional benefits to those induced by heat-acclimatization. In other words, the improvement in performance allowed by a heat training camp is not magnified by hypoxic stress. This tends to be more and more confirmed.

A transfer effect:
Heat-acclimatization predisposes to heat endurance performance, of course, but it also prepares for temperate AND hypoxic environments! This transfer seems to be explained by the increase in plasma volume (better muscle oxygenation), the lower muscle oxygenation in heat (thus the possible increase in red blood cell mass) and the cellular stress imposed by heat (better tolerance at altitude: changes in HSP72, but not HSP90a or mRNA).

because heat also expresses its benefits in other environments.


We would advise athletes to acclimatize to heat rather than altitude before a performance at sea level in a neutral atmosphere (<200m altitude, <20°C). This position is based on the coach’s key concerns.


5.a. Eat better.

Before the camp, favour foods rich in iron to promote the fixation of O2 (red meat, fish, egg, parsley, spinach, beans, fig, almond, hazelnut), carbohydrates to anticipate their increased consumption in altitude (cereals, dry vegetables, potatoes, fruits) and antioxidants to limit cellular damage induced by free radicals (garlic, almond, eggplant, beetroot, coffee, lemon, wheat germ oil). Start between 2 and 5 days before the camp.

During the camp, take carbohydrate snacks before and immediately after the sessions. These timings are adequate to limit respiratory infections, frequent during stages. In terms of hydration, drink ~1.5L of water in addition to the usual volume over the day.

Accentuate the intake of carbohydrates but not those of lipids because the rise in internal temperature amplifies glycolysis but reduces lipolysis. In addition, your hydration should at least compensate for the more abundant sweating losses. Choose the “double weighing” method (weight before the session – weight after + volume of water ingested) and drink 150% of this amount in recovery.

5.b. Adjust the load.

The training load data is crucial during stages because it allows you to adjust the session duration and intensity. On average, for a given intensity, the HR in running increases by 5 to 15 bpm at 2000m compared to the plain, and identically at 35°C compared to 20°C. Thus, a good benchmark for the coach will be to calibrate the training intensity on the athlete’s usual heart rate in order to maximize the practice time and therefore the physiological stress of acclimatization.

In terms of benchmarks, measurements on the plain or in cool conditions will then help to determine the level of the athlete’s acclimatization: a decreasing heart rate combined with a stable (or lower) exertion will be a positive marker of the athlete’s acclimatization.

5.c. Wait your time.

Know that the individuals who suffer the most from hypoxia or heat are those who progress the most! Example: in altitude, the hemoglobin mass increases more strongly in individuals with the lowest initial levels. The same goes for the heat: athletes who really suffer during the first 2-3 acclimatisation sessions should wait 4-5 sessions before giving up.

5.d. Do’s and don’ts.

During recovery, do not schedule a hypoxia session. This would be useless for 3 reasons:
– Physio adaptations take time to develop when the athlete is passive (which is why he often spends the whole night in a hypoxic tent). So there would be little effect
– Muscular oxygenation is a recovery factor. In hypoxia, it is reduced.
– This practice generally increases the feeling of fatigue and recovery time.

In recovery, favour passive exposure to heat as this allows the athlete to benefit from the increased temperature of the previous training. The time spent in heat can then be shortened by 5′ to 15′ compared to a normal exposure.

-> You now have all the keys in hand to make your choices for the season. This one’s just getting started. So we wish you all the best with Gutai!