Though different definitions exist of what constitutes altitude training, we will, for the sake of clarity, consider altitudes in the region of m to m above sea level. In simple terms, the oxygen inhaled from the air people breathe affects the energy their muscles receive to perform physical activities. Oxygen is carried around the body within red blood cells and helps the molecules in muscles perform their functions. The higher the altitude, the lower the atmospheric pressure, which makes it harder for the body to transfer the oxygen into the blood.
This is why people can often feel lethargic at altitude. In response to this situation, the brain triggers the increased production of the hormone erythropoietin EPO , encouraging the body to make more red blood cells to better transport the oxygen available.
This means, over time, the body begins to transport the limited oxygen better than when it first arrived at altitude. The body will, over time, return to normal levels of red blood cell production if the process is not repeated and the time taken for this to happen will vary from one athlete to the next. It takes time for the body to adapt to higher altitude and many of the effects do not occur until after a prolonged period of time.
Various studies suggest there is no increase in red blood cell count within the first seven to 10 days, meaning usually athletes choose to spend a minimum of three to four weeks at altitude. Some athletes choose to be based at high altitude throughout the year e. Boulder in Colorado, Iten in Kenya and Addis Ababa in Ethiopia , coming down to sea level for shorter periods around a racing season. Early on in any stay at altitude individuals will likely find themselves to be lethargic as their body responds to the lower atmospheric pressure.
This failure has been attributed to reduced training loads at altitude. One approach developed by Levine and Stray-Gundersen, called "living high-training low" has been shown to improve sea level performance over events lasting minutes.
This strategy combines altitude acclimatization 2, m with low altitude training to get the optimal effect. The opposite strategy, "living low-training high" is proposed by Dr. Hoppeler in this debate. These observations indicate that the total dose of hypoxia resulting from the combination of altitude and time of exposure seems to determine the hematological response to altitude training.
As Garvican-Lewis et al. Attempts to optimize different doses of altitude exposure are still underway. This optimization would make it possible to establish a minimum threshold efficacy of hypoxic exposure.
This model integrates the role of exposure time and altitude. However, Millet et al. As Millet et al. Such an approach could help to individualize training.
Undoubtedly, standardization of hypoxic dose metrics would help establish clear guidelines for altitude training prescription, which includes the potential for a targeted approach based on individual response patterns, whilst providing a stronger framework for interpreting and comparing different training protocols employed in research and practice. The positive effects of altitude training in athletes are temporary Wehrlin et al. Even if immediately after or a few days after a return to sea level, hematological variables are elevated, some data indicate that nearly all of the hypoxia-induced changes may be lost within 1—2 weeks Pottgiesser et al.
Chapman et al. This loss of effect may be why some studies did not detect improvements in hematological variables despite applying the recommended hypoxic dose. When the only post-altitude measurement is made after 14 days, the effects may not be observed Heinicke et al. On the other hand, in a study conducted by Brocherie et al. Interestingly, even if hypoxia promotes erythropoiesis, it remains uncertain whether this will lead to an increase in RBC, as in certain conditions the reticulocytes may be targeted for early destruction Hahn and Gore, This is likely linked to the phenomenon of neocytolysis.
Neocytolysis is a physiological process that controls red cell mass by down-regulation if it is excessive. Because the absence of hypoxic stress after return to sea level causes a decline of the EPO level, neocytolysis is accelerated, which consequently leads to selective hemolysis of the youngest circulating red blood cells Alfrey et al.
A significant decrease in tHb mass , fall of erythrocyte survival time, lower iron turnover and reduction of bone marrow production of erythroid cell lines are observed after the return to sea level of both native highlanders and those individuals who stayed at altitude temporarily Levine, ; Prommer et al. These results suggest that neocytolysis may be one of the causes of the lack of improvement in hematological variables after return to sea level even if altitude training favored erythropoiesis.
For athletes, the duration of maintaining the post-altitude training effects is a very important issue, because it determines the moment of the return to sea level before the competition.
Dick claimed that the maximum performance occurs between 15 and 24—28 days, whereas the first week following altitude training is characterized by poor performance. Conversely, Heinicke et al. Furthermore, Turner et al. This problem is yet to be explained, and practical application should be individually adapted to athletes. Based on the majority of the reviewed literature, it would seem logical to suppose that the conflicting and inconsistent reports concerning tHb mass after altitude training might be, at least partially, attributed to different methodological approaches used by researchers.
It was indicated that not all techniques had the same precision and were suitable for the research concerning training in hypoxia Wehrlin et al. Furthermore, Garvican et al. The rate of EPO production is related to the level of hypoxic stress, but the increase in EPO induced by altitude is characterized by inter-individual variability.
The difference of EPO level after exposure to the same altitude can be up to several hundred percent between subjects Table 2. The study conducted by Li et al. The individual EPO response may affect the level of hematological adaptation and performance in athletes after altitude training. The athletes performed a 4-week training block at altitudes between 1, and 3, m. Based on the improvements in sea-level performance during a 5, m run, the study group was retrospectively divided into responders and non-responders to altitude training.
It was found that the responders had significantly larger increases in mean EPO concentration after 30 h of hypoxia compared to the non-responders. After 14 days of exposure to altitude, EPO remained elevated only in responders. It is likely that for non-responders, greater hypoxic stimuli may be needed to induce a sufficiently large release in EPO and augment red cell production Chapman et al.
Table 2. Individual variability of EPO levels after exposure to moderate altitude. Individual variability in response to altitude training is also important in the context of the efficiency of anti-doping tools such as the Athlete Biological Passport ABP.
The results of a meta-analysis conducted by Lobigs et al. However, better knowledge of the effects of altitude training on blood markers can make it easier for experts to make a judgment on a dubious ABP profile.
We believe that the information presented in our review may also be a guide for experts concerning the impact of altitude training on erythropoietic response and hematological variables. Various mechanisms have so far been suggested as responsible for individual variability, but many factors associated with the altitude response seem to be genetically determined Chapman et al. Attempts to find genetic determinants of the hypoxic response are in progress.
For instance, Witkowski et al. Further analysis Jedlickova et al. In this study, based on the level of increase in the EPO level after hypoxic exposure, athletes were divided into low responders, intermediate responders, and high responders. The obtained data demonstrated that the bp allele of the EPO gene was most prevalent in the group of high responders. The bp allele showed a significant correlation with the EPO hypoxic response. Tanimoto et al.
In turn, Liu and Hu demonstrated a higher SaO 2 during exercise performed in hypoxia in subjects with the CT genotype. Furthermore, following 4 weeks of hypoxic exposure and hypoxic training, changes in maximal oxygen uptake VO 2 max were higher for the CT genotype compared to the CC genotype.
However, there are data that do not confirm the above observations. Hennis et al. Also, Richalet et al. Presence of the I allele of the ACE gene reduces enzymatic activity and, consequently, decreases the level of angiotensin II. This suggests that the DD genotype may be associated with a greater increase in EPO level after hypoxic exposure. The responders and non-responders had similar genotypic profiles. These results led to the conclusion that polymorphism of the ACE gene does not influence the EPO response in athletes during exposure to hypoxia.
It turns out that the I allele of the ACE gene is identified in mountaineers more often than the D allele Martin et al. Woods et al. Furthermore, Bigham et al. However, other research, conducted by Patel et al. Current reports on genetic determinants of the hypoxic response are few and ambiguous. Therefore, further research is needed, because such data may be useful in predicting the response to altitude training.
In addition to an insufficient hypoxic dose, the effectiveness of altitude training may be disturbed by other factors. One of them is insufficient body iron stores. At high altitude, when erythropoiesis is activated, the iron demand increases. Erythropoiesis stimulation leads to reduced production of hepcidin, a peptide hormone regulating body iron homeostasis Robach et al.
These changes allow for enhanced iron absorption and release of iron stores Goetze et al. The reduced serum ferritin levels observed at altitude in several studies Roberts and Smith, ; Stray-Gundersen et al.
However, if iron stores are insufficient or a diet at altitude is inadequate for the nutritional demands Michalczyk et al. This interpretation is consistent with data obtained by Stray-Gundersen et al. Therefore, ferritin levels must be monitored on a regular basis before and during the hypoxic exposure if the aim of altitude training is to improve blood oxygen-carrying capacity. It is also recommended to supplement iron in athletes during altitude training to prevent anemia from occurring Michalczyk et al.
The hematological response to altitude training may also be limited by inadequate choice of training loads. Type of training and choice of training loads affect the direction and scope of hormonal changes Lehmann et al. Testosterone T and cortisol C are steroid hormones that regulate anabolic and catabolic changes in the body.
Although there are reports in which an increase in resting cortisol Vasankari et al. Another factor which is likely to impair the erythropoietic response is injury or infection McLean et al. However, in the most recent study, Turner et al. In view of the ambiguous results of previous studies, regulation of EPO production by inflammatory cytokines and the relationship of injury and infection with the response to hypoxia remain unclear and require further research.
Data on the impact of initial tHb mass level on improvement of tHb mass after altitude training are still inconclusive. It is likely that a physiological limit of the total circulating quantity of tHb mass exists. Elite endurance athletes, in response to many years of sea level training, may have maximized their tHb mass level. Therefore, they might have a limited scope for increases in hematological variables following altitude training Gore et al. However, recent data presented by Hauser et al.
Therefore, it seems that even elite athletes with higher initial tHb mass can expect tHb mass improvement after LH-TL. This observation is supported by findings presented in studies by Millet et al. Another question concerns the possibility of different physiological responses between hypobaric and normobaric hypoxia. Opinions are still divided Millet et al.
Interesting results were obtained by Saugy and colleagues during a two-stage project. The first stage Saugy et al. The same experiment was repeated, but using a crossover study and reduction of the possible group effect, and indicated that the post-LH-TL hematological responses and performance improvements were similar for hypobaric and normobaric stimuli Saugy et al.
However, the small number of previous studies comparing hypobaric and normobaric hypoxia does not allow us to state clearly whether the type of hypoxia significantly affects the difference in hematological responses after altitude training. It should be noted that the blood volume BV adjustments during acclimatization to altitude have two phases.
In the early phase, which begins within hours of altitude exposure and lasts for the first 3—4 weeks, plasma volume PV decreases, causing hemoconcentration.
One of the mechanisms leading to a reduction in PV at altitude is dehydration associated with increased respiratory water loss due to enhanced ventilation and increased urinary water loss observed at altitudes Sawka et al.
In this first phase of acclimatization, PV is decreased while erythrocyte volume remains stable, resulting in increased [Hb] and reduced BV. During the second phase of acclimatization, as a result of continuous exposure to altitude for several weeks or months, erythrocyte volume increases. This phase can be accelerated by an intensive program of aerobic training at high altitudes Sawka et al.
Changes in BV during and after hypoxic exposure can cause errors in interpretation of research results and evaluation of the actual effects of altitude training may be impeded. Hemoconcentration may explain the increase in hematological variables, especially in the early days of altitude training. It is also likely that differences in EPO levels following hypoxic exposure partly result from the decrease in PV rather than from significant diversity of EPO production.
Monitoring of urine specific gravity during altitude training is essential to eliminate the hydration effect on EPO values. If this condition is not met, it may turn out that the variability range of EPO level, especially after hypoxic exposure for over 12 h, may differ from the actual variability in EPO production. Due to the individual variability of EPO production and many factors affecting the hypoxia response, identification of responders and non-responders among athletes prior to altitude training seems to be useful in optimizing the training process.
As suggested by Chapman et al. In studies conducted by Friedmann et al. These findings indicate that short hypoxic exposure can be used to identify athletes who respond to altitude training. Based on the results of previous research Figure 2 , it can be assumed that the EPO level after 24 h of exposure to moderate altitude should be at least doubled in relation to baseline. However, as we observed on the basis of the reviewed studies, even a significant increase in EPO level during altitude training does not guarantee improvement of hematological variables after the return to sea level see Table 1.
This observation is in line with previous reports in which correlations were not found between the EPO levels following exposure to altitude and tHb mass Rusko et al. Therefore, it is doubtful whether the improvements of hematological variables after altitude training can be predicted using the EPO response to acute hypoxic exposure.
Determining efficacious methods to identify better and worse responders prior to altitude training remains a topic for further research.
Regardless of attempts to find an effective tool for identification of responders and non-responders prior to altitude training, daily measurement of SpO 2 at rest during altitude training may by a useful and inexpensive method to analyze the hematological response to chronic hypoxia. Millet et al. It is known that the increase in EPO levels is directly proportional to the level of hypoxia and decline in SpO 2 Eckardt et al.
In our recent study Czuba et al. Consequently, the effect of hypoxia on blood EPO levels became weaker with increasing acclimation, which was caused by the improvement in blood oxygen-carrying capacity. Since the desaturation level depends not only on altitude but is highly individualized and also dependent on the training background of athletes or previous altitude experiences Chapman et al.
Despite the knowledge developed over the years, EPO still remains the subject of researchers' interest. As noted by Lundby , new discoveries in recent years have appeared in the field of the functions and synthesis of EPO. However, in the context of altitude training, what seems to be the most important is that the elevated EPO production by the kidneys in hypoxia is a key factor enabling subsequent improvement of hematological variables.
However, even a significant increase in EPO level during altitude training does not guarantee improvement of hematological variables after the return to sea level. The most recent publications reviewed show that the effectiveness of altitude training depends on many variables. The hypoxic dose should be at least h at an altitude of 2, to 2, m Rusko et al. The measurement of SpO 2 during altitude training will allow measurement of the time the athlete has spent at a given saturation level.
EPO production, like many other factors associated with the response to altitude, may be genetically determined, but further research is needed to identify the genetic determinants of the hypoxic response. All authors have read and approved the final version of the manuscript. All authors made a significant contribution to this study. This study has been conducted in the framework of the grant awarded by the National Science Centre of Poland, No. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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