Is Hypoxic/Altitude Training an Important Topic in the Field of Hypoxia?

Hypoxia is an essential topic in medical or biological sciences. The main aims of the present study were to examine the most important medical articles (i.e., the top 100 most cited) on hypoxia. We examine how the Nobel-prize awarded hypoxia inducible factor (HIF)-pathway discovery in the early 1990s has changed the thematic composition of this body of literature, with a special emphasis on the studies linking hypoxia and cancer. We searched Pubmed for articles with the terms #Hypox, #Altitude, or #Mountain in the title that have been published in biomedical journals and ranked the articles on their number of citations in Web of Science. A second search was performed in all journals for articles related to hypoxia and cancer. Strikingly, only 12 of the top-100 most-cited articles on hypoxia and only 3 articles of the top-100 articles related to cancer were published before 1995. Moreover, only 5 articles from prior 1995 reached 1000 citations, while 27 articles published in 1995 or later were cited more than 1000 times, most of them on the HIF-1 pathway. Eighty percent of the top-100 articles were related to the HIF pathway, while there were no articles on the application of hypoxia either for therapeutic use (i.e., hypoxic conditioning in patients) or for performance enhancement (i.e., altitude training in athletes). In conclusion, the early-1990s discovery of the HIF pathway and of its molecular regulation has shifted the focus of hypoxia research towards molecular mechanisms and consequences of tissue hypoxia, most notably in cancer. The importance of studies focusing on clinical and performance applications of systemic hypoxia is relatively lower.


Introduction
Between 1775 and 1777, Scheele [103] and Priestley [93] found a previously unknown gas in the air that was named "oxygen" by Lavoisier [64]. This fundamental finding paved the way for the understanding of the primary importance of the metabolic and physiological adaptations to oxygen availability and consequently to "The history of hypoxia" as elegantly reported by Richalet [98]. The Nobel prize in physiology or medicine for 2019 awarded to William Kaelin Jr, Sir Peter Ratcliffe, and Gregg Semenza for their work on the molecular mechanisms underlying how cells adapt to different levels of oxygen supply is the most prestigious event of this long journey.
Since the 1950s, the term "hypoxia" (i.e., "abnormal condition resulting from a decrease in the oxygen supplied to or utilized by body tissue"; Webster's New World College Dictionary, 4th ed 2010; from [98]) has been increasingly deployed in the medical or biological scientific literature: 1. In environmental physiology, to deepen the understanding of the physiological (i.e., cardiovascular and respiratory adaptations) and pathophysiological (i.e., acute mountain sickness) responses to severe hypoxia (e.g., in aviation physiology [34]) or to high altitude, as pioneered at the end of 19th-early twentieth century in Europe [97] and USA [134]). 2. With the development of cellular or molecular biology, to understand how variations in oxygen delivery lead to changes in gene transcription and to adaptive responses in cells and tissues. The importance of the isolation of the hypoxia inducible factor (HIF) has been unanimously recognized and celebrated by bestowal of the Nobel Prize [83]. The subsequent growing understanding of its role in cellular pathways and its molecular regulation, e.g., by the von Hippel-Lindau (VHL) protein, contributed to the recognition of various potential (clinical) applications, especially with regard to associated drug targets in various cancers [125]. Various HIF molecules and subunits (HIF-1α, HIF-1β, HIF-2α, HIF-2β, HIF-3α, HIF-3β) have been characterized and our understanding of their differential roles has increased substantially during the last decades. For example, the unique target genes of HIF-1α and HIF-2α and their different functions in glycolytic regulation, glucose consumption, or hypoxia responses are being continuously unraveled [50].
Numerous applications of hypoxia have also been investigated and published in the scientific literature: 3. In the clinical field, the continuous exposure to altitude or systemic intermittent hypoxic conditioning/training (not to be confused with pathology-associated "intermittent hypoxia" as in sleep apnea, for example) has been proposed to be an effective treatment strategy against different diseases, conferring various health benefits: e.g., cardioprotection [77]; neuroprotection [13]; and reduction of obesity [135]; cancer mortality [14], and cardiovascular diseases [28,29]. 4. Since the 1960s for enhancing performance in athletes, initially only in endurance sports due to the hematological benefit of prolonged exposure to altitude [23,74] and more recently in intermittent (i.e., team or racket) sports [36,82].
To our knowledge, there is no detailed analysis of the scientific contributions related to hypoxia. The main aims of the present study were to examine the most important (i.e., the most-cited) articles in this field and to examine how the HIF-pathway discovery may have changed the thematic composition of this body of literature, with a special emphasis on the studies linking hypoxia and cancer. Finally, the relative representation in hypoxia research of five different topics (pathophysiological responses to high altitude; physiological adaptive responses to hypoxia; molecular responses to hypoxia; clinical/therapeutic use of hypoxia; altitude training) was analyzed.

Methods
We first identified biomedical (biology, medicine, physiology, or "sport sciences") journals in PubMed (https:// pubmed. ncbi. nlm. nih. gov/) with potential articles on altitude or hypoxia. All journals with the terms #Med, #Physiol, #Biol, #Exerc, and #Cell in the journal name were first selected. For inclusion of the highest ranked general scientific and medical journals, we added journals with #Sci, #Lancet, #Jama, #Nature, #PNAS, and #BMJ in their names.
This yielded a list of 1726 journals or source titles. From this list, we removed those that were not closely related to human physiology or medicine (e.g., plants, veterinary, zoology, animals) reducing the list to 1668 journals.
Next we searched Pubmed for articles with the terms Hypox*, Altitude, or Mountain* in the title. This yielded 29,888 articles on the 15th march 2021.
The next step was to determine the number of citations for these articles. We performed a complete search for all the articles on Web of Science (https:// www. webof scien ce. com/) on 16-17th march 2021. Basic information, including source journal, publication year, citations per year, and total number of citations, was extracted.
These publications were then ranked by the number of times cited and a list of the top-100 cited articles was created. Finally, we extracted from this list all articles related to cancer (i.e., titles containing the terms "tumour"/"tumor" or "cancer"), yielding a list of the top-100 articles on hypoxia but without a primary focus on cancer ( Table 1).
Each of these 100 articles was then analyzed by two independent reviewers (GPM and JB) to classify them according to five distinct categories, regarding their main purpose: A second search in Pubmed was performed in all journals for articles related to hypoxia and cancer, i.e., articles with #Hypox or #Altitude and #cancer in the title. This search yielded 3173 articles. As described for Table 1, we ranked   these articles by citations. We added to this list the articles related to cancer extracted from Table 1 to create Table 2 with the top-25 articles related to hypoxia and cancer. A third search was performed only in "sport sciences" journals, with "exercise," "sport," "applied physiology," or "rehabilitation" in the journal title. This search yielded 3113 articles.

Results
The number of citations of the (bio)medical articles on hypoxia began to increase in the 1960s, while the rise in citations on "hypoxia and cancer" started later, in the end 90 s. The total number of citations is > 68,000 and > 18,000 in 2020, respectively (Fig. 1). The articles related to "cancer" are not displayed (see Table 2) * Shows the review articles   Table 1 displays the top-100 most-cited articles on hypoxia and Table 2 shows the top-25 articles on hypoxia related to cancer.
The mean number of citations of the top-100 articles on hypoxia (Table 1)  Of the top-100 articles on hypoxia, only 7 were on the pathophysiological responses and risks of hypoxia, 13 on the physiological responses to hypoxia, and most of them (80) on the molecular responses, mainly related to the HIF-1 pathway.
We did not identify any articles on the application of hypoxia either for therapeutic use (i.e., hypoxic conditioning in patients) or for performance enhancement (i.e., altitude training in athletes) in the top 100. However, there is a large increase in the number of citations in the "sport sciences" journals ( Fig. 1), starting in the 1960s with an important rise in the last twenty years.
Wang et al. [130] published the most-cited article in 1995. Consequently, to display the impact of the discovery of the HIF pathway on the body of literature related to hypoxia, we arbitrarily chose to analyze two distinct periods An outstanding point is that the Nobel laureate Gregg Semenza published 27 of these most-cited articles on hypoxia, including 10 with GL Wang as a co-author (including the most cited article in the list).

Discussion
The presented descriptive overview of the current biomedical literature provides some insights into the evolution of the research related to hypoxia: 1. The mechanistic investigation of how the body adapts to hypoxia is an important topic in biology and medicine since "the responses to oxygen in the cells and organisms is one of the most central physiological adaptation that animals have" (statement of the Nobel assembly). 2. The HIF pathway discovery in the 1990s, which was made possible with the increasing effectiveness of the molecular biology methods, opens the way to many Fig. 1 A Total citations number per year (1950-2020) of articles on "hypoxia," "hypoxia and cancer," and "hypoxia in sport sciences"; B The respective contribution in the top-100 articles on hypoxia of the molecular, pathophysiology, and physiology approaches is shown. No articles related to clinical or sport (altitude training) applications reached the top-100 medical or pharmacological applications in many diseases, especially cancer. Since the literature on clinical applications of hypoxia is still relatively scarce, harnessing related strategies seems to be still at an early stage and requires future research. 3. Altitude training for sport performance enhancement is an important topic in sport sciences but attracts relatively limited attention compared to the overall hypoxia research field.
There is a long-term interest in medical sciences for hypoxia. This is shown by the increase in the total number of citations starting in the 1960s. Great studies have been published on pathophysiological and systemic physiological responses to acute hypoxia or to adaptations to prolonged exposure. One perfect example is the study by Shweiki et al. [120] on angiogenesis that is the most-cited article (3721 citations) prior 1995.
The scientific importance of the HIF pathway is confirmed by the large majority of publications based on molecular approaches related to HIF. Importantly, 97 of the top-100 articles on hypoxia have been published since 1995 with 27 having > 1000 citations. Thus, the early 1990s period-characterized by a switch from non-molecular to highly molecular-focused research-could be regarded as the period "when mountains moved." This is further supported indirectly by the recent (end 1990s) increase in citations of the articles on hypoxia and cancer. Unsurprisingly, there is a large number of review articles (i.e., 17 in top-100 on "hypoxia"; 11 in the top-25 articles on "hypoxia and cancer") in the most-cited ones. This has to be taken into account since the publication of original data is associated with different citation patterns than the publication of review articles.
Surprisingly, although several molecular articles on cancer suggested clinical potentialities and the fact that the Nobel committee awarded the prize also for the potential clinical applications, to date none of the clinical articles reached the top-100 position yet. While there is an increasing body of literature on the pros and cons of hypoxic conditioning, most of these papers have been published very recently. Similarly, the articles on the application of hypoxia in athletes exhibit a comparable minor impact, although the most-cited article of this field by Levine and Stray-Gundersen [66] reached 493 citations. Of interest is the discrepancy between the continuous increase of citations in "sport sciences" journals ( Fig. 1) and the fact that the most-cited articles are almost exclusively in-vitro/in-cellulo molecular studies (Table 1). Our analysis shows that there is no decline in the (applied) physiology studies but the highly cited molecular studies "dilute" their numbers. Similar trends likely occur in many other "biomedical" fields.
One obvious limitation of this type of bibliometric analysis is the relevance of the citations number as metrics for assessing the research impact. Important articles for specific fields (e.g., mainly relevant for clinicians) may be cited more rarely than their (theoretical or practical) impact would suggest.
In conclusion, the discovery of the HIF pathway and of its molecular regulation in the early 1990s has revolutionized the research on hypoxia with the emergence of a considerably growing interest for its link to cancer. However, studies on clinical applications of hypoxia remain of comparatively lower interest yet but are promising.
Funding Open Access funding provided by Université de Lausanne.

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Conflict of interest
On behalf of all authors, the corresponding author states that there is no conflict of interest.

Ethical Approval Not applicable.
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