In the course of clustering, we identified four basic groups, from which we highlighted the most relevant 18 topics.
We present the 18 topic according to the following four aspect: the number of publications in the cluster (the size of the topic), the distribution of the publications based on publication date, the citation measure of the publications and the analysis of rank of publications within each topic.
The 18 topics are the following:
1) Football and physiology
2) Carbohydrate metabolism
3) Muscle physiology: alkalosis and acidosis
4) Muscle power enhancement and dietary supplementation
5) Fluid balance and hydration
6) Dietary intake and nutrition knowledge
7) Assessment of energy intake of athletes
8) Bone health, female athlete triad
9) Fluid intake strategy
10) Weight management
11) Nutritional Strategies and human skeletal muscle
12) Dietary nitrate supplementation
13) Oxidative stress and dietary supplement use
14) Dietary supplement use and doping
15) Oxidative stress, inflammation and dietary antioxidants
16) Training adaptation and nutritional strategies
17) Gut microbiota
18) Celiac disease
Scientometric Characteristics Of Topics
Size distribution of topics
Based on the absolute and relative size of the topics, that is based on the number of publications belonging to a cluster, we can get an insight into the significance of the explored topics and research trends in sport nutrition science. These data are displayed in Fig. 3., which, besides the number of publications in each cluster, also shows their composition in terms of the core publications identified through the targeted search as well as of the publications citing, and those cited by the core sample. In terms of weight (size), the most dominant topic, with ca. 300 publications, is muscle power enhancement and dietary supplementation, followed by topics with about 200–250 publications: Carbohydrate metabolism; Oxidative stress and dietary supplement use; Nutritional modulation of training adaptation; Dietary nitrate supplementation; Nutritional strategies and human skeletal muscle. This group is closely followed by the topic of Bone health, female athlete triad; Dietary intake and nutrition knowledge; and Assessment of energy intake of athletes, with about 200 publications. The group with 100–150 publications is led by the topics Fluid balance and hydration; and Weight management; moreover, here belong the themes of Football and physiology, the also hydration-related fluid intake strategy, as well as Dietary supplement use and doping. In the 50–100 range, the dominant topics with almost 100 publications are Muscle physiology: alkalosis and acidosis; Oxidative stress, inflammation and dietary antioxidants; also here belong the smaller but well-defined topics of Gut microbiota and Celiac disease. The full size of the topics greatly correlates with their role and importance in the core sample; that is, size ranking also reflects the extent to which the cluster is of “sport physiological” nature.
Chronological Distribution Of Topics
The distribution by publication date of the publications belonging to the identified topics, to put it another way, the chronological distribution of each cluster facilitates the assessment of the trends and relevance of each topic, of their “popular” or “winding down” nature. The distribution of the size of the topics is summarized in Fig. 4., which shows the number of documents belonging to each topic starting from 2010. Based on this, the trend graphs in the table provide an overview of the chronological dynamics of the topics. The trend graphs attest that each topic shows a growing tendency (that is, more and more articles appeared on the given topic in the past decade); each cluster is at its maximum size in the last examined year (2018). The differences between the topics appear in the characteristics and pattern of the increase. According to the latter, with a little simplification, we observe two types of increase: (1) the topic increases gradually from the beginning or middle of the decade, (2) after a relative “stagnation”, the topic shows a rising slope in the last few years. Type (1), gradual increase is most characteristic of the topics Muscle physiology: alkalosis and acidosis; Nutritional strategies and human skeletal muscle, Nutritional modulation and training adaptation. Type (2), sudden increase with a more moderate rising slope, spread over the last four-five years, appears in relation with the topics Football and physiology; Energy intake and nutrition knowledge, Nutritional Strategies and human skeletal muscle, fluid balance and hydration, Fluid intake strategy, Oxidative stress and dietary supplement use; Dietary supplement use and doping, Oxidative stress, inflammation and dietary antioxidants. In case of topics with a Type (2) the slope is rising and normally peaking in the last two years, these are the followings: muscle power enhancement and dietary supplementation, Assessment of energy intake of athletes, Weight management, Gut microbiota and Celiac disease. A “unique” cluster, with a somewhat different pattern from the two basic types is Carbohydrate metabolism, which, along with some fluctuation, shows a steady output in the topic (also with a slight increase); another is Bone health and Female Athlete Triad, which also shows a steady output with a rising slope in the last examined year.
Citation Measures Of Topics And Journal Ranks
Apart from chronological trends, the relevance and scientific significance of topics can be investigated through the use of these two scientometric indicators (1) the citation measures (of the publications) of individual topics, and (2) the prestige or rank of publications. Citation measures provides information about the scientific impact of the publications of a topic, which is a fundamental approach to the sport nutrition science- relevance of topics.
Instead of the raw citation number, publications are characterized by their position in the citation measures rank order for the research field, called percentile-measure. The clusters with the biggest impact are those whose average, that is, the characteristic value of the relevant articles, is situated closest to 0. Those with a mean below the value of 25 can be considered to have a high general citation index (they belong to the most cited, 25%, citation quartile). Based on this, practically all the identified clusters fall into the high citation measure range, both in terms of their mean, and of the majority (minimum 50%) of their publications. The topics with the biggest impact are: Nutritional Strategies and human skeletal muscle; Dietary nitrate supplementation. A similarly high composite impact is shown by the topics Football and physiology; Carbohydrate metabolism; Bone health and Female Athlete Triad; Weight management; Gut Microbiota.
The rank of publications in this case refers to the rank and recognition of the publishing journals, with the latter representing, in terms of the topics, the “quality” of knowledge transferred in them. To describe the rank and the “quality” of the knowledge content of topics, we applied the citation quartile system. The basis of the approach is the classification of publications into four quality classes according to the rank of the publishing journal. The so-called Q1 journals belong to the upper 25% of the journal rank of the field, Q2 journals belong to the upper 25–50% quartile, Q3 journals are in the lower 25–50%, and Q4, in the bottom 25%. Figure 5. shows the distribution of the articles of the identified clusters among the four classes for each theme. In this case also, we can state that almost all clusters include publications of high quality, inasmuch as in average, 50% of their publications are ranked Q1, and the majority of the rest of the publications belong to Q2, which can also be considered a satisfactory quality rank. Carbohydrate metabolism; Nutritional Strategies and human skeletal muscle; Dietary nitrate supplementation stand out (Fig. 5).
In what follows, utilizing the scientometric assessment of topics, we select the three most significant topic, and showcase their internal structure in terms of key thematic relations based on our text mining and core document methodologies. We illustrated the conceptual network of topics with a dendrogram, and we analyzed the three topic based on their interconnections from the aspects of sport nutrition, sport physiology, type of sport, dietary supplements and performance diagnostics. The collection of the 18 topics and their core papers are in an additional file. Using an advanced bibliometric method, we obtained the so-called core papers for each research topic, the most representative publications in the cluster [see Additional file 1].
Description of the nutritional strategies and human skeletal muscle cluster
This topic focuses on the relationship between sport performance and the metabolic pathways of skeletal muscle cells. Its most frequent issue is the role of PGC-1 alpha transcriptional regulation especially in relation to mitochondrial biogenesis, citrate synthesis, endurance performance, phosphocreatine as the creatine substrate of performance enhancement, the relation of insulin, free fatty acids and fatigue, protein and carbohydrates (first of all fructose), and also in relation to energy expenditure and regeneration. Depending on the type of the examined sport, endurance training stands out, but high-intensity interval training is also present with almost the same frequency. Of the biological components, the role of P38 MAPK protein kinase is worth emphasizing (Fig. 6).
Description of the dietary nitrate supplementation topic
The complex (sport-) physiological focus of the topic is on the relation network of blood circulation, oxidative metabolism and nitrogen metabolism - and supplementation (in relation to sport activities). Its primary topics are oxygen consumption (related to the concept of efficiency), fatigue, inorganic nitrate substrate related to vascular function, the stimulus-muscle contraction connection, vegetarianism, the enzyme that produces nitrogen oxide substrate, the relation network of hypertension and vasodilatation, inflammation and beta-alaine, and bloodstream. Depending on the type of physical activity, the theme is characterized by endurance training (especially related to oxidative stress and its biomarkers), but also the concept of diving response emerges, first of all related to oxygen uptake – and consumption, but also more loosely to respiratory control. Within further substrata, sodium nitrite, and, with lower frequency, creatine-nitrate emerges. Regarding indices, the use of the VO2 performance indicator becomes apparent (Fig. 7).
Description of the carbohydrate metabolism topic
From a sport physiological angle, this theme also revolves around the improvement of skeletal muscle function, mostly, however, in relation to carbohydrate metabolism. Its pivotal concepts and issues are lipid oxidation during exercise metabolism processes, nitrogen balance (especially after eating), lean body mass (-increase), aerobic capacity related mostly to fatigue and endurance training, glycemic index, muscle glycogen, and endurance training (and carbohydrate absorption). The last one of these is characteristic for the type of the examined sport; in terms of branch of sport, cycling (time check training) and running emerge. For the examined substrates and dietary supplements, as well as biologically active components, the role of lipid oxidation (time check cycling), and, less frequently of catecholamine (running), sport drinks (cycling) and fructose (endurance training) as well as creatine kinase emerges (Fig. 8).