Introduction: It is known that telomere length and mitochondrial DNA copy number are affected by physical fitness level. However, it remains unclear to what extent the level of aerobic fitness induced by long-term exercise training influences telomere length and mitochondrial DNA copy number in adolescent athletes. In addition, the inclusion of these data in the same resource may lead to a more coherent interpretation of the study results in young athletes. Furthermore, analysis using non-invasive methods such as saliva is likely to provide rapid feedback to athletes, coaches, and other sports professionals. Accordingly, the purpose of this study was to examine the association of cardiorespiratory fitness with salivary telomere length and mitochondrial DNA copy number in adolescent endurance athletes. Methods: Thirteen healthy adolescent cross-country skiers [males (n=7) and females (n=6), age: 16.6±1.1 years, height: 164.9±7.5 cm, body mass: 56.1±6.9 kg, body mass index: 20.8±1.5 kg/m2, body fat: 16.5±6.1 %, maximum oxygen uptake: 65.6±6.1 ml/kg fat-free mass/min (mean±SD)] took part in this study. All participants were athletes who had been regularly engaged in endurance training for at least two years and were active at the national level. They trained at least six times a week for two to three hours per session. Saliva specimens were taken using a saliva collection kit prior to determining cardiorespiratory fitness. Saliva was used to extract DNA, which was then analyzed for telomere length and mitochondrial DNA copy number using real-time PCR. The maximal oxygen uptake of each participant was measured on a treadmill using the incremental load method to determine their aerobic capacity. Results: A significant positive correlation was observed between cardiorespiratory fitness and mitochondrial DNA copy number (r=0.775, p<0.01), but not telomere length (r=0.351, p=0.24), in adolescent endurance athletes. Discussion: There is still a lack of information as to whether there is a causal relationship of cardiorespiratory fitness (enhanced by endurance exercise training) with telomere length and/or mitochondrial copy number in humans. However, a summary of previous studies suggests that moderate-intensity exercise may maintain telomere length and increase mitochondrial copy number, rather than low- or extremely high-intensity exercise (below or above an optimal threshold). In this study, the main findings of this study imply that training volume (increased aerobic capacity) in adolescent endurance athletes may have been at optimal levels, resulting in the maintenance of telomere length and the increase in mitochondrial copy number. Moreover, the results of the present study suggest that mitochondrial copy number from saliva samples may be a much more relevant biomarker than telomere length for monitoring cardiorespiratory fitness in aerobically trained young athletes. Impact/Application to the field: In sports settings, rapid and informative feedback regarding the condition of athletes is desirable. Thus, mitochondrial copy number appears to be a more promising candidate than telomere length as a more sensitive and important biomarker for determining physical fitness in a non-invasive manner using saliva. Declaration: My co-authors and I acknowledge that we have no conflict of interest of relevance to the submission of this abstract.
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