Aspects of regular long-term endurance exercise in adolescents, with focus on cardiac size and function, hormones, and the immune system

The long-term effects of starting high-intensity training at younger ages are largely unknown. The present studies focused on adolescents who had performed regular endurance exercise for several years at an elite level and compared those subjects with a control group of adolescents of similar age and sex who had not engaged in regular exercise. The knowledges generated by this research will contribute to further understanding of some of the physiological effects of strenuous regular exercise during adolescence. Aim: The overall aim of this research was to investigate endurance-trained adolescents, focusing on cardiac size and function, hormones associated with growth and metabolism, and impact on the immune system. Methods: All participants underwent echocardiography at rest as well as immediately and 15 minutes after amaximal cardiopulmonary exercise test. Blood samples were taken at rest and analyzed for biomarkers such as hormones, immune cell surface markers, and secreted cytokines and chemokines. The study design was crosssectional (Papers I, III, and IV) and comparative, with a quantitative approach in all four studies. The evaluationin Paper II used a pre-post test design with measurements of cardiac parameters before and after a maximal treadmill test. The studies in Papers I–III compared endurance-trained (active group) and untrained (controls) adolescents matched by age and sex, whereas the analysis in Paper IV considered differences between the sexes in the endurance-trained adolescents. Results: Compared with controls, the endurance-trained adolescents showed increased size of all four heart chambers, as well as improved cardiac systolic function at rest. Considering cardiac changes from baseline to immediately after exercise, the systolic and diastolic patterns were similar in both groups, although the diastolic function was more enhanced in the active group. Strong associations between peak oxygen uptake and cardiac size and function could be seen both at rest and after exercise. Circulating hormones at rest did not differ between the two groups. No correlation between insulin-like growth factor 1 and cardiac size was found among the endurance-trained adolescents. Compared with endurance-trained girls, endurance-trained boys exhibited an elevated response to a potent type 1 diabetes-related autoantigen. Conversely, compared with the trained boys, the trained girls showed an increased number of circulating immune cells and increased secretion of C-peptide and proinsulin. Conclusions: There are many benefits associated with regular exercise, and the present research did not provide any data to oppose that statement. Factors such as increased cardiac size at rest and exercise-related effects on cardiac functions do occur and therefore should be expected in endurance-trained adolescents with high peak oxygen uptake. Indeed, this should be interpreted as a sign of physiological adaptation and not as pathophysiology. The greater cardiac dimensions observed in these subjects could not be related to increased resting levels of hormones associated with growth and metabolism. The endurance-trained adolescents did show some sex-related differences with regard to their immune response at rest