Discussion
muscle damage
The main purpose of this research was to investigate the potential effect of vitamin D supplementation, sunlight radiationradiation, and home isolation on seasonal changes of 25(OH)D concentration and selected biomarkers during a one1-year training cycle in young soccer players. It was assumed that the highest drops in 25(OH)D concentration would occur in the autumn-–winter season when the sunlight radiation is low. Therefore, vitamin D supplementation could effectively prevent its deficiency and negative changes in related biomarkers. Furthermore, due to the unpredicted occurrence of the COVID-19 pandemic during the project, forced home isolation was recognized as an additional factor potentially influencing the results of analyzed blood parameters.To our bestthe best of our knowledge, this is the first research analyzing this topic in a one1-year training cycle in young soccer players. Previous studies by Andelković et al. [The results presented in our research confirmed the hypothesis concerning the seasonal changes in 25(OH)D concentration. The highest drops of this biomarker were noted in the autumn-–winter seasons. However, the results obtained after home isolation did not confirm the hypothesis of the longitudinal and preventive effecteffects of vitamin D supplementation. The analysis of selected blood parameters exhibited significant variations during the project, and the differences between GS and GP were not statistically significant.The seasonal changes of 25(OH)D concentration in athletes were previously demonstrated not only in the countries of Northern Europe [Several biomarkers related to seasonal changes in 25(OH)D, morphology, lipid profileprofile, and muscle damage were investigated in this research. In the previous research from this project [More dynamic changes in four time points along the one1-year training cycle were registered in RBC and HGB. The significant increase of HGB at the end of autumn (T2, Systematic monitoring of blood parameters allows applying the training loads precisely and avoids the detraining effect in professional soccer players [The results of the current study did not exhibit any significant changes in the lipid profile of the tested players (Creatine kinase is one of athletes’ most widely used muscle damage markers [Furthermore, Radzimiński et al. [Previous research [Biomarkers such as PTH, PP, and Ca are important parameters that complemented the results of our annual project and were described in the previous study [In the present research project, some advantages and one limitation are worth mentioning. In our opinion, the study duration and homogeneity of the group (players from one club, who were analyzed in terms of their nutrition) with the use of a double-blind design could be considered strengths of the current investigation. The main limitation of the project was the occurrence of the COVID-19 pandemic, which made it impossible to perform all the measurements according to the original design. The number of participants who completed the project was relatively low (