Conclusions
The results presented in this research confirmed the hypothesis of seasonal changes in 25(OH)D concentration during four seasons. It was exhibited that these changes are dependent on the sunlight radiation expressed in UV units. The applied 8-week vitamin D supplementation had no extended effect on the level of 25(OH)D concentration. Moreover, the home isolation due to the COVID-19 pandemic reduced the athletes’ sunlight exposure and could negatively influence endogenous vitamin D synthesis. Although the seasonal changes in 25(OH)D concentration were significant, they did not affect most of the analyzed biomarkers. Only a number of WBC was significantly related to 25(OH)D concentration. Changes in parameters such as HGB, ASTAST, and CK were statistically significant for the whole group (