Introduction
The seasonal changes in 25(OH)D (vitamin D) concentration in blood plasma mainly concernsconcern the inhabitants of countries where the sunlight radiation varies according to the season (e.g. Northern Europe, Canada,Canada, and northern states of the USA) [Several authors [Vitamin D supplementation in young athletes is one of the most important factors protecting against its deficits. Skalska et al. [Numerous studies have analyzed the correlations between seasonal variations in vitamin D [The type of sport is another important factor determining the amplitude of changes in 25(OH)D concentration. The essential issue is whether the training sessions are conducted indoors or outdoors. The meta-analysis by Farrokhyar et al. [When the COVID-19 pandemic was announced at the beginning of March, outdoor physical activity was prohibited due to the pandemic restrictions introduced in most European countries. As a result, numerous athletes were forced to perform home-based training [Due to the important role of vitamin D in physiological processes during physical effort in young athletes with deficiency of this biomarker during some seasons, the project considering its changes collectively with chosen blood parameters in young soccer players was introduced.This research aimed to investigate the influence of vitamin D supplementation, sunlight radiationradiation, and home isolation during the COVID-19 pandemic on seasonal changes in 25(OH)D and selected biomarkers in young soccer players along a one1-year training cycle. It was hypothesized that due to the lower sunlight radiation, the highest drop in 25(OH) concentration will occur in the autumn and winter season. Therefore, vitamin D supplementation might effectively prevent its deficiency and negative changes in related blood parameters. However, the home-isolation factor was not part of the design because the COVID-19 pandemic occurred in March 2020 during the project.