Study design
The study employed a cross-sectional study design. The research was conducted on 40 young soccer players between the middle of September 2019 and the end of August 2020. It was assumed that the amount of sunlight where participants were staying would not influence the synthesis of vitamin D in the period from early autumn to the end of the winter. Moreover, the radiation of sun rays could increase vitamin D synthesis from the beginning of spring. During the analyzed period, all the players performed the same training program involving technical and tactical drills, official and friendly matches, or exercises developing aerobic capacity, speed, strength, and power. Within a one1-year training cycle, the volume and the intensity of the training varied depending on the part of the season. The competition season training process was planned according to the next game and included phases of intentionally increasing or decreasing the training load. A typical weekly training load in the competition period contained six training sessions for all players, three for individual training (defenders, middle players, and strikers)strikers), and one league game (12–15 h a week). During the preseason, a large number of general training drills were used along with the soccer-specific exercise. Therefore, the preparation period contained a higher training volume, and several drills were performed with high intensity (Training load (min) taking into account type (overall and special drills) and intensity (aerobic, or anaerobic) of exercises of typical weekly training during competition and preparation period of one1-year training season of young soccer players.