Results
The typical training volume in a competitive season was 720 minutes720 min a week and mainly involved aerobic intensity exercise (53.5% of totalthe total time) and mixed aerobic-–anaerobic intensity drills (34.7%). During the pre-season period (partially due to the sports camp occurrence), the weekly training volume increased to 810 minutes810 min. However, the percentage distribution of the training loads was similar to the in-season period (The scope of ultraviolet (UV) radiation during the analyzed period is presented in The analysis of the selected blood parameters in the total groups of 24 players in four time-pointstime points from four different seasons demonstrated that the largest difference was in blood plasma’s 25(OH)D concentration. The highest level of this biomarker was noted at the final stage of the project (in the late summer). Moreover, this value was significantly (Changes in selected blood parameters in young soccer players (*Significantly different from T1; The two-way ANOVA demonstrated the significant (Correlations between 25(OH)D concentration and white blood cells in young soccer players (Changes in selected blood parameters in young soccer players in the supplemented (GS) and placebo group (GP) in the T2, T3T3, and T4.*Significantly different from T2; †Significantly different from T3; WBC-: white blood cells; RBC-: red blood cells; HGB-: hemoglobin; HCT-: hematocrit; PLT-: thrombocytes; TC-: total cholesterol; HDL-: C-high-density cholesterol; LDL-C-: low-density cholesterol; TG-: triglycerides; ALT-: alanine aminotransferase; AST-: asparagine aminotransferase; CK-: creatine kinase; pη2-: partial eta square; OP-: observed power.