2.5. Statistical Analysis
A power analysis was performed before the commencement of this study using G*Power [All data were analyzed using intent-to-treat (ITT) analysis. Results are presented as mean ± standard deviation (M ± SD). Outliers were analyzed using box plots and defined as values beyond the 1.5-fold interquartile range (IQ).The normality and homogeneity of variances were assessed using Shapiro–Wilk’s and Levene’s tests. One-way ANOVA with Tukey’s post hoc analysis was conducted to analyze the differences between the four groups at a specific measurement time point. Repeated-measures ANOVA with Bonferroni-adjusted post hoc analysis was conducted to detect differences in 25(OH)D concentrations within the groups over time.Repeated-measures analysis of covariance (ANCOVA) in conjunction with a Bonferroni post hoc test was performed to evaluate the differences in 25(OH)D concentrations among the groups over time, controlling for confounding variables, such as BMI, training volume, and age.The Pearson correlation coefficient was calculated to identify correlations between 25(OH)D concentrations and possible influencing factors, such as BMI, training and competition volume, or nutritive vitamin D uptake.The Eta-squared statistic (η