2.6. Statistical Analysis
The minimal total sample size for a medium effect size at the power of 0.8 and significance level of 0.05 was calculated using GPower ver. 3.1.9.2. and provided a minimal population of 28 subjects.All data are presented as the mean ± standard deviation. To assess the impact of the ultramarathon combined with vitamin D supplementation on amino acid levels, a two-way (2 × 3) analysis of variance (ANOVA) with repeated measures was performed. The group (S, supplemented; C, control) was the first factor and was represented by participants administered a high dose of vitamin D or placebo, accordingly. The second factor was the time point concerning the ultramarathon run completion: I, 24 h before the run; II, immediately after the run; and III, 24 h after the run. If a significant interaction effect of the two factors was observed, Tukey’s post hoc test was performed.The assumptions of ANOVA, a normal distribution (Shapiro–Wilk test), and homogeneity of variance (Levene test) were verified and held. To evaluate the magnitude of the effect, the eta-squared statistic (η