3. Results
A total of 90 athletes with an average training volume of 7.4 ± 4.1 h per week, an average age of 26 ± 5 years, and an average BMI of 23 ± 3 were recruited for the first vitamin D measurement. At baseline, 67 athletes (74%) had an insufficient vitamin D status (25(OH)D level < 30 ng/mL); whereas, a total of 23 (26%) had a sufficient vitamin D status (25(OH)D level ≥ 30 ng/mL). One-way ANOVA revealed no significant difference in the mean baseline 25(OH)D concentrations between the INDV, STAD, and CON groups (17.5 ± 7.6 vs. 17.0 ± 7.2 vs. 21.9 ± 6.3 ng/mL). Compared to these, the baseline 25(OH)D concentration of the SUF group was significantly higher at 40.5 ± 8.2 ng/mL (Within the entire 10-week study protocol, the mean 25(OH)D level significantly increased by 20.4 ng/mL in the INDV group (In total, 50% of the INDV study group volunteers reached the target concentration of 40 ng/mL at the pre-calculated time point and 92% reached 30 ng/mL. However, after the total 10-week supplementation protocol, only 30% showed a 25(OH)D concentration of at least 40 ng/mL and 78% showed a 25(OH)D concentration of 30 ng/mL or more.In the STAD group, 17% showed a vitamin D level of 40 ng/mL or higher after 5 weeks and 38% after 10 weeks. On average, 41 ± 12 days elapsed until the insufficient vitamin D status was corrected in the INDV group.In the CON group, 25(OH)D concentration decreased over the 10-week study period (−3.8 ng/mL). The 25(OH)D concentration significantly decreased in the first 5 weeks from 21.9 ± 6.3 to 17.7 ± 6.2 ng/mL (In the SUF group, 25(OH)D significantly decreased in the first 5 weeks (−8.9 ng/mL, Repeated-measures ANCOVA in the supplementation groups revealed a significant difference in 25(OH)D concentration over time, independent of the group, when controlling for training volume (F(2,60) =5.24, There was a negative correlation between the weekly average training volume and the increase in 25(OH)D during the 10-week supplementation protocol (r = −0.34, In the CON and SUF groups, there was a negative correlation between the baseline concentration and the decline in 25(OH)D during the 10-week observation period (r = −0.45, Nutritional vitamin D intake was calculated using the means of the different chosen foods available in Germany. The average daily vitamin D uptake was 3.6 ± 2.3 μg/day.Dietary vitamin D intake did not affect the total 25(OH)D concentration (r = −0.23, 