**3. Results**

The characteristics of the studied group and vitamin D status throughout the intervention are presented in Table 1. While comparing the serum 25(OH)D level, it was stated that the results di ffered throughout the study (*p* = 0.0001; Kruskal-Wallis ANOVA), but for multiple comparisons, it was stated that the results after 8 weeks of intervention did not di ffer from the results at baseline (*p* = 0.7964). However, during phase I of the study (weeks I-IV), a significant decrease of serum 25(OH)D level was observed (*p* < 0.0001 for the comparison of t0 and t4 results), whereas during phase II (weeks V-VIII), a significant increase was noted (*p* = 0.0005 for the comparison of t4 and t8 results).


**Table 1.** Characteristics of the studied group accompanied by vitamin D status throughout the study.

\* non-parametric distribution (verified using Shapiro–Wilk test; *p* ≤ 0.05); 25(OH)D—25-hydroxyvitamin D; t0—baseline; t4—after 4 weeks of intervention; t8—after 8 weeks of intervention.

The dietary vitamin D intake in the studied group throughout the study is presented in Table 2. It was observed that, at baseline, the dietary intake of vitamin D was significantly lower than those assessed after four weeks of intervention (*p* < 0.0001), as well as after eight weeks of intervention (*p* < 0.0001). At the same time, the dietary intakes of vitamin D assessed after four weeks of intervention and those assessed after eight weeks of intervention did not di ffer (*p* = 0.5758).

**Table 2.** Dietary vitamin D intake in the studied group throughout the study.


\* non-parametric distribution (verified using Shapiro–Wilk test; *p* ≤ 0.05); t0—baseline; t4—after 4 weeks of intervention; t8—after 8 weeks of intervention.

The assessment of the adequacy of vitamin D status throughout the study is presented in Table 3. While comparing the share of participants characterized by adequate and inadequate serum 25(OH)D level, it was stated that it did not di ffer between baseline assessment and assessment after eight weeks of intervention (*p* = 1.0000, chi2). At the same time, the share of respondents characterized by inadequate serum 25(OH)D levels increased during phase I of the study (*p* = 0.0002 for the comparison of t0 and t4 shares, chi2), but decreased during phase II (*p* = 0.0002 for the comparison of t4 and t8 shares, chi2).

**Table 3.** Assessment of adequacy of vitamin D status throughout the study while compared with the reference values.


\* reference values of 50 nmol/L and 250 nmol/L [11,25,36,37].

The analysis of correlations between age or BMI and vitamin D status throughout the study are presented in Table 4. It was stated that neither age nor BMI influenced 25(OH)D level during the study, as well as its changes, apart from the baseline 25(OH)D level which was significantly correlated with BMI (*p* = 0.0419; R = −0.2980).


**Table 4.** Analysis of correlations between age, or BMI, and vitamin D status throughout the study.

\* Spearman rank correlation coefficient for non-parametric distributions (for parametric distribution Pearson correlation coefficient applied); 25(OH)D—25-hydroxyvitamin D; t0—baseline; t4—after 4 weeks of intervention; t8—after 8 weeks of intervention.

The analysis of correlations between vitamin D status before the intervention and its later levels and changes throughout the study is presented in Table 5. It was stated that the 25(OH)D level before the intervention was associated with its levels during the intervention (*p* < 0.0001). It was also associated with 25(OH)D changes from the baseline values (*p* < 0.0001), but not with the following changes (from t4 to t8).

**Table 5.** Analysis of correlations between vitamin D status before intervention and its later levels and changes throughout the study.


\* Spearman rank correlation coefficient for non-parametric distributions (for parametric distribution Pearson correlation coefficient applied); 25(OH)D—25-hydroxyvitamin D; t0—baseline; t4—after 4 weeks of intervention; t8—after 8 weeks of intervention.

The comparison of the vitamin D status throughout the study between subgroups characterized by various baseline statuses is presented in Table 6.

For the serum 25(OH)D level, all differences were statistically significant (*p* ≤ 0.05) and higher levels were observed for participants characterized by adequate baseline status. However, for the serum 25(OH)D level, for participants characterized by inadequate baseline status, lower decreases from t0 to t4 and higher increases from t4 to t8 were noted, when compared to participants with adequate baseline status.


**Table 6.** Comparison of vitamin D status throughout the study between subgroups characterized by various baseline statuses.

\* Distribution differs from normal; \*\* Student's t-test (if normal distribution) or Mann–Whitney U test (if distribution differs from normal); 25(OH)D—25-hydroxyvitamin D; t0—baseline; t4—after 4 weeks of intervention; t8—after 8 weeks of intervention.

25(OH)D change from t0 to t4 (nmol/L) −5.0 ± 13.0 −8.5 (−21.2–32.9) −17.4 ± 24.0 −16.4 (−53.7–41.3) 0.0501 25(OH)D change from t4 to t8 (nmol/L) 15.9 ± 12.6 15.1 (−0.3–37.0) 8.4 ± 11.2 7.1 (−18.5–38.8) 0.0381 25(OH)D change from t0 to t8 (nmol/L) 10.9 ± 16.0 8.2 (−10.3–52.0) −9.0 ± 23.9 −6.7 (−52.0–53.0) \* 0.0005

> Change of serum level
