**5. Conclusions**

Available data on the efficacy of vitamin supplementation during weight loss are inconclusive and mostly limited to adults. Our study shows that there is a limited or no effect of vitamin D supplementation on body weight reduction in children and adolescents with vitamin D insufficiency. Being aware and understanding the potential limitation of our study—wide age group, one dose of vitamin D, small sample of the study we believe that further research in this field is needed.

**Author Contributions:** M.B. participated in the study design and coordination of the study, performed the statistical data analysis, drafted the paper and revised and approved the manuscript; A.J., M.S.-F., P.M., and P.W. participated in the study design, patient recruitment, data collection, revised and approved the manuscript; P.S. participated in the study design and revised and approved the manuscript; A.S.-S. participated in the study design and coordination of the study, data analysis, drafted, revised and approved the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** The study was awarded a financial grant from a charitable foundation (Fundacja Nutricia) Grant no RG–1/2015. The Fundacja Nutricia did not take part in any element of study design or study realization such as the study design or; collection, management, analysis, and interpretation of data; writing of the report; and the decision to submit the report for publication.

**Acknowledgments:** All authors would like to sincerely thank all participant, parents and members of the "6–10–14 for Health" team for their input into the study. Authors would also like to thank University Clinical Centre administration for help in performing the study.

**Conflicts of Interest:** The authors declare no conflicts of interest.
