## 5 Conclusions  
In summary, the presented data revealed the usefulness and safety of chronic inhalation with both the bioactive form of vitamin D3 (1,25(OH)\({}_{2}\)-vitamin D3) as well as its precursor (25(OH)-vitamin D3) in the restoration of the physiological calcitriol level in the lung tissue and serum of mice with vitamin D deficiencies. At the same time, it indicated that the desired effect required the use of a 20-times lower concentration of calcitriol than its precursor, which provides important advice for future studies. Moreover, the obtained results clearly demonstrated that the calcitriol serum level cannot be used to predict this metabolite concentration in lung tissue. It needs to be highlighted that the presented study is the first one that revealed the physiological concentration of calcitriol in the lung tissue of the mice strain C57 BL, which is commonly used in research, in particular, dedicated to pulmonary fibrosis. Furthermore, in the light of the increasing negative health consequences of vitamin D3 deficiencies, the information provided in the paper regarding the induction of this disorder in mice could be useful for scientists focused on this phenomenon.  
We hope that the presented results will encourage further research on the use of inhaled vitamin D3 metabolites in the prevention and treatment of lung diseases associated with calcitriol deficiency, especially since in the case of these diseases this route of drug administration is one of the cheapest, fastest, safest, and most convenient, as well the most acceptable methods by patients with respiratory disorders.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app131910672/s1](https://www.mdpi.com/article/10.3390/app131910672/s1)]([https://www.mdpi.com/article/10.3390/app131910672/s1](https://www.mdpi.com/article/10.3390/app131910672/s1)), Table S1: Alterations in calcitriol concentrations in murine lung tissue and serum in response to inhalations with 1,25(OH)2-vitamin D3 or 25(OH)-vitamin D3 as well as diets with standard or reduced amounts of chobecaliferol. Data are presented as a mean of calcitriol concentration \(\pm\) SEM.  
Conceptualization, M.K.L.; methodology, M.K.L.; formal analysis, M.C. and M.K.L.; investigation, M.C., J.A., I.L. and M.K.L.; writing--original draft preparation, M.K.L.; writing--review and editing, M.K.L.; visualization, J.A., I.L. and M.K.L.; supervision, M.K.L.; project administration, M.K.L.; funding acquisition, M.K.L. All authors have read and agreed to the published version of the manuscript.  
The research was supported by a grant from the National Science Centre, Poland [Grant No. UMO-2020/38/E/NZF/00366, 2021].  
The experimental protocols were approved by the Local Ethics Committee for Animal Experimentation in Lublin, Poland (Resolution Nos. 28/2021, 25/2022, and 73/2022).  
Not applicable.  
The data supporting reported results can be found in the laboratory databases of the Institute of Rural Health. The data are available on request from the corresponding author.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.