*4.4. Statistical Analysis*

All analyses were performed in duplicate. The normality of the quantitative variables was evaluated using the Shapiro–Wilk *W* test. The comparison of anthropometric indices at baseline between Synergy 1 and the placebo group was performed using a parametric Student's *t*-test. As the VOC data showed non-normal distribution, quantitative variables were expressed as median values (P25–P75). Differences in the concentration of individual VOCs between Synergy 1 and the placebo group were tested with the non-parametric Mann–Whitney *U* test. VOC concentrations within the group, before and after the intervention, were compared using the Wilcoxon signed-rank test. Results were considered statistically significant at the 5% critical level (*p* < 0.05). Exploratory data analysis using PCA was carried out to interpret the complex data and to determine if the differences between experimental groups could be seen. Both univariate and multivariate analyses were performed using XLSTAT for Excel software.

### **5. Conclusions**

In summary, this pilot study indicated that oligofructose-enriched inulin, applied as a supplement of GFD for 12 weeks, had a moderate impact on the concentrations of VOCs in the headspace above the urine of children and adolescents with CD. It is possible that the prolongation of the study may result in a more dominant effect. Further studies are needed to confirm the effect of prebiotics on gut integrity and, consequently, on the profile of VOCs in different biological fluids. Moreover, the origin of the VOCs in the human body requires further examination.

**Author Contributions:** N.D. conceived and designed the study presented here; U.K.-K. conceived the original clinical trial; N.D., E.J.-C., and U.K.-K. designed the original study; N.D. and E.J.-C. collected samples; N.D. performed all experiments, was responsible for methodology, analyzed and interpreted the data, prepared visualization of data, acquired funding, and administrated the PRELUDIUM Project; N.D., E.J.-C., and U.K.-K. provided resources; N.D. wrote the manuscript; N.M.R. helped in the interpretation of the data and critically revised the manuscript; all authors reviewed and approved the final version of the manuscript.

**Funding:** This research was funded by the Polish National Science Centre through project PRELUDIUM 11 (project number: 2016/21/N/NZ9/01510).

**Acknowledgments:** We wish to sincerely thank all patients who participated in this study. The samples were collected in the study supported by statutory funds of the Department of Chemistry and Biodynamics of Food of the Institute of Animal Reproduction and Food Research, Polish Academy of Science (GW20).

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