**5. Conclusions**

We conclude that feed supplementation with potato fibre can enhance the apparent digestibility of Zn and Cu from the gu<sup>t</sup> of piglets, but it significantly decreased apparent total tract digestibility of Fe and Mn, and Fe apparent digestibility in the ileum. On the other hand, it seems that Zn glycinate is less soluble in the gu<sup>t</sup> than Zn sulphate, resulting in decreased Zn absorption and digestibility in the lower small intestine. Moreover, ZnGly can affect in vitro solubility of other TEs in the different phases of simulated digestion, which could lead to the changed absorbability of TEs. Apparent digestibility of Zn and Cu

was not affected by Zn or fibre source in the separate small intestinal segments, while Mn digestibility in the duodenum increased by ZnGly supplementation.

Although in vitro solubility of TEs did not provide a good prediction of mineral bioaccessibility because of rapidly decreased solubility of Zn, Fe, and Mn in the simulated small intestinal phase, using a combination of the results from in vitro and in vivo experiments might be used for predicting the relative bioavailability of TEs and might help to improve understanding of important factors involved in digestion and absorption processes involving TEs.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/agriculture12040517/s1, Figure S1: In vitro solubility of Zn from both Zn sources (ZnSO4, ZnGly); Table S1: Individual significant differences between parameters, in vitro solubility of Zn from both Zn sources (ZnSO4, ZnGly).

**Author Contributions:** Conceptualization, L'.G. and M.B.; methodology, L'.G. and K.T.; validation, L'.G. and M.T. (Marcin Taciak); formal analysis, L'.G., K.T., M.B. and A.T.; investigation, L'.G. and M.B.; data curation, L'.G. and M.T. (Margaréta Takácsová ); writing—original draft preparation, K.T. and L'.G.; writing—review and editing, L'.G. and M.T. (Marcin Taciak); project administration, L'.G. and K.C.; funding acquisition, K. ˇ C. All authors have read and agreed to the published version of ˇ the manuscript.

**Funding:** This research was funded the Slovak Research and Development Support Agency APVV, gran<sup>t</sup> number 17-0297 and by the Slovak Grant Agency VEGA, gran<sup>t</sup> number 2/0008/21. This study is based upon work from COST Action FA1401 (PiGutNet), supported by COST (European Cooperation in Science and Technology).

**Institutional Review Board Statement:** The study was conducted according to the guidelines of European Union Directive 2010/63/EU for animal experiments, and approved by the Local Animal Experimentation Ethics Committee (resolution number WAW2\_21/2016, Warsaw University of Life Sciences-SGGW, Warsaw, Poland) and Polish Law on Animal Protection.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The datasets used and analyzed in this survey are available from the corresponding author upon reasonable request.

**Acknowledgments:** We thank Andrew Billingham for providing language support that greatly improved the manuscript. Thanks go to Renata Gerocova and Peter Jerga for their excellent assistance in data collection.

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