**5. Conclusions**

In the present study, data related to the metabolomics of saliva and serum in trained standardbred horses have been provided for the first time. Several molecules are common to metabolic pathways already described in human athletes (e.g., energetic metabolism). However, the findings of 2-hydroxyisobutyrate and 3- hydroxybutyrate in serum and GABA in equine saliva, as well as their modifications following exercise, provide new insights about the physiology of exercise in athletic horses. Furthermore, the use of metabolomic analysis allowed to identify metabolites like glycerol that might represent novel biomarkers for fitness evaluation in sport horses. However, further investigations are needed to validate the metabolites herein found as indices of fitness levels in equine species.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2218-1989/10/7/298/s1, Table S1: Serum metabolites with no significant difference in concentrations before and after exercise (μmol/L; mean ± SD), Table S2: Salivary metabolites with no significant difference in concentrations before and after exercise (μmol/L; mean ± SD).

**Author Contributions:** Conceptualization, F.L; L.L and M.B; methodology, F.L., C.Z. and L.L.; formal analysis, L.L. and C.Z.; investigation, E.L. and S.S.; resources, S.S and B.T.; writing—original draft preparation, F.L., L.L. and M.B.; writing—review and editing, F.L., L.L., M.B. and B.T.; supervision, F.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** Chenglin Zhu gratefully acknowledges financial support from Chinese Scholarship Council (grant number 201606910076).

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