In terms of the metabolites, exercise could elevate the kynurenine aminotransferase level in skeletal muscles through the PGC-1α1-PPARα/δ pathway, which then reduced the blood level of kynurenine, a compound able to permeate the blood-brain barrier and cause depression (Agudelo et al., 2014). This evidence concerns the gene PPARA and depressive symptom measurement.