GSTM1 and chronic kidney disease: Systemic oxygen delivery factors contribute to explain the variation in aerobic exercise capacity also in CKD, but besides stroke volume, hemoglobin (Hb) level and peak heart rate seem to be important determinants (Chinnappa et al., 2018; Howden et al., 2015; Odden et al., 2004; Wallin et al., 2018).