Individual variability in skeletal muscle metabolic response to endurance training in humans

Regular endurance training increases skeletal muscle aerobic capacity, but there is considerable variability between individuals in the magnitude of the training response. The present study investigated inter-individual variability in adaptations of enzymes involved in fatty acid and glucose metabolism. The local ethics committee approved the study and 20 untrained young women gave written informed consent to participate. Biopsies were obtained from the vastus lateralis muscle before and after six weeks of endurance cycle training (3 x 45 min sessions/wk). Western blot analyses were used to measure the expression of a) the mitochondrial enzymes succinate dehydrogenase (SDH), cytochrome C oxidase (COX1) and ATP synthase (ATPsyn); b) the proteins involved in fatty acid metabolism 3-hydroxyacyl CoA dehydrogenase (HAD) and transporter CD36 (CD36) and c) those involved in glucose metabolism; phosphofructokinase (PFK) and glucose transporter 4 (GLUT4). As the data were not normally distributed non-parametric statistical tests were used. The maximal oxygen uptake (10%) and protein expression of SDH (47%), COX1 (52%), ATPsyn (63%), HAD (69%) and CD36 (86%) were all significantly elevated (all P < 0.05), while PFK (29%) and GLUT4 (18%) did not change with training. There was, however, considerable inter-individual variability in training response of each protein and this was examined in relation to the delta SDH, since fatty acid and glucose metabolism converge in the mitochondria. Delta SDH correlated with the training-induced changes in COX1 (r = 0.63), HAD (r = 68), CD36 (r = 0.85), PFK (r = 0.86) and GLUT4 (r = 0.80) (all P < 0.01), but not with delta ATPsyn (r = 0.38, P = 0.099). High and Low groups (n = 6 per group) were classified as those individuals having a delta SDH in the upper or lower third, respectively, of the mean of the group as a whole. The absence of a significant group x protein interaction (repeated measures ANOVA) indicated that the changes in expression of enzymes involved in fatty acid and glucose metabolism showed similar responses within groups, but different responses between groups (Fig. 1). This was also true if delta COX1 was used instead of delta SDH as a marker of mitochondrial adaptations. In conclusion, the data indicate a coordinated response of mitochondrial enzymes and proteins associated with fat metabolism in response to endurance training. The variable response between individuals may indicate genetic differences in the coordinating signal(s)