Improvements in Aerobic Fitness With Exercise Training: the Role of Myokines

For both healthy adults and patients with cardiovascular disease (CVD), aerobic fitness (V̇O2max) is a stronger predictor of the risk of future chronic disease and premature death than other established risk factors such as hypertension, smoking, or Type 2 diabetes. Considering the large medical and economic burden of physical inactivity-related chronic disease it is important to improve the understanding of the regulation of V̇O2max to enable optimisation of interventions aimed at increasing V̇O2max in the current predominantly sedentary population. Furthermore, there is a need to identify novel drug targets to aid pharmacological intervention in those individuals who are unwilling or unable to improve V̇O2max through exercise. Currently, only exercise training is a viable method for increasing V̇O2max. However, although exercise training on average improves V̇O2max, \~10-20% of people who follow fully supervised, standardised training interventions do not demonstrate a measurable increase in V̇O2max. Low response to training is a clinically relevant concern, but the large variability in response to exercise training also provides an opportunity to dissect out the molecular mechanisms responsible for adaptations to V̇O2max by contrasting low vs. high responders to training. Using this approach it has previously been shown that in skeletal muscle samples obtained pre- and post- training, 86 genes are differentially regulated in high compared to low responders for V̇O2max. Strikingly, out of these 86 genes, 13 genes encode proteins that have been reported to be released by muscle during or after exercise (i.e., 'messenger proteins' termed myokines). This strongly suggests low responders to exercise training fail to sufficiently upregulate the production and release of these myokines, and that this is (at least partly) the reason why these people do not manage to improve their V̇O2max as much as high responders. However, in order to confirm a mechanistic role of these myokines in increasing V̇O2max, it needs to be demonstrated that beside the change in gene expression, the change in the plasma levels of these myokines is also impaired in low responders for V̇O2max.