Exercise training, physical fitness and the growth hormone-insulin-like growth factor-1 axis and cytokine balance.

The effect of exercise training on anabolic hormones and inflammatory mediators is particularly important during childhood and puberty, since during this period there is a spontaneous increase in anabolic hormones that leads to the marked puberty-related growth spurt. Therefore, any training-associated hormonal and/or inflammatory effect during this critical period may have profound consequences on growth and development, especially if the effect is maintained for long periods. Several studies suggest the hypothesis that a sudden imposition of a training program which is associated with substantial increase in energy expenditure leads initially to an increase in pro-inflammatory cytokines, and as a consequence, to decreases in IGF-1 levels. Further, if the training adaptation is successful, the proinflammatory cytokines fall, and with that decrease, the suppression of IGF-1 diminishes, an anabolic 'rebound' in the GH-IGF-1 axis may ensue, and IGF-1 level exceed the pretraining level. Exactly how and when this switch takes place, and whether the initial catabolic-type stage is necessary for the ultimate anabolic adaptation remains unknown. Consistent with the two phases hypothesis, longer periods of training were indeed associated with stable or with increases in circulating GH and IGF-1 levels. Despite the early training-associated decrease in circulating IGF-1 levels, there is an increase in muscle mass and fitness may improve, suggesting that the local tissue effect of exercise on growth factors differ from systemic effects. Total caloric intake as well as macronutrient content, consumption and timing influence the anabolic and inflammatory response to training. Finally, changes in the balance of anabolic and catabolic hormones and inflammatory mediators during the training season may help elite athletes and their coaches in their training and preparation for competition.