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Seven years ago, irisin was fi rst announced as a new and exciting myokine, which was secreted by the skeletal muscle in response to exercise and contributed for some of the health benefi ts promoted by physical activity. It was suggested that exercise stimulates the increase of the amount of the coactivator PGC-1α, which subsequently increases the expression of FNDC5, a membrane protein that is proteolytically cleaved to form irisin. Once released into the bloodstream, irisin connects to a yet unknown receptor on the surface of white fat cells and promotes a special process known as "browning" of white adipose tissue, which increases thermogenesis and the energy expenditure. Since its discovery, great hopes have been raised, and the association between irisin and these abovementioned functions led this myokine to be considered a potential therapeutic weapon in the fi ght against obesity and other related metabolic disorders, such as diabetes. However, concerns about its presence, regulation and the yet-to-be fully understood functions associated with inconstant results, put the future of irisin in doubt. Therefore, caution needs to be taken in expressing optimism, and the near future will be a challenge for irisin's ability to survive as a useful tool in the treatment of metabolic diseases. Meanwhile, new associations between irisin, neoplastic, cardiovascular and neurodegenerative diseases are being deduced and further studies will help to clarify the role of irisin in humans.

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The GH and insulin-like growth factor-I (IGF-I) axis is not only involved in the regulation of somatic growth, but also in glucose metabolism. During fasting and stress, GH secretion is increased and these conditions may be viewed as the natural metabolic domain for GH action. GH decreases glucose uptake in adipose tissue and regulates the glucose transporter I (GLUT-I) in adipose-tissue-derived cell lines. 2 GH may antagonize adipocyte insulin action. At hepatic level, GH increases glycogenolysis, thereby increasing endogenous glucose production (EGP), which could possibly be as a result of insulin antagonism. GH-deficient children have reduced fasting plasma glucose (FPG) levels, impaired glucose tolerance (IGT), and increased insulin sensitivity due to increased glucose utilization and diminished EGP. 3,4 GH replacement increase FPG, insulin levels and EGP. 4 GH-deficient adults have elevated fasting insulin levels and show a positive correlation between fasting plasma insulin and both fat mass and waist girth, suggesting the presence of insulin resistance. GH replacement initially increases insulin resistance even more, during the first 1-6weeks of therapy, but long-term studies suggest that this subsequently reverts to unchanged insulin-sensitivity. 5 Developmental models of GH deficiency and excess indicate that GH is positively associated with β-cell mass. The reduction in GH levels observed with age and weight gain may contribute to the age-related decline in pancreatic β-cell function. Humans with long-term adult-onset GH deficiency, or with developmental isolated GHD, show IGT, 6,7 and may have an increase prevalence to diabetes mellitus (DM). 8