Serum leptin, insulin-like growth factor-I components and sex-hormone binding globulin. Relationship with sex, age and body composition in healthy population.

Leptin plays an important role in the regulation of food intake and thermogenesis, regulates long term energy balance and reproductive function and its concentrations are closely linked to body mass index. Leptin secretion is influenced by many factors and the age-related changes in different hormones might modify circulating leptin concentrations. Sex dimorphism in leptin concentrations has been clearly shown in previous studies and its concentrations were lower in men than in women in all decades of life. Insulin growth factor-I (IGF-I) is a peptide growth factor that is present in all types of physiologic fluids and is also produced by connective tissue cell types and its autocrine/paracrine secretion is nearly always present within tissues. There is a physiological decline of the growth hormone (GH)/IGF-I axis with ageing and in addition, insulin, thyroid hormones and the supply of dietary energy may directly regulate the circulating levels of the IGFs and growth hormone binding protein (GHBP). Furthermore, there is no doubt that GH participates in the regulation of body composition, and with advanced age there is a decrease in muscle and an increase in adiposity associated with a decline in GH and total IGF-I. The biological activities of the IGF ligands are modulated by the family of high affinity GHBP. Sex hormone binding globulin (SHBG) concentrations are thought to be regulated primarily through opposing actions of sex steroids on hepatic SHBG production, with oestrogen stimulating and androgen inhibiting SHBG production, and thyroid hormones are also a potent stimulator of SHBG production concentrations. Some studies support an independent IGFBP3 contribution to SHBG variability and these findings are compatible with the hypothesis that some of the anabolic effects ascribed to the GH/IGF axis may be caused by SHBG-mediated changes in testosterone activity or SHBG/total testosterone index.