Insulin: the other anabolic hormone of puberty

To determine the effect of puberty on insulin action, we used the euglycaem ic-hyperinsulinaemic clamp technique in combination with stable isotopes an d indirect calorimetry in lean preadolescents, adolescents and adults. Thes e studies indicated that the insulin resistance induced by normal puberty a lters glucose metabolism but is insufficient to adversely affect insulin-st imulated protein metabolism or to inhibit Lipolysis. Using the hyperglycaem ic clamp technique, we evaluated the impact of the insulin resistance on in sulin secretion in pre-adolescents, adolescents and young adults. These stu dies revealed that the insulin and C-peptide responses to a standardized in travenous hyperglycaemic stimulus were two- to threefold greater in adolesc ents than in pre adolescent children and adults. As growth hormone (GPI), i nsulin-like growth factor I (IGF-I) and insulin levels normally peak during puberty, we examined the influence of insulin on IGF-I regulation by measu ring basal GH, total and free IGF-I, and IGF binding protein (IGFBP) levels in lean adolescents and young adults. During the clamp studies, the adoles cents exhibited low levels of IGFBP-1 and -2 as well as a reduced insulin-i nduced suppression of IGFBP-1, compared with lean adults. Thus, we postulat e that the insulin resistance of puberty induces compensatory hyperinsulina emia, which in turn suppresses circulating levels of IGFBP-1, which in turn leads to increased levels of free IGF-I.