In biochemical terms, GH stimulates protein synthesis and increases fat breakdown to provide the energy necessary for tissue growth. It also antagonizes (opposes) the action of insulin. GH may act directly on tissues, but much of its effect is mediated by stimulation of the liver and other tissues to produce and release insulin-like growth factors, primarily insulin-like growth factor 1 (IGF-1; formerly called somatomedin). The term insulin-like growth factor is derived from the ability of high concentrations of these factors to mimic the action of insulin, although their primary action is to stimulate growth. Serum IGF-1 concentrations increase progressively with age in children, with an accelerated increase at the time of the pubert