IGF-1 is naturally produced in the liver as a result of GH (Growth Hormone) metabolism in the presence of insulin. Muscle tissue can also produce IGF-1 by way of an intracellular response. In fact, one of the benefits of training sets that result in an intense burn, or stretch position training, is the production of natural IGF-1. It is also a side effect of oral 17-ALFA ALKYLATED STEROIDS, which cause a higher release of IGF-1 from the liver. IGF-1 receptors exist throughout muscles and organs such as the heart, spleen, small intestines, and kidneys with a higher concentration of receptors exerting effects upon organs. IGF-1 is extremely anabolic, far more so than GH or Insulin.