““In the bloodstream, virtually all insulin-like growth factors are attached to small proteins that ferry them around to various tissues where they might be needed. But the IGFs, when attached to these proteins, are too large and unwieldy to pass through the walls of blood vessels and get to the tissues and cells where the IGF might be used. At any one time, only a small percentage of IGF in the circulation is left unbound to stimulate the growth of cells. These binding proteins constitute yet another of the mechanisms used by the body to regulate hormonal signals and growth factors. Insulin appears to depress the concentration of IGF-binding proteins, and so high levels of insulin mean more IGF itself is available to effect cell growth—including that of malignant cells. Anything that increases insulin levels will therefore increase the availability of IGF to the cells, and so increase the strength of the IGF proliferation signals. (Insulin has been shown to affect estrogen this way, too, one way in which elevated levels of insulin may potentially cause breast cancer.) The””
