THE 411 ON IGF-1
As the name implies, IGF-1 is a protein with a similar structure to insulin. Its presence in the body was first discovered in 1957 when scientists realized that growth hormone (GH) seemed to have another chemical that mediated its anabolic effects—somatomedin C, or as it’s now known, insulin-like growth factor-1 (IGF-1).
Originally, it was thought that IGF-1 was produced and released from the liver. From there, it is carried in the blood by one of its binding proteins to various tissues, such as muscle cells, where it instigates growth. While there are a host of IGF-1 binding proteins (IGFBPs), the two most important ones are IGFBP-1 and IGFBP-3. Most of the IGF-1 released from the liver is bound to IGFBP-3. This binding protein appears to facilitate IGF-1’s anabolic actions. IGFBP-1, on the other hand, appears to negate IGF-1’s anabolic actions.
It was later realized that IGF-1 is also produced in the muscle cells themselves. Muscle actually produces three differ ent kinds of IGF-1: 1) IGF-1Ea, IGF-1Eb, and IGF-1Ec. IGF-1ea is similar to the IGF-1 produced by the liver, while IGF-1Ec is better known as mechano-growth factor (MGF) due to the fact that it is expressed in muscle following mechanical stress, for example, from weightlifting. While the exact mechanism behind how these growth factors are produced in the muscle is still debatable, both circulating GH and testosterone levels may play a key role in increasing their expression in muscle.
The main way that circulating IGF-1 is believed to kick-start muscle growth is by binding to its receptor found on the membrane of muscle cells, where it sets off a cascade of events that leads to an increase in muscle protein synthesis—that is, a buildup of the protein that makes muscle and therefore results in growth. Binding IGF-1 to its receptor also decreases muscle protein breakdown. Muscle protein is constantly being shredded and built up. A decrease in muscle protein breakdown with a boost in its buildup (synthesis) leads to growth. Circulating IGF-1 has also been found to provide a brain-boosting affect, which may be the link between exercise and better brain function.