Anabolic Hormones Part 3: Insulin-Like Growth Factors
What is it?
I will give a very condensed explanation of what insulin-like growth factors (IGFs) are. In a process that takes anywhere from 8 to 29 hours to complete, growth hormone stimulates liver cell DNA to synthesize IGFs. The IGFs then attach to binding proteins in order to be transported by the blood stream to specific tissues throughout the body. The stress created from resistance training influences the hormonal response of IGFs so that they can stimulate growth of muscle, nerve, and bone tissue remodeling. Many of these hormonal pathways are still not understood as the interactions between hormones and their different pathways are so diverse.
Exercise Responses of Insulin-Like Growth Factors
IGF-1 has been the most studied in the context of exercise because of its immense role in protein anabolism. There is still so much to this process that is still unknown because of the 8 to 29 hour window of operation from IGFs. Researchers do believe that it is possible to initiate IGF synthesis caused by the disruption or breakdown of cellular material in the body from resistance training, including fat cells and muscle cells.
Having said that, there have also been studies that have shown the release of IGFs from fat cells caused by non exercise related stress. IGFs are stored it fairly large quantities in our fat cells but in much lower amounts in our muscle cells.
One thing that is for certain however, is that cases have been shown where if an individual's IGF concentrations are low prior to a resistance workout, the levels have risen post-workout.
Training Adaptations of IGFs
Responses of IGFs to heavy resistance training is still unknown, but levels of IGFs prior to a workout appear to affect the levels post-workout. For example, if an individual has lower levels of IGF prior to their workout, then they would see an increase due to resistance training. However, if their levels are high prior to their workout, they would see no increase, as if their body has already reached a level suited to their needs.
Mechano Growth Factor (MGF)
IGFs that are produced directly in the muscle cells are known as mechano growth factors (MGFs). These are produced in response to muscular overload and stretch (resistance training). It has been suggested that while IGFs promote the initiation of protein synthesis, MGFs initiate the growth or hypertrophy of the muscle cells themselves. MGFs have received great interest from bodybuilders as the direct injection of MGFs into the muscles appear to be a primary anabolic hormone for muscle hypertrophy.
Baechle, Thomas R. and Earle, Roger W. Essentials of Strength Training and Conditioning Third Edition