Last week we discussed the anabolic hormone testosterone. This week, part 2 of 3, will focus on growth hormone.
What is Growth Hormone?
Also known as "somatotropin", growth hormone comes from the pituitary gland. It has many different uses and anabolic effects within the human body which can have direct effects or mediated through the production of insulin-like growth factor-1 (IGF-1) by the liver and fat cells.
What Does It Do?
Growth hormone is present and very important in the growth and development of children, but has also been seen as a result of resistance training. The tissues that growth hormone interact with include bones, immune cells, skeletal muscles, fat cells, and liver tissues. The main physiological roles are:
-Decreases glucose utilization
-Decreases glycogen synthesis
-Increases amino acid transport across cell membranes
-Increases protein synthesis
-Increases utilization of fatty acids
-Increases fat breakdown
-Increases availability of glucose and amino acids
-Enhances immune cell function
How Does it Work?
As one of the most potent anabolic hormones, growth hormone may me mediated by secondary hormones, but for the most part, it acts directly on target tissues. It interacts with tissues and stimulates the release of IGFs which greatly increase the conditions that promote tissue repair.
Growth hormone is released from hepatic (liver) cells as well as non-hepatic cells (fat, white blood cells, muscle, etc.) and enters peripheral circulation. It is transported in this way so that it can attach to specific binding proteins on cells.
Pharmacological Growth Hormone
Due to the varied uses of growth hormone in the human body, it is still unclear as to why pharmacological growth hormone acts differently than exercised-induced growth hormone. One thing that has been researched and documented however is that injecting growth hormone into an individual may increase muscle hypertrophy (size) but this may compromise the quality of muscle being built. For still-unknown reasons, exercised-induced hypertrophy is a much more favorable technique for gaining mass while also maintaining muscle quality and strength.
Program Design
If an individual is training for hypertrophy purposes, their program design is very important in order to reach their goals. Studies have shown that increased blood lactate concentrations increase the level of growth hormone. One thing to note however, is that when individuals were studied using very light weights and exercising in a much higher repetition range, serum levels of growth hormone did not change. There appears to be an intensity limit for individuals to reach in order to stimulate growth hormone production.
On the other hand, individuals who were training with heavy resistance (10 rep maximum) with short rest periods between sets (1 minute) saw greater growth hormone concentration increases when compared to individuals training in the 5 rep maximum range with longer rest periods. This has been studied and documented before and should be of no surprise as strength or power training (very high reps, 5 or fewer reps) works very well to increase and individual's overall strength, but a 6-12 rep range is ideal for muscular growth.
Growth Hormone in Women
Studies have shown that women have higher blood levels of growth hormone throughout their menstrual cycle compared to men. This not only proves that growth hormone plays many different physiological roles within the human body, but also that it is not the single limiting-factor when it comes to muscular growth. This may be due to the fact that women generally have lower levels of testosterone, but other factors must also be at play when determining what causes muscular hypertrophy.
Summary
Although there have been many studies done on the effects of both exercised-induced, as well as pharmacological growth hormone, there are still many variants and observations that need to be done. It is understood that there are multiple interactions that growth hormone plays a role in that determines muscular growth and strength as it has been shown that simply increasing concentrations does not necessarily increase muscle size/strength. Also, understanding how growth hormone concentrations change throughout not only an individual's day to day life, but also over long periods of resistance training can help us further understand the full role this anabolic hormone plays in our bodies.
-Tyler Robbins
B.Sc. CSCS
Sources:
Baechle, Thomas R. and Earle, Roger W. Essentials of Strength Training and Conditioning Third Edition
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