Written by: Rob Egbers University of Michigan Medical School
Reviewed for medical accuracy by: Kestrson S., M.D.
Added to knowledge base: 10/16/08
Growth hormone supplementation has been widely publicized lately due to recent allegations against Jason Grimsley and David Segui of Major League Baseball fame. People choose to take the supplement for a promise of augmented athletic performance or to turn back the clock on life, without chance of detection by governing agencies. Before jumping on the bandwagon, you should know what growth hormone is and what its effects are.
What is Growth Hormone?
The polypeptide human growth hormone, hGH, is one of the most important naturally occurring hormones for normal growth to adult size. It is normally secreted in a pulsatile fashion 6-12 times per day, meaning concentrations fluctuate throughout the day. Growth hormone secretion peaks during puberty and decreases with age. Secretion is increased by sleep, stress, starvation, puberty, exercise, hypoglycemia (low blood sugar), and other hormones (ie growth hormone releasing hormone).
hGH secretion is inhibited by somatostatin (from the hypothalamus), somatomedins (hormones released by the hGH target organs in response to a rise in hGH), obesity, hyperglycemia (high blood sugar) and pregnancy. It is important to understand that like many other hormone systems in the body, hGH and GHRH inhibit their own secretion as part of a negative feedback system.
What is the effect of Growth Hormone?
hGH is taken up by skeletal muscle and has many effects. It acts directly to decrease glucose uptake into cells, increase hepatic gluconeogenisis, increase lipolysis (fat release from adipose cells), increase protein synthesis in muscle cells, increase lean body mass and increase release of insulin like growth factor (IGF-1) from the liver and locally in skeletal muscle. IGF-1 increases bone length in children, increases protein synthesis in muscle and increases lean body mass. Both growth hormone and IGF-1 are anabolic, non-androgenic compounds. This means that it promotes protein synthesis without stimulating male secondary sex characteristics.
Are there diseases of Growth Hormone secretion?
Yes. A deficiency in hGH can result in failure to grow and mild obesity. Currently the FDA has approved growth hormone therapy in the treatment of childhood growth deficiencies and wasting diseases such as AIDS.
There are also diseases of excess growth hormone secretion. Excess hGH before puberty can cause gigantism and after puberty causes an increase in bone growth, organ size and glucose intolerance.
How can I increase my levels of hGH?
As already mentioned, exercise can naturally increase growth hormone secretion. Research has shown that exercise counteracts the negative feedback mechanisms of GHRH and hGH allowing levels to rise unchecked by this feedback system. Research has also shown that repeated exercise sessions led to an increase in hGH after each session and over a 24 hour period. Results suggest that load and frequency of the resistance exercise are important determinants of hGH concentration while length of the workout is not.
Over the short term, endurance exercise also increases growth hormone release. Significant increases in plasma growth hormone was noted after 10 minutes of exercise above the lactic acid threshold (ie high intensity). Multiple exercise sessions further increased the amount of growth hormone released per pulse rather than leading to feedback inhibition. However, in response to chronic endurance exercise (greater than 3 weeks), some studies found a decreased growth hormone response to additional exercise bouts. This suggests that the hGH response to endurance exercise is adaptive and may plateau with time.
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